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* Re: MVCC and all that...
@ 2025-09-09 17:57 Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
0 siblings, 1 reply; 21+ messages in thread
From: Ellen Allhatatlan @ 2025-09-09 17:57 UTC (permalink / raw)
To: Merlin Moncure <[email protected]>; +Cc: pgsql-general
> Note: your link is wrong, corrected here:
Extra hyphen - sorry about and thanks for pointing it out!
> What the article is driving at is that postgres does not use rollback logs to handle updated records in the MVCC implementation. There are absolutely performance tradeoffs in that decision and, if you do a lot of development against postgresql, those tradeoffs should influence how you design databases. The author then cherry picked the 'worst case' case, large unconstrained updates.
Hmm... I was wondering about that - even though he stressed that there
was (paraphrasing) no right or wrong - just different design
decisions!
> The article is a bit of a cheezy dig on postgres. Another example is the complaint about autonomous transactions with another cherry picked example to make postgres look back. In the real world, these would not matter much, and can be worked around (if you want to see my take on how to deal with it, see here: https://github.com/leaselock/pgasync).
OK - so, I was wrong in my original assumption that somehow (and it
wasn't simply because of the phraseology - sweep vs vacuum) I thought
that PG and FB had a similar MVCC implementation vs. Oracle and MySQL
(InnoDB) (and OrioleDB). I'll do a deep dive into their docco and see
what they actually do! I'm actually very interested in the
benchmarking side of database technology - but I do know the old adage
- there are lies, damned lies, statistics and *_then_* there are
database benchmarks (as seen with the link I posted!).
Thanks for your input.
Best regards,
El!
> merlin
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
@ 2025-09-09 22:55 ` Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
0 siblings, 1 reply; 21+ messages in thread
From: Merlin Moncure @ 2025-09-09 22:55 UTC (permalink / raw)
To: Ellen Allhatatlan <[email protected]>; +Cc: pgsql-general
On Tue, Sep 9, 2025 at 11:57 AM Ellen Allhatatlan <
[email protected]> wrote:
> > Note: your link is wrong, corrected here:
>
> Extra hyphen - sorry about and thanks for pointing it out!
>
> > What the article is driving at is that postgres does not use rollback
> logs to handle updated records in the MVCC implementation. There are
> absolutely performance tradeoffs in that decision and, if you do a lot of
> development against postgresql, those tradeoffs should influence how you
> design databases. The author then cherry picked the 'worst case' case,
> large unconstrained updates.
>
> Hmm... I was wondering about that - even though he stressed that there
> was (paraphrasing) no right or wrong - just different design
> decisions!
>
> > The article is a bit of a cheezy dig on postgres. Another example is
> the complaint about autonomous transactions with another cherry picked
> example to make postgres look back. In the real world, these would not
> matter much, and can be worked around (if you want to see my take on how to
> deal with it, see here: https://github.com/leaselock/pgasync).
>
> OK - so, I was wrong in my original assumption that somehow (and it
> wasn't simply because of the phraseology - sweep vs vacuum) I thought
> that PG and FB had a similar MVCC implementation vs. Oracle and MySQL
> (InnoDB) (and OrioleDB). I'll do a deep dive into their docco and see
> what they actually do! I'm actually very interested in the
> benchmarking side of database technology - but I do know the old adage
> - there are lies, damned lies, statistics and *_then_* there are
> database benchmarks (as seen with the link I posted!).
>
Sure. I think you'll find that postgres approach to MVCC is somewhat
unusual relative to other players in this space, which is to write 'old'
records or changes in a rollback log; if the transaction commits it is
discarded but if it rolls back, the rollback log is written back to the
heap. This makes rollbacks potentially very painful. I suspect the
postgres approach can also do better in cases of highly contended records,
but that's just a guess.
For small (one or a small number of records) updates, the approach doesn't
make a whole lot of difference especially if you are aware of and exploit
HOT. For very large updates however, it absolutely does, and one might try
to avoid them using various strategies. I very much appreciate fast
rollbacks though.
merlin
merlin
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
@ 2025-09-10 00:41 ` Justin <[email protected]>
2025-09-10 01:11 ` Re: MVCC and all that... Ron Johnson <[email protected]>
2025-09-10 07:41 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-10 15:55 ` Re: MVCC and all that... Ron Johnson <[email protected]>
2025-09-10 21:28 ` Re: MVCC and all that... Nico Williams <[email protected]>
0 siblings, 4 replies; 21+ messages in thread
From: Justin @ 2025-09-10 00:41 UTC (permalink / raw)
To: Merlin Moncure <[email protected]>; +Cc: Ellen Allhatatlan <[email protected]>; pgsql-general
I read through the article its click bait/flame war just waiting to happen.
Article is a list of cherry picked PG drawbacks that can be mitigated or
worked around.
On the bulk updating. I'm shaking my finger at any one that locks up 25%
of a table with an update or delete. That is asking for problems in a
production database with a high TPS rate.
The author brings up threaded vs multi-process. That's an old old old old
old conversation that has been shown there is no clear better way.
Number of open connections. so firebird can do 1000 open sessions with a
smaller memory footprint, still can not have 1000 simultaneous running
sessions unless we have 1000 CPU's. Where is the win here?? We should be
managing resources better on the application side, not opening thousands of
connections that sit idle doing nothing.
On autonomous transactions we have procedures now that allow transactions
inside of transactions that can be committed and rollbacked. that has been
around for several years now.
Backup argument is cherry picking and not discussing pgBackrest and other
solutions or the use of tablespaces to isolate databases in a cluster at
the disk layer or disk snapshots.
"PostgreSQL has a relatively simple, but fast query planning algorithm"
Compared to what.... What feature is PG missing these days... the only
thing I know it can't do is change the plan in the middle of the
execution stage. Which is not a query planner thing but the execution
layer saying to itself I am taking too long maybe go back to the planning
stage... Query Hints that have been discussed endlessly. Adding hints
adds its own problems and has become a big mess for databases that support
it.
Multiple transactions per connection. I am asking WHY is that a feature.
when one can have multiple sessions, what is the difference? running
multiple transactions in single or multiple sessions means moving part of
transaction logic into the application space. What do we gain here.....
No application packaging. This Oracle thing that firebird has duplicated
at some level. we can simulate this with namespace/schemas.
I can keep going on here.
There are litigmate points here
Compression,
not being able to return partials result sets from functions
XID being 32 bit
anonymous functions in PG have several limitation not just input
arguments (not sure i see the need for that)
Temporary tables are a pain and cause issues for big databases
The article is unfair in many places..
On Tue, Sep 9, 2025 at 6:55 PM Merlin Moncure <[email protected]> wrote:
> On Tue, Sep 9, 2025 at 11:57 AM Ellen Allhatatlan <
> [email protected]> wrote:
>
>> > Note: your link is wrong, corrected here:
>>
>> Extra hyphen - sorry about and thanks for pointing it out!
>>
>> > What the article is driving at is that postgres does not use rollback
>> logs to handle updated records in the MVCC implementation. There are
>> absolutely performance tradeoffs in that decision and, if you do a lot of
>> development against postgresql, those tradeoffs should influence how you
>> design databases. The author then cherry picked the 'worst case' case,
>> large unconstrained updates.
>>
>> Hmm... I was wondering about that - even though he stressed that there
>> was (paraphrasing) no right or wrong - just different design
>> decisions!
>>
>> > The article is a bit of a cheezy dig on postgres. Another example is
>> the complaint about autonomous transactions with another cherry picked
>> example to make postgres look back. In the real world, these would not
>> matter much, and can be worked around (if you want to see my take on how to
>> deal with it, see here: https://github.com/leaselock/pgasync).
>>
>> OK - so, I was wrong in my original assumption that somehow (and it
>> wasn't simply because of the phraseology - sweep vs vacuum) I thought
>> that PG and FB had a similar MVCC implementation vs. Oracle and MySQL
>> (InnoDB) (and OrioleDB). I'll do a deep dive into their docco and see
>> what they actually do! I'm actually very interested in the
>> benchmarking side of database technology - but I do know the old adage
>> - there are lies, damned lies, statistics and *_then_* there are
>> database benchmarks (as seen with the link I posted!).
>>
>
> Sure. I think you'll find that postgres approach to MVCC is somewhat
> unusual relative to other players in this space, which is to write 'old'
> records or changes in a rollback log; if the transaction commits it is
> discarded but if it rolls back, the rollback log is written back to the
> heap. This makes rollbacks potentially very painful. I suspect the
> postgres approach can also do better in cases of highly contended records,
> but that's just a guess.
>
> For small (one or a small number of records) updates, the approach doesn't
> make a whole lot of difference especially if you are aware of and exploit
> HOT. For very large updates however, it absolutely does, and one might try
> to avoid them using various strategies. I very much appreciate fast
> rollbacks though.
>
> merlin
>
>
>
>
>
> merlin
>
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
@ 2025-09-10 01:11 ` Ron Johnson <[email protected]>
3 siblings, 0 replies; 21+ messages in thread
From: Ron Johnson @ 2025-09-10 01:11 UTC (permalink / raw)
To: pgsql-general
On Tue, Sep 9, 2025 at 8:41 PM Justin <[email protected]> wrote:
> I read through the article its click bait/flame war just waiting to happen.
>
> Article is a list of cherry picked PG drawbacks that can be mitigated or
> worked around.
>
> On the bulk updating. I'm shaking my finger at any one that locks up 25%
> of a table with an update or delete. That is asking for problems in a
> production database with a high TPS rate.
>
> The author brings up threaded vs multi-process. That's an old old old old
> old conversation that has been shown there is no clear better way.
>
> Number of open connections. so firebird can do 1000 open sessions with a
> smaller memory footprint, still can not have 1000 simultaneous running
> sessions unless we have 1000 CPU's. Where is the win here?? We should be
> managing resources better on the application side, not opening thousands of
> connections that sit idle doing nothing.
>
> On autonomous transactions we have procedures now that allow transactions
> inside of transactions that can be committed and rollbacked. that has been
> around for several years now.
>
> Backup argument is cherry picking and not discussing pgBackrest and other
> solutions or the use of tablespaces to isolate databases in a cluster at
> the disk layer or disk snapshots.
>
> "PostgreSQL has a relatively simple, but fast query planning algorithm"
> Compared to what.... What feature is PG missing these days... the only
> thing I know it can't do is change the plan in the middle of the
> execution stage. Which is not a query planner thing but the execution
> layer saying to itself I am taking too long maybe go back to the planning
> stage... Query Hints that have been discussed endlessly. Adding hints
> adds its own problems and has become a big mess for databases that support
> it.
>
> Multiple transactions per connection. I am asking WHY is that a feature.
> when one can have multiple sessions, what is the difference? running
> multiple transactions in single or multiple sessions means moving part of
> transaction logic into the application space. What do we gain here.....
>
> No application packaging. This Oracle thing that firebird has duplicated
> at some level. we can simulate this with namespace/schemas.
>
>
> I can keep going on here.
>
> There are litigmate points here
> Compression,
> not being able to return partials result sets from functions
> XID being 32 bit
>
Would converting them to 64 bits require changing the on-disk structure of
database files?
> anonymous functions in PG have several limitation not just input
> arguments (not sure i see the need for that)
>
Aren't transience and "ad hockery" the whole point of anonymous
procedures? Thus, I don't see the point of passing them parameters, either.
(When I *do* need something similar, I build the DO block as a bash string
variable with environment variables as "parameters", and then execute it
via psql -c "$sql" More like a template, TBH. It's great for purging old
data from tables, since I can bypass records who's DELETE statements fail
due to a FK constraint.
> Temporary tables are a pain and cause issues for big databases
>
I'd like to see GLOBAL TEMPORARY tables. Each connection gets its own
private copy of the table, so that applications don't need to carry around
CREATE TEMPORARY TABLE code with them.
--
Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
<Redacted> lobster!
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
@ 2025-09-10 07:41 ` Ellen Allhatatlan <[email protected]>
2025-09-10 14:46 ` Re: MVCC and all that... Adrian Klaver <[email protected]>
2025-09-10 15:29 ` Re: MVCC and all that... Justin <[email protected]>
3 siblings, 2 replies; 21+ messages in thread
From: Ellen Allhatatlan @ 2025-09-10 07:41 UTC (permalink / raw)
To: Justin <[email protected]>; +Cc: Merlin Moncure <[email protected]>; pgsql-general
Hi, and thanks for your input,
Just before I reply - if you (at least here in Ireland - Google's
answers vary per location, unlike Duckduckgo's) search for "firebird
mvcc mechanism" the "AI assistant" tells me twice that FB's MVCC
implementation is "like PostgreSQL's"... I'll investigate further and
report back. Igor Rogov's book looks like a good place to start!
> I read through the article its click bait/flame war just waiting to happen.
> Article is a list of cherry picked PG drawbacks that can be mitigated or worked around.
Pity - I took the guy at his word when he said that PostgreSQL was
just different, not better or worse.
> On the bulk updating. I'm shaking my finger at any one that locks up 25% of a table with an update or delete. That is asking for problems in a production database with a high TPS rate.
OK - I'm going to run the benchmarks myself and see what happens - but
I"m sure he didn't pick that test for nothing - come to think of it,
the table stable structure is bizarre!
> The author brings up threaded vs multi-process. That's an old old old old old conversation that has been shown there is no clear better way.
This is where things become interesting. Firebird actually has 3
process/threading models - and they manage to maintain these with a
team that is *_much_* smaller than the PostgreSQL one - FB is a minnow
compared to PG!
AIUI, Michael Stonebraker suggested that the process model
would/should be "upgraded" to a threaded one at some point in the
system's developement?
> Number of open connections. so firebird can do 1000 open sessions with a smaller memory footprint, still can not have 1000 simultaneous running sessions unless we have 1000 CPU's. Where is the win here?? We should be managing resources better on the application side, not opening thousands of connections that sit idle doing nothing.
Agreed on that point.
> On autonomous transactions we have procedures now that allow transactions inside of transactions that can be committed and rollbacked. that has been around for several years now.
OK.
> Backup argument is cherry picking and not discussing pgBackrest and other solutions or the use of tablespaces to isolate databases in a cluster at the disk layer or disk snapshots.
OK again. I'm just wondering if the single file per database isn't a
fundamental architectural flaw in itself? AIUI, you could have
mulitple files (back in 32-bit land) "chained" - but (again AIUI) the
same table could be spread over x files - all "intermingled"... weird.
> "PostgreSQL has a relatively simple, but fast query planning algorithm" Compared to what.... What feature is PG missing these days... the only thing I know it can't do is change the plan in the middle of the execution stage. Which is not a query planner thing but the execution layer saying to itself I am taking too long maybe go back to the planning stage... Query Hints that have been discussed endlessly. Adding hints adds its own problems and has become a big mess for databases that support it.
I know - personally, I'm in favour of the PostgreSQL approach - rather
than improve the hints, improve the planner!
Plus, if you really want to, you can go here:
https://www.postgresql.org/docs/current/runtime-config-query.html and,
for example
SET enable_seqscan = OFF;
Plus, there is/are extension(s) which allow one to provide hints - I
did think this was a bit of a whopper alright!
> Multiple transactions per connection. I am asking WHY is that a feature. when one can have multiple sessions, what is the difference? running multiple transactions in single or multiple sessions means moving part of transaction logic into the application space. What do we gain here.....
No idea - I'll take your word for it!
> No application packaging. This Oracle thing that firebird has duplicated at some level. we can simulate this with namespace/schemas.
Again, I'm not too sure of my ground here - but I do know that Oracle
(and SQL Server) are ahead in this domain.
> There are litigmate points here
> Compression,
> not being able to return partials result sets from functions
> XID being 32 bit
There's a lot of talk about 64 bit ones - FB has 48 bit ones AIUI -
that could kick the can down the road for PostgreSQL at the price of 2
bytes per record - is it worth it to alleviate the difficulties
associated with VACUUM-ing?
> anonymous functions in PG have several limitation not just input arguments (not sure i see the need for that)
> Temporary tables are a pain and cause issues for big databases
> The article is unfair in many places..
Accepted now - thanks for your input.
--
El!
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 07:41 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
@ 2025-09-10 14:46 ` Adrian Klaver <[email protected]>
2025-09-10 15:08 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
1 sibling, 1 reply; 21+ messages in thread
From: Adrian Klaver @ 2025-09-10 14:46 UTC (permalink / raw)
To: Ellen Allhatatlan <[email protected]>; Justin <[email protected]>; +Cc: Merlin Moncure <[email protected]>; pgsql-general
On 9/10/25 00:41, Ellen Allhatatlan wrote:
> Hi, and thanks for your input,
>
>> The author brings up threaded vs multi-process. That's an old old old old old conversation that has been shown there is no clear better way.
>
> This is where things become interesting. Firebird actually has 3
> process/threading models - and they manage to maintain these with a
> team that is *_much_* smaller than the PostgreSQL one - FB is a minnow
> compared to PG!
Though I would like to know what happened in mid 2010?:
https://github.com/FirebirdSQL/firebird/graphs/contributors
>
>> Backup argument is cherry picking and not discussing pgBackrest and other solutions or the use of tablespaces to isolate databases in a cluster at the disk layer or disk snapshots.
>
> OK again. I'm just wondering if the single file per database isn't a
> fundamental architectural flaw in itself? AIUI, you could have
> mulitple files (back in 32-bit land) "chained" - but (again AIUI) the
> same table could be spread over x files - all "intermingled"... weird.
>
What are you referring to above?
At any rate from the article:
"pg_basebackup always dumps the entire cluster, when restoring one
database, your entire PostgreSQL cluster will be restored, which will
contain only one database. The files of all your other databases will be
zero size."
AFAIK there is no option to restore one database with pg_basebackup.
I think the author is referring to pgBackRest:
https://pgbackrest.org/user-guide.html#restore/option-db-include
" The test1 database, despite successful recovery, is not accessible.
This is because the entire database was restored as sparse, zeroed files."
--
Adrian Klaver
[email protected]
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 07:41 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-10 14:46 ` Re: MVCC and all that... Adrian Klaver <[email protected]>
@ 2025-09-10 15:08 ` Ellen Allhatatlan <[email protected]>
2025-09-10 15:11 ` Re: MVCC and all that... Ron Johnson <[email protected]>
0 siblings, 1 reply; 21+ messages in thread
From: Ellen Allhatatlan @ 2025-09-10 15:08 UTC (permalink / raw)
To: Adrian Klaver <[email protected]>; +Cc: Justin <[email protected]>; Merlin Moncure <[email protected]>; pgsql-general
> Though I would like to know what happened in mid 2010?:
> https://github.com/FirebirdSQL/firebird/graphs/contributors
Yes, indeed, WTF? I'm not a member of the FB Illuminati - so I can't say!
> > OK again. I'm just wondering if the single file per database isn't a
> > fundamental architectural flaw in itself? AIUI, you could have
> > mulitple files (back in 32-bit land) "chained" - but (again AIUI) the
> > same table could be spread over x files - all "intermingled"... weird.
> What are you referring to above?
I'm sorry - the single file flaw I was referring to occurs in FB and
has nothing to do with PG.
FB dbs are single files - or were - 32 bit - up to 2GB and then there
was another file. I don't know what happens for 64 bit - (note to self
- find out)!
So, you have table X - it has 2M rows (say, 0.5 GB) in the first file
(along with all the other tables). The 2GB limit is hit, more data is
added. 0.7 GB is added to table X - these records go into a new
database file - the table is split in two - you have 2 "extents" of
2GB with X split 0.5 - in extent1, 0.7 in extent2. All mixed up with
other tables as well!
That was the architectural flaw to which I was referring. Nothing to
do with PG, backups or anything like that - again, apologies for any
confusion - my phraseology wasn't the best! And I should have put what
I wrote elsewhere anyway!
--
El!
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 07:41 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-10 14:46 ` Re: MVCC and all that... Adrian Klaver <[email protected]>
2025-09-10 15:08 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
@ 2025-09-10 15:11 ` Ron Johnson <[email protected]>
2025-09-10 15:15 ` Re: MVCC and all that... Adrian Klaver <[email protected]>
0 siblings, 1 reply; 21+ messages in thread
From: Ron Johnson @ 2025-09-10 15:11 UTC (permalink / raw)
To: pgsql-general
On Wed, Sep 10, 2025 at 11:08 AM Ellen Allhatatlan <
[email protected]> wrote:
[snip]
> So, you have table X - it has 2M rows (say, 0.5 GB) in the first file
> (along with all the other tables). The 2GB limit is hit, more data is
> added. 0.7 GB is added to table X - these records go into a new
> database file - the table is split in two - you have 2 "extents" of
> 2GB with X split 0.5 - in extent1, 0.7 in extent2. All mixed up with
> other tables as well!
>
> That was the architectural flaw to which I was referring. Nothing to
> do with PG
>
You're gonna be in for a very rude surprise after adding 1GB of data to a
PG table...
--
Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
<Redacted> lobster!
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 07:41 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-10 14:46 ` Re: MVCC and all that... Adrian Klaver <[email protected]>
2025-09-10 15:08 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-10 15:11 ` Re: MVCC and all that... Ron Johnson <[email protected]>
@ 2025-09-10 15:15 ` Adrian Klaver <[email protected]>
0 siblings, 0 replies; 21+ messages in thread
From: Adrian Klaver @ 2025-09-10 15:15 UTC (permalink / raw)
To: Ron Johnson <[email protected]>; pgsql-general
On 9/10/25 08:11, Ron Johnson wrote:
> On Wed, Sep 10, 2025 at 11:08 AM Ellen Allhatatlan
> <[email protected] <mailto:[email protected]>> wrote:
> [snip]
>
> So, you have table X - it has 2M rows (say, 0.5 GB) in the first file
> (along with all the other tables). The 2GB limit is hit, more data is
> added. 0.7 GB is added to table X - these records go into a new
> database file - the table is split in two - you have 2 "extents" of
> 2GB with X split 0.5 - in extent1, 0.7 in extent2. All mixed up with
> other tables as well!
>
> That was the architectural flaw to which I was referring. Nothing to
> do with PG
>
> You're gonna be in for a very rude surprise after adding 1GB of data to
> a PG table...
Yes, but the table is split not the database as a whole per:
https://www.postgresql.org/docs/current/storage-file-layout.html
>
> --
> Death to <Redacted>, and butter sauce.
> Don't boil me, I'm still alive.
> <Redacted> lobster!
--
Adrian Klaver
[email protected]
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 07:41 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
@ 2025-09-10 15:29 ` Justin <[email protected]>
2025-09-10 17:28 ` Re: MVCC and all that... Álvaro Herrera <[email protected]>
2025-09-11 11:38 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
1 sibling, 2 replies; 21+ messages in thread
From: Justin @ 2025-09-10 15:29 UTC (permalink / raw)
To: Ellen Allhatatlan <[email protected]>; +Cc: Merlin Moncure <[email protected]>; pgsql-general
On Wed, Sep 10, 2025 at 3:41 AM Ellen Allhatatlan <
[email protected]> wrote:
> Hi, and thanks for your input,
>
> Just before I reply - if you (at least here in Ireland - Google's
> answers vary per location, unlike Duckduckgo's) search for "firebird
> mvcc mechanism" the "AI assistant" tells me twice that FB's MVCC
> implementation is "like PostgreSQL's"... I'll investigate further and
> report back. Igor Rogov's book looks like a good place to start!
>
> > I read through the article its click bait/flame war just waiting to
> happen.
> > Article is a list of cherry picked PG drawbacks that can be mitigated or
> worked around.
>
> Pity - I took the guy at his word when he said that PostgreSQL was
> just different, not better or worse.
>
> > On the bulk updating. I'm shaking my finger at any one that locks up
> 25% of a table with an update or delete. That is asking for problems in a
> production database with a high TPS rate.
>
> OK - I'm going to run the benchmarks myself and see what happens - but
> I"m sure he didn't pick that test for nothing - come to think of it,
> the table stable structure is bizarre!
>
My point here is not to criticize that article as being wrong, PG will
bloat the table and be slower than firebird PG. My criticism is this
argument/example is not reflective of the real world. What DBA/Programmer
would not be in trouble locking 25% of a table for a prolonged period of
time doing an update. This approach would all but guarantee an outage or
other issues across the database. This argument is bogus and very much
against best practices updating large amounts of data in a single
transaction creates blockers for other sessions.
>
> > The author brings up threaded vs multi-process. That's an old old old
> old old conversation that has been shown there is no clear better way.
>
> This is where things become interesting. Firebird actually has 3
> process/threading models - and they manage to maintain these with a
> team that is *_much_* smaller than the PostgreSQL one - FB is a minnow
> compared to PG!
>
> AIUI, Michael Stonebraker suggested that the process model
> would/should be "upgraded" to a threaded one at some point in the
> system's developement?
>
I am going to need a source on this. Process vs Threads: pro and cons are
very well documented and proven today.
>
>
> > Backup argument is cherry picking and not discussing pgBackrest and
> other solutions or the use of tablespaces to isolate databases in a
> cluster at the disk layer or disk snapshots.
>
> OK again. I'm just wondering if the single file per database isn't a
> fundamental architectural flaw in itself? AIUI, you could have
> mulitple files (back in 32-bit land) "chained" - but (again AIUI) the
> same table could be spread over x files - all "intermingled"... weird.
>
Single file approach vs multiple file approach. This boils down to
preference; there is no clear plus and minus to either approach. the Path
to the actual data has to go through several Maps.
Multiple Files. Disk Internal Structure, => File System( ZFS EXT FAT
etc..), => Table mapped to a File or group of Files,=> DataPage mapped to
data type and columns,
Single File Disk Internal Structure => File System( ZFS EXT FAT etc..),
Single File => Mapping Tables to locations in the single file => DataPage
mapped to data type and columns.
Single file has the advantage removing IO context switching at the OS
level, however it moves the Context switch to the database side
jumping around in the single file itself to find the necessary data.
This does not reduce IO
>
> > "PostgreSQL has a relatively simple, but fast query planning algorithm"
> Compared to what.... What feature is PG missing these days... the only
> thing I know it can't do is change the plan in the middle of the
> execution stage. Which is not a query planner thing but the execution
> layer saying to itself I am taking too long maybe go back to the planning
> stage... Query Hints that have been discussed endlessly. Adding hints
> adds its own problems and has become a big mess for databases that support
> it.
>
>
My criticism here "The claim postgresql query planner is simple" is not
backed up with actual examples and data. Looks at a specific case which
does not even involve the query planner.
>
> > Multiple transactions per connection. I am asking WHY is that a
> feature. when one can have multiple sessions, what is the difference?
> running multiple transactions in single or multiple sessions means moving
> part of transaction logic into the application space. What do we gain
> here.....
>
> No idea - I'll take your word for it!
>
Making a logical argument here, the statement this feature is a plus
without any examples to backup the claim. I'm trying to show a counter
example why would this be a plus when we can do the same thing just
differently. It ignores the obvious issue of moving the transaction logic
into the app has big drawbacks.
>
> Accepted now - thanks for your input.
>
>
> --
>
> El!
>
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 07:41 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-10 15:29 ` Re: MVCC and all that... Justin <[email protected]>
@ 2025-09-10 17:28 ` Álvaro Herrera <[email protected]>
1 sibling, 0 replies; 21+ messages in thread
From: Álvaro Herrera @ 2025-09-10 17:28 UTC (permalink / raw)
To: Justin <[email protected]>; +Cc: Ellen Allhatatlan <[email protected]>; Merlin Moncure <[email protected]>; pgsql-general
On 2025-Sep-10, Justin wrote:
> On Wed, Sep 10, 2025 at 3:41 AM Ellen Allhatatlan <
> [email protected]> wrote:
> > > The author brings up threaded vs multi-process. That's an old old old
> > old old conversation that has been shown there is no clear better way.
> >
> > This is where things become interesting. Firebird actually has 3
> > process/threading models - and they manage to maintain these with a
> > team that is *_much_* smaller than the PostgreSQL one - FB is a minnow
> > compared to PG!
> >
> > AIUI, Michael Stonebraker suggested that the process model
> > would/should be "upgraded" to a threaded one at some point in the
> > system's developement?
>
> I am going to need a source on this. Process vs Threads: pro and cons are
> very well documented and proven today.
My recollection is that this is correct -- Stonebraker and team used a
process model because it was the expedient thing to do. Decades later
we're still using it, but there's an ongoing effort to implement a
threaded model; there are patches already being committed for that. See
https://wiki.postgresql.org/wiki/Multithreading
Heikki Linnakangas gave two talks on this effort, one at pgconf.eu 2023
titled "Multithreaded PostgreSQL?" and another at pgconf.eu 2024 titled
"Multithreaded PostgreSQL!". You kinda see a progression there.
I expect the next one should be "Multithreaded PostgreSQL!!1yksitoista!"
or something.
--
Álvaro Herrera Breisgau, Deutschland — https://www.EnterpriseDB.com/
"Find a bug in a program, and fix it, and the program will work today.
Show the program how to find and fix a bug, and the program
will work forever" (Oliver Silfridge)
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 07:41 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-10 15:29 ` Re: MVCC and all that... Justin <[email protected]>
@ 2025-09-11 11:38 ` Ellen Allhatatlan <[email protected]>
2025-09-11 16:52 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
1 sibling, 1 reply; 21+ messages in thread
From: Ellen Allhatatlan @ 2025-09-11 11:38 UTC (permalink / raw)
To: Justin <[email protected]>; +Cc: Merlin Moncure <[email protected]>; pgsql-general
>> AIUI, Michael Stonebraker suggested that the process model
>> would/should be "upgraded" to a threaded one at some point in the
>> system's developement?
> I am going to need a source on this. Process vs Threads: pro and cons are very well documented and proven today.
Ask, and it will be given to you;
seek, and you will find;
knock, and it will be opened to you!
Matthew 7:7
From here: https://dsf.berkeley.edu/papers/ERL-M85-95.pdf (bottom of page 13) -
5.1. Process Structure - Stonebraker expresses regret that the project
doesn't have the resources to use the "server model" - i.e. threading:
"The server model has many performance benefits (e.g., sharing of open
file descriptors and buffers and optimized task switching and message
sending overhead) in a large machine environment in which high
performance is critical. However, this approach requires that a fairly
complete special-purpose operating system be built. In constrast, the
process-per-user model is simpler to implement but will not perform as
well on most conventional operating systems. We decided after much
soul searching to implement POSTGRES using a process-per-user model
architecture because of our limited programming esources. POSTGRES is
an ambitious undertaking and we believe the additional complexity
introduced by the server architecture was not worth the additional
risk of not getting the system running. Our current plan then is to
implement POSTGRES as a process-per-user model on Unix 4.3 BSD"
No date, but the latest date in the references was 1986.
and from here https://dsf.berkeley.edu/papers/ERL-M90-34.pdf (middle
of page 29):
"A last aspect of our design concerns the operating system process
structure. Currently, POSTGRES runs as one process for each active
user. This was done as an expedient to get a system operational as
quickly as possible. We plan on converting POSTGRES to use lightweight
processes available in the operating systems we are using. These
include PRESTO for the Sequent Symmetry and threads in Version 4 of
Sun/OS."
Again, no date, but this time the latest reference was from 1989 -
(threading advancing?).
The use of "because of our limited programming esources" (1st ref.)
and "expedient" (2nd) clearly indicates to me that Stonebraker was
eyeing up the threading model and saw the per-process one as an
albatross around the neck of the project!
I hope my sources are sufficient?
--
El!
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 07:41 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-10 15:29 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-11 11:38 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
@ 2025-09-11 16:52 ` Merlin Moncure <[email protected]>
2025-09-11 17:55 ` Re: MVCC and all that... Pavel Stehule <[email protected]>
2025-09-12 19:16 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
0 siblings, 2 replies; 21+ messages in thread
From: Merlin Moncure @ 2025-09-11 16:52 UTC (permalink / raw)
To: Ellen Allhatatlan <[email protected]>; +Cc: Justin <[email protected]>; pgsql-general
On Thu, Sep 11, 2025 at 5:38 AM Ellen Allhatatlan <
[email protected]> wrote:
> "A last aspect of our design concerns the operating system process
> structure. Currently, POSTGRES runs as one process for each active
> user. This was done as an expedient to get a system operational as
> quickly as possible. We plan on converting POSTGRES to use lightweight
> processes available in the operating systems we are using. These
> include PRESTO for the Sequent Symmetry and threads in Version 4 of
> Sun/OS."
>
Technical discussions from the 80's are more or less historically
interesting only. At that time, support for threads was pretty immature
relative to today, and the general state of operating system technology was
pretty crude by modern standards. Process spinup via fork() might also
have been much more performance relevant that it is today, and various
synchronization primitives might have been pretty lousy as well.
I find the threads/processes debate to be pretty silly in general. Things
have changed a lot, IPC has improved a lot, and I would argue the decision
to use/not use SSL is much more important to database session startup than
the database spinning up a process vs a thread. The mythology around this
architectural decision is pervasive and mostly incorrect IMO, and there are
many high quality solutions to work around this connection poolers,
pgbouncer, etc, which are essentially employed against all databases in
some or another way, and are essentially universally employed in scenarios
where scale and reliable performance is important.
Maybe Microsoft is the odd man out here as its weird non-standard process
model made porting multi process servers (including postgres) difficult and
imperformant. Those issues are (mostly) long since worked out though.
Having said that, I suspect 3rd party vendor support for postgres/microsoft
being relatively limited would be much more based in business calculation
rather than technical constraints.
Final thoughts on this: firebird (fmrly interbase) did not achieve the
level of success in the market that postgres, even though they may have
been similarly positioned. My take: that disparity in success has more to
do with postgres having a more open development model, stronger community,
and (especially) timing; postgres was pretty well established in the open
source world when Borland open sourced it around the year 2000. Firebird
had (and has) some neat stuff, in particular a nice embedding option and
strong windows support, but the market was already pretty crowded at that
time.
merlin
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 07:41 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-10 15:29 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-11 11:38 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-11 16:52 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
@ 2025-09-11 17:55 ` Pavel Stehule <[email protected]>
1 sibling, 0 replies; 21+ messages in thread
From: Pavel Stehule @ 2025-09-11 17:55 UTC (permalink / raw)
To: Merlin Moncure <[email protected]>; +Cc: Ellen Allhatatlan <[email protected]>; Justin <[email protected]>; pgsql-general
Hi
> Final thoughts on this: firebird (fmrly interbase) did not achieve the
> level of success in the market that postgres, even though they may have
> been similarly positioned. My take: that disparity in success has more to
> do with postgres having a more open development model, stronger community,
> and (especially) timing; postgres was pretty well established in the open
> source world when Borland open sourced it around the year 2000. Firebird
> had (and has) some neat stuff, in particular a nice embedding option and
> strong windows support, but the market was already pretty crowded at that
> time.
>
Still around 2005 Firebird was more popular than Postgres (in the Czech
Republic). I think after this time, the Firebird had not some wave of
popularity (and Postgres did - RoR, PostGIS, ora2pg, json, ...), and it was
too joined with windows desktop and Delphi. I have not any idea about
current situation, and current performance of Firebird, but 15 years ago
(Firebird 2.x), Postgres was better for web applications running on Linux
server with more than thousands users. I did some benchmarks at 2005, and
Firebird was really fast on Windows, and slower on Linux (slower than
Postgres).
> merlin
>
>
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 07:41 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-10 15:29 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-11 11:38 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-11 16:52 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
@ 2025-09-12 19:16 ` Ellen Allhatatlan <[email protected]>
2025-09-13 01:54 ` Re: MVCC and all that... Justin <[email protected]>
1 sibling, 1 reply; 21+ messages in thread
From: Ellen Allhatatlan @ 2025-09-12 19:16 UTC (permalink / raw)
To: Merlin Moncure <[email protected]>; +Cc: Justin <[email protected]>; pgsql-general
You, (Merlin Moncure) said:
> Technical discussions from the 80's are more or less historically interesting only.
I agree with your technical points - and the fact that I brought up "history".
I was replying to Justin in this context:
I wrote:
> AIUI, Michael Stonebraker suggested that the process model
> would/should be "upgraded" to a threaded one at some point in the
> system's developement?
To which Justin replied:
> I am going to need a source on this. Process vs Threads: pro and cons are very well documented and proven today.
Hence the history lesson - to provide the "source" from Michael Stonebraker
HTH.
> merlin
--
El!
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 07:41 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-10 15:29 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-11 11:38 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-11 16:52 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-12 19:16 ` Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
@ 2025-09-13 01:54 ` Justin <[email protected]>
0 siblings, 0 replies; 21+ messages in thread
From: Justin @ 2025-09-13 01:54 UTC (permalink / raw)
To: Ellen Allhatatlan <[email protected]>; +Cc: Merlin Moncure <[email protected]>; pgsql-general
On Fri, Sep 12, 2025 at 3:18 PM Ellen Allhatatlan <
[email protected]> wrote:
> You, (Merlin Moncure) said:
>
> > Technical discussions from the 80's are more or less historically
> interesting only.
>
> I agree with your technical points - and the fact that I brought up
> "history".
>
> I was replying to Justin in this context:
>
> I wrote:
>
> > AIUI, Michael Stonebraker suggested that the process model
> > would/should be "upgraded" to a threaded one at some point in the
> > system's developement?
>
> To which Justin replied:
>
> > I am going to need a source on this. Process vs Threads: pro and cons
> are very well documented and proven today.
>
> Hence the history lesson - to provide the "source" from Michael Stonebraker
>
>
> Thank you for the documents.
The reason PostgreSQL was not developed using threads was that it was not
technically feasible at the time. The entire idea of PostgreSQL was
experimental. One of the things you want to do is test out as many ideas
as you can feasibly do .
Jump forward 20-40 years into the future, what have we learned, What was
thought would be a clear advantage for threads has been shown not to be a
clear advantage.
Each approach has pluses and minuses. I agree with many others that the
time spent trying to get threads to work in PostgreSQL is just not worth it
when we should be spending our time other issues
moving XID to 64 bit and moving away from the current file format and stop
storing row versions in the tables, would pay far more dividends in
performance than threading will...
The append only storage design aka Storing row versions in the table is a
Stonebraker idea, which would allow for Time-Travel Queries, I am pretty
sure this proved to be unworkable.
PostgreSQL table layout and MVCC being tracked in the tables has been shown
to be problematic and a performance bottleneck . Just look at all the time
the community has spent making AutoVacuum better and all the IO spent
keeping the XID from wrapping around, or HOT updates or the FILL FACTOR
setting.
The question I should have asked is what is Stonebraker's current thought
on process vs threading today.
Thank you
Justin
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
@ 2025-09-10 15:55 ` Ron Johnson <[email protected]>
3 siblings, 0 replies; 21+ messages in thread
From: Ron Johnson @ 2025-09-10 15:55 UTC (permalink / raw)
To: pgsql-general
On Tue, Sep 9, 2025 at 8:41 PM Justin <[email protected]> wrote:
[snip]
> Multiple transactions per connection. I am asking WHY is that a feature.
> when one can have multiple sessions, what is the difference? running
> multiple transactions in single or multiple sessions means moving part of
> transaction logic into the application space. What do we gain here.....
>
If I interpret "Multiple transactions per connection" correctly, they're a
great way to emulate WITH HOLD cursors.
For example:
1. In channel #1, open a Read Only cursor. (Yes, this was long ago.)
2. Fetch a row.
3. Switch to channel #2, begin R/W txn, modify the table, commit.
4. Switch back to channel #1
5. Go to step 2.
Not being an application developer anymore, I've never needed to use WITH
HOLD cursors or wish for multiple channels in PG.
--
Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
<Redacted> lobster!
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
@ 2025-09-10 21:28 ` Nico Williams <[email protected]>
2025-09-10 22:20 ` Re: MVCC and all that... Justin <[email protected]>
3 siblings, 1 reply; 21+ messages in thread
From: Nico Williams @ 2025-09-10 21:28 UTC (permalink / raw)
To: Justin <[email protected]>; +Cc: Merlin Moncure <[email protected]>; Ellen Allhatatlan <[email protected]>; pgsql-general
On Tue, Sep 09, 2025 at 08:41:02PM -0400, Justin wrote:
> The author brings up threaded vs multi-process. That's an old old old old
> old conversation that has been shown there is no clear better way.
This is relevant to the next part:
> Number of open connections. so firebird can do 1000 open sessions with a
> smaller memory footprint, still can not have 1000 simultaneous running
> sessions unless we have 1000 CPU's. Where is the win here?? We should be
> managing resources better on the application side, not opening thousands of
> connections that sit idle doing nothing.
When a service is written in such a way as to minimize the memory
footprint of each request/client then it can process more of them
assuming it's only memory-bound. Why? Because less memory per thing ==
less bandwidth use, and also less thrashing of caches and higher cache
hit ratios.
Minimizing request/client state means not spreading any of it on the
stack, thus not requiring a stack per-client. This means not
thread-per-client (green or otherwise) or process-per-client. It means
essentially some flavor of continuation passing style (CPS). For a
query plan executor that's really: the query plan, all the in-flight I/O
requests, all cached data still needed to continue processing the plan.
If you have a Duff's device style / CPS style implementation, then
nothing on the stack needs to be preserved while waiting for I/Os, and
the state of the query plan is effectively minimized.
But for a database with storage I/O costs the memory footprint doesn't
matter quite so much because most likely it will be I/O bound rather
than CPU- or memory-bound.
> "PostgreSQL has a relatively simple, but fast query planning algorithm"
> Compared to what.... What feature is PG missing these days... the only
> thing I know it can't do is change the plan in the middle of the
> execution stage. Which is not a query planner thing but the execution
> layer saying to itself I am taking too long maybe go back to the planning
> stage... Query Hints that have been discussed endlessly. Adding hints
> adds its own problems and has become a big mess for databases that support
> it.
I would really like out-of-band hints. These would be hints not
specified in the SQL itself but to be sent separately and which address
table sources or joins by name, like this:
psql> SELECT .. FROM x x1 JOIN y y1 ON .. JOIN y y2 ON .. WHERE ..;
...> \hint y1 indexed by ..
...> \hint y2 indexed by ..
...> ;
> Multiple transactions per connection. I am asking WHY is that a feature.
> when one can have multiple sessions, what is the difference? running
> multiple transactions in single or multiple sessions means moving part of
> transaction logic into the application space. What do we gain here.....
I agree it's not really important. Moreover interleaving multiple
queries over one TCP connection will lead to having to manage how much
bandwidth each query consumes so as not to drown out the others.
> No application packaging. This Oracle thing that firebird has duplicated
> at some level. we can simulate this with namespace/schemas.
And extensions.
> XID being 32 bit
This is a huge problem.
> Temporary tables are a pain and cause issues for big databases
Yes. PG badly needs GLOBAL TEMP.
Another thing that would be nice is if PG could have tables that are not
heaps.
Nico
--
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 21:28 ` Re: MVCC and all that... Nico Williams <[email protected]>
@ 2025-09-10 22:20 ` Justin <[email protected]>
2025-09-10 23:07 ` Re: MVCC and all that... Nico Williams <[email protected]>
2025-09-11 00:11 ` Re: MVCC and all that... Ron Johnson <[email protected]>
0 siblings, 2 replies; 21+ messages in thread
From: Justin @ 2025-09-10 22:20 UTC (permalink / raw)
To: Nico Williams <[email protected]>; +Cc: Merlin Moncure <[email protected]>; Ellen Allhatatlan <[email protected]>; pgsql-general
On Wed, Sep 10, 2025 at 5:28 PM Nico Williams <[email protected]> wrote:
> On Tue, Sep 09, 2025 at 08:41:02PM -0400, Justin wrote:
> > The author brings up threaded vs multi-process. That's an old old old old
> > old conversation that has been shown there is no clear better way.
>
> This is relevant to the next part:
>
> > Number of open connections. so firebird can do 1000 open sessions with
> a
> > smaller memory footprint, still can not have 1000 simultaneous running
> > sessions unless we have 1000 CPU's. Where is the win here?? We should be
> > managing resources better on the application side, not opening thousands
> of
> > connections that sit idle doing nothing.
>
> When a service is written in such a way as to minimize the memory
> footprint of each request/client then it can process more of them
> assuming it's only memory-bound. Why? Because less memory per thing ==
> less bandwidth use, and also less thrashing of caches and higher cache
> hit ratios.
>
> Minimizing request/client state means not spreading any of it on the
> stack, thus not requiring a stack per-client. This means not
> thread-per-client (green or otherwise) or process-per-client. It means
> essentially some flavor of continuation passing style (CPS). For a
> query plan executor that's really: the query plan, all the in-flight I/O
> requests, all cached data still needed to continue processing the plan.
> If you have a Duff's device style / CPS style implementation, then
> nothing on the stack needs to be preserved while waiting for I/Os, and
> the state of the query plan is effectively minimized.
>
> But for a database with storage I/O costs the memory footprint doesn't
> matter quite so much because most likely it will be I/O bound rather
> than CPU- or memory-bound.
>
I am not following you here, Databases are going to be bound somewhere at
some point, Disk,IO, Network IO, Memory, or CPU bound. Which one is
causing the bottle neck just depends on the workload and size of the
database.
The number of idle sessions does not really matter it is just
wasting resources across the entire application stack.
> > "PostgreSQL has a relatively simple, but fast query planning algorithm"
> > Compared to what.... What feature is PG missing these days... the only
> > thing I know it can't do is change the plan in the middle of the
> > execution stage. Which is not a query planner thing but the execution
> > layer saying to itself I am taking too long maybe go back to the
> planning
> > stage... Query Hints that have been discussed endlessly. Adding hints
> > adds its own problems and has become a big mess for databases that
> support
> > it.
>
> I would really like out-of-band hints. These would be hints not
> specified in the SQL itself but to be sent separately and which address
> table sources or joins by name, like this:
>
> psql> SELECT .. FROM x x1 JOIN y y1 ON .. JOIN y y2 ON .. WHERE ..;
> ...> \hint y1 indexed by ..
> ...> \hint y2 indexed by ..
> ...> ;
>
>
I humbly disagree, the point of SQL being a 4th generation language is, I
tell it what I want, not how to go get what I want.
Thank you,
Justin
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 21:28 ` Re: MVCC and all that... Nico Williams <[email protected]>
2025-09-10 22:20 ` Re: MVCC and all that... Justin <[email protected]>
@ 2025-09-10 23:07 ` Nico Williams <[email protected]>
1 sibling, 0 replies; 21+ messages in thread
From: Nico Williams @ 2025-09-10 23:07 UTC (permalink / raw)
To: Justin <[email protected]>; +Cc: Merlin Moncure <[email protected]>; Ellen Allhatatlan <[email protected]>; pgsql-general
On Wed, Sep 10, 2025 at 06:20:18PM -0400, Justin wrote:
> I am not following you here, Databases are going to be bound somewhere at
> some point, Disk,IO, Network IO, Memory, or CPU bound. Which one is
> causing the bottle neck just depends on the workload and size of the
> database.
>
> The number of idle sessions does not really matter it is just
> wasting resources across the entire application stack.
Idle sessions do cost something, but I agree it's not much.
The point is that if you've got a thread- or process-per-client/request
architecture then you can easily make the per-client memory footprint
(and cache pressure) of the service much higher than if per-client/req
state was minimized (by not spreading it over the stack). If you do
that you might go from I/O-bound to memory-bound, though frankly I doubt
it. Instead the likely cost of PG's architecture is just that you need
more memory per (active) client, which is probably a fine price to pay
considering how featureful PG is and how active the PG community is.
> > I would really like out-of-band hints. These would be hints not
> > specified in the SQL itself but to be sent separately and which address
> > table sources or joins by name, like this:
> >
> > psql> SELECT .. FROM x x1 JOIN y y1 ON .. JOIN y y2 ON .. WHERE ..;
> > ...> \hint y1 indexed by ..
> > ...> \hint y2 indexed by ..
> > ...> ;
>
> I humbly disagree, the point of SQL being a 4th generation language is, I
> tell it what I want, not how to go get what I want.
This debate will go on and on. And query planners will continue to plan
some queries very poorly. And telling users to deal with it sucks for
them.
Nico
--
^ permalink raw reply [nested|flat] 21+ messages in thread
* Re: MVCC and all that...
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Re: MVCC and all that... Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Re: MVCC and all that... Justin <[email protected]>
2025-09-10 21:28 ` Re: MVCC and all that... Nico Williams <[email protected]>
2025-09-10 22:20 ` Re: MVCC and all that... Justin <[email protected]>
@ 2025-09-11 00:11 ` Ron Johnson <[email protected]>
1 sibling, 0 replies; 21+ messages in thread
From: Ron Johnson @ 2025-09-11 00:11 UTC (permalink / raw)
To: pgsql-general
On Wed, Sep 10, 2025 at 6:20 PM Justin <[email protected]> wrote:
> On Wed, Sep 10, 2025 at 5:28 PM Nico Williams <[email protected]>
> wrote:
>
>> [snip]
> I would really like out-of-band hints. These would be hints not
>> specified in the SQL itself but to be sent separately and which address
>> table sources or joins by name, like this:
>>
>> psql> SELECT .. FROM x x1 JOIN y y1 ON .. JOIN y y2 ON .. WHERE ..;
>> ...> \hint y1 indexed by ..
>> ...> \hint y2 indexed by ..
>> ...> ;
>>
>
> I humbly disagree, the point of SQL being a 4th generation language is,
> I tell it what I want, not how to go get what I want.
>
And the point of high-level languages is to eliminate GOTO, and yet GOTO
still exists.
The real problem is requiring the application to carry the hints.
My old RDBMS let you CREATE HINT to which you (the DBA) would put a SELECT,
UPDATE or DELETE statement (similar to PREPARE), and what indices to use.
That way, the application doesn't carry them, and "you" can update them as
needed.
Rarely useful, but occasionally *VERY* useful.
--
Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
<Redacted> lobster!
^ permalink raw reply [nested|flat] 21+ messages in thread
end of thread, other threads:[~2025-09-13 01:54 UTC | newest]
Thread overview: 21+ messages (download: mbox mbox.gz follow: Atom feed)
-- links below jump to the message on this page --
2025-09-09 17:57 Re: MVCC and all that... Ellen Allhatatlan <[email protected]>
2025-09-09 22:55 ` Merlin Moncure <[email protected]>
2025-09-10 00:41 ` Justin <[email protected]>
2025-09-10 01:11 ` Ron Johnson <[email protected]>
2025-09-10 07:41 ` Ellen Allhatatlan <[email protected]>
2025-09-10 14:46 ` Adrian Klaver <[email protected]>
2025-09-10 15:08 ` Ellen Allhatatlan <[email protected]>
2025-09-10 15:11 ` Ron Johnson <[email protected]>
2025-09-10 15:15 ` Adrian Klaver <[email protected]>
2025-09-10 15:29 ` Justin <[email protected]>
2025-09-10 17:28 ` Álvaro Herrera <[email protected]>
2025-09-11 11:38 ` Ellen Allhatatlan <[email protected]>
2025-09-11 16:52 ` Merlin Moncure <[email protected]>
2025-09-11 17:55 ` Pavel Stehule <[email protected]>
2025-09-12 19:16 ` Ellen Allhatatlan <[email protected]>
2025-09-13 01:54 ` Justin <[email protected]>
2025-09-10 15:55 ` Ron Johnson <[email protected]>
2025-09-10 21:28 ` Nico Williams <[email protected]>
2025-09-10 22:20 ` Justin <[email protected]>
2025-09-10 23:07 ` Nico Williams <[email protected]>
2025-09-11 00:11 ` Ron Johnson <[email protected]>
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