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* Insert performance vs Table size
@ 2005-06-27 11:24 Praveen Raja <praveen.raja@netlight.se>
2005-06-27 11:40 ` Re: Insert performance vs Table size Jacques Caron <jc@directinfos.com>
0 siblings, 1 reply; 8+ messages in thread
From: Praveen Raja @ 2005-06-27 11:24 UTC (permalink / raw)
To: pgsql-performance
Hi all
I'm wondering if and how the size of a table affects speed of inserts
into it? What if the table has indexes, does that alter the answer?
Thanks
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: Insert performance vs Table size
2005-06-27 11:24 Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
@ 2005-06-27 11:40 ` Jacques Caron <jc@directinfos.com>
2005-06-27 11:50 ` Re: Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
0 siblings, 1 reply; 8+ messages in thread
From: Jacques Caron @ 2005-06-27 11:40 UTC (permalink / raw)
To: Praveen Raja <praveen.raja@netlight.se>; +Cc: pgsql-performance
Hi,
At 13:24 27/06/2005, Praveen Raja wrote:
>I'm wondering if and how the size of a table affects speed of inserts
>into it? What if the table has indexes, does that alter the answer?
Many parameters will affect the result:
- whether there are any indexes (including the primary key, unique
constraints...) to update or not
- whether there are any foreign keys from or to that table
- the size of the rows
- whether the table (or at least the bits being updated) fit in RAM or not
- whether the table has "holes" (due to former updates/deletes and vacuum)
and how they are placed
- and probably a bunch of other things...
Obviously, if you have an append-only (no updates, no deletes) table with
no indexes and no foreign keys, the size of the table should not matter
much. As soon as one of those conditions is not met table size will have an
impact, probably small as long as whatever is needed can be held in RAM, a
lot bigger once it's not the case.
Hope that helps,
Jacques.
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: Insert performance vs Table size
2005-06-27 11:24 Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
2005-06-27 11:40 ` Re: Insert performance vs Table size Jacques Caron <jc@directinfos.com>
@ 2005-06-27 11:50 ` Praveen Raja <praveen.raja@netlight.se>
2005-06-27 12:05 ` Re: Insert performance vs Table size Jacques Caron <jc@directinfos.com>
0 siblings, 1 reply; 8+ messages in thread
From: Praveen Raja @ 2005-06-27 11:50 UTC (permalink / raw)
To: 'Jacques Caron' <jc@directinfos.com>; +Cc: pgsql-performance
Just to clear things up a bit, the scenario that I'm interested in is a
table with a large number of indexes on it (maybe 7-8). In this scenario
other than the overhead of having to maintain the indexes (which I'm
guessing is the same regardless of the size of the table), does the size
of the table play a role in determining insert performance (and I mean
only insert performance)?
-----Original Message-----
From: Jacques Caron [mailto:jc@directinfos.com]
Sent: 27 June 2005 13:40
To: Praveen Raja
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] Insert performance vs Table size
Hi,
At 13:24 27/06/2005, Praveen Raja wrote:
>I'm wondering if and how the size of a table affects speed of inserts
>into it? What if the table has indexes, does that alter the answer?
Many parameters will affect the result:
- whether there are any indexes (including the primary key, unique
constraints...) to update or not
- whether there are any foreign keys from or to that table
- the size of the rows
- whether the table (or at least the bits being updated) fit in RAM or
not
- whether the table has "holes" (due to former updates/deletes and
vacuum)
and how they are placed
- and probably a bunch of other things...
Obviously, if you have an append-only (no updates, no deletes) table
with
no indexes and no foreign keys, the size of the table should not matter
much. As soon as one of those conditions is not met table size will have
an
impact, probably small as long as whatever is needed can be held in RAM,
a
lot bigger once it's not the case.
Hope that helps,
Jacques.
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: Insert performance vs Table size
2005-06-27 11:24 Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
2005-06-27 11:40 ` Re: Insert performance vs Table size Jacques Caron <jc@directinfos.com>
2005-06-27 11:50 ` Re: Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
@ 2005-06-27 12:05 ` Jacques Caron <jc@directinfos.com>
2005-06-28 09:50 ` Re: Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
0 siblings, 1 reply; 8+ messages in thread
From: Jacques Caron @ 2005-06-27 12:05 UTC (permalink / raw)
To: Praveen Raja <praveen.raja@netlight.se>; +Cc: pgsql-performance
Hi,
At 13:50 27/06/2005, Praveen Raja wrote:
>Just to clear things up a bit, the scenario that I'm interested in is a
>table with a large number of indexes on it (maybe 7-8).
If you're after performance you'll want to carefully consider which indexes
are really useful and/or redesign your schema so that you can have less
indexes on that table. 7 or 8 indexes is quite a lot, and that really has a
cost.
> In this scenario
>other than the overhead of having to maintain the indexes (which I'm
>guessing is the same regardless of the size of the table)
Definitely not: indexes grow with the size of the table. Depending on what
columns you index (and their types), the indexes may be a fraction of the
size of the table, or they may be very close in size (in extreme cases they
may even be larger). With 7 or 8 indexes, that can be quite a large volume
of data to manipulate, especially if the values of the columns inserted can
span the whole range of the index (rather than being solely id- or
time-based, for instance, in which case index updates are concentrated in a
small area of each of the indexes), as this means you'll need to have a
majority of the indexes in RAM if you want to maintain decent performance.
>does the size of the table play a role in determining insert performance
>(and I mean
>only insert performance)?
In this case, it's really the indexes that'll cause you trouble, though
heavily fragmented tables (due to lots of deletes or updates) will also
incur a penalty just for the data part of the inserts.
Also, don't forget the usual hints if you are going to do lots of inserts:
- batch them in large transactions, don't do them one at a time
- better yet, use COPY rather than INSERT
- in some situations, you might be better of dropping the indexes, doing
large batch inserts, then re-creating the indexes. YMMV depending on the
existing/new ratio, whether you need to maintain indexed access to the
tables, etc.
- pay attention to foreign keys
Jacques.
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: Insert performance vs Table size
2005-06-27 11:24 Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
2005-06-27 11:40 ` Re: Insert performance vs Table size Jacques Caron <jc@directinfos.com>
2005-06-27 11:50 ` Re: Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
2005-06-27 12:05 ` Re: Insert performance vs Table size Jacques Caron <jc@directinfos.com>
@ 2005-06-28 09:50 ` Praveen Raja <praveen.raja@netlight.se>
2005-06-28 10:43 ` Re: Insert performance vs Table size Jacques Caron <jc@directinfos.com>
2005-06-28 14:25 ` Re: Insert performance vs Table size Tom Lane <tgl@sss.pgh.pa.us>
0 siblings, 2 replies; 8+ messages in thread
From: Praveen Raja @ 2005-06-28 09:50 UTC (permalink / raw)
To: 'Jacques Caron' <jc@directinfos.com>; +Cc: pgsql-performance
I assume you took size to mean the row size? What I really meant was
does the number of rows a table has affect the performance of new
inserts into the table (just INSERTs) all other things remaining
constant. Sorry for the confusion.
I know that having indexes on the table adds an overhead but again does
this overhead increase (for an INSERT operation) with the number of rows
the table contains?
My instinct says no to both. If I'm wrong can someone explain why the
number of rows in a table affects INSERT performance?
Thanks again
-----Original Message-----
From: Jacques Caron [mailto:jc@directinfos.com]
Sent: 27 June 2005 14:05
To: Praveen Raja
Cc: pgsql-performance@postgresql.org
Subject: RE: [PERFORM] Insert performance vs Table size
Hi,
At 13:50 27/06/2005, Praveen Raja wrote:
>Just to clear things up a bit, the scenario that I'm interested in is a
>table with a large number of indexes on it (maybe 7-8).
If you're after performance you'll want to carefully consider which
indexes
are really useful and/or redesign your schema so that you can have less
indexes on that table. 7 or 8 indexes is quite a lot, and that really
has a
cost.
> In this scenario
>other than the overhead of having to maintain the indexes (which I'm
>guessing is the same regardless of the size of the table)
Definitely not: indexes grow with the size of the table. Depending on
what
columns you index (and their types), the indexes may be a fraction of
the
size of the table, or they may be very close in size (in extreme cases
they
may even be larger). With 7 or 8 indexes, that can be quite a large
volume
of data to manipulate, especially if the values of the columns inserted
can
span the whole range of the index (rather than being solely id- or
time-based, for instance, in which case index updates are concentrated
in a
small area of each of the indexes), as this means you'll need to have a
majority of the indexes in RAM if you want to maintain decent
performance.
>does the size of the table play a role in determining insert
performance
>(and I mean
>only insert performance)?
In this case, it's really the indexes that'll cause you trouble, though
heavily fragmented tables (due to lots of deletes or updates) will also
incur a penalty just for the data part of the inserts.
Also, don't forget the usual hints if you are going to do lots of
inserts:
- batch them in large transactions, don't do them one at a time
- better yet, use COPY rather than INSERT
- in some situations, you might be better of dropping the indexes, doing
large batch inserts, then re-creating the indexes. YMMV depending on the
existing/new ratio, whether you need to maintain indexed access to the
tables, etc.
- pay attention to foreign keys
Jacques.
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: Insert performance vs Table size
2005-06-27 11:24 Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
2005-06-27 11:40 ` Re: Insert performance vs Table size Jacques Caron <jc@directinfos.com>
2005-06-27 11:50 ` Re: Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
2005-06-27 12:05 ` Re: Insert performance vs Table size Jacques Caron <jc@directinfos.com>
2005-06-28 09:50 ` Re: Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
@ 2005-06-28 10:43 ` Jacques Caron <jc@directinfos.com>
1 sibling, 0 replies; 8+ messages in thread
From: Jacques Caron @ 2005-06-28 10:43 UTC (permalink / raw)
To: Praveen Raja <praveen.raja@netlight.se>; +Cc: pgsql-performance
Hi,
At 11:50 28/06/2005, Praveen Raja wrote:
>I assume you took size to mean the row size?
Nope, the size of the table.
> What I really meant was
>does the number of rows a table has affect the performance of new
>inserts into the table (just INSERTs) all other things remaining
>constant. Sorry for the confusion.
As I said previously, in most cases it does. One of the few cases where it
doesn't would be an append-only table, no holes, no indexes, no foreign keys...
>I know that having indexes on the table adds an overhead but again does
>this overhead increase (for an INSERT operation) with the number of rows
>the table contains?
It depends on what you are indexing. If the index key is something that
grows monotonically (e.g. a unique ID or a timestamp), then the size of the
table (and hence of the indexes) should have a very limited influence on
the INSERTs. If the index key is anything else (and that must definitely be
the case if you have 7 or 8 indexes!), then that means updates will happen
all over the indexes, which means a lot of read and write activity, and
once the total size of your indexes exceeds what can be cached in RAM,
performance will decrease quite a bit. Of course if your keys are
concentrated in a few limited areas of the key ranges it might help.
>My instinct says no to both. If I'm wrong can someone explain why the
>number of rows in a table affects INSERT performance?
As described above, maintaining indexes when you "hit" anywhere in said
indexes is very costly. The larger the table, the larger the indexes, the
higher the number of levels in the trees, etc. As long as it fits in RAM,
it shouldn't be a problem. Once you exceed that threshold, you start
getting a lot of random I/O, and that's expensive.
Again, it depends a lot on your exact schema, the nature of the data, the
spread of the different values, etc, but I would believe it's more often
the case than not.
Jacques.
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: Insert performance vs Table size
2005-06-27 11:24 Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
2005-06-27 11:40 ` Re: Insert performance vs Table size Jacques Caron <jc@directinfos.com>
2005-06-27 11:50 ` Re: Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
2005-06-27 12:05 ` Re: Insert performance vs Table size Jacques Caron <jc@directinfos.com>
2005-06-28 09:50 ` Re: Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
@ 2005-06-28 14:25 ` Tom Lane <tgl@sss.pgh.pa.us>
1 sibling, 0 replies; 8+ messages in thread
From: Tom Lane @ 2005-06-28 14:25 UTC (permalink / raw)
To: Praveen Raja <praveen.raja@netlight.se>; +Cc: 'Jacques Caron' <jc@directinfos.com>; pgsql-performance
"Praveen Raja" <praveen.raja@netlight.se> writes:
> I know that having indexes on the table adds an overhead but again does
> this overhead increase (for an INSERT operation) with the number of rows
> the table contains?
Typical index implementations (such as b-tree) have roughly O(log N)
cost to insert or lookup a key in an N-entry index. So yes, it grows,
though slowly.
regards, tom lane
^ permalink raw reply [nested|flat] 8+ messages in thread
* Re: Insert performance vs Table size
@ 2005-06-27 11:30 李江华 <ljh1469@alibaba-inc.com>
0 siblings, 0 replies; 8+ messages in thread
From: 李江华 @ 2005-06-27 11:30 UTC (permalink / raw)
To: Praveen Raja <praveen.raja@netlight.se>; pgsql-performance
Praveen Raja:
I think the size of a table don't affect the speed of inserts into it.Because PostgreSQL just doing something like "append" on the data files.
But the index do speed-down the inserts. Because PostgreSQL should maintain the index when doing inserts.
hope this is useful for your question.
======= 2005-06-27 19:24:06 you wrote：=======
>Hi all
>
>I'm wondering if and how the size of a table affects speed of inserts
>into it? What if the table has indexes, does that alter the answer?
>
>Thanks
>
>
>
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Best regards!
李江华
Seamus Dean
Alibaba.com
TEL:0571-85022088-2287
ljh1469@alibaba-inc.com
2005-06-27
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2005-06-27 11:24 Insert performance vs Table size Praveen Raja <praveen.raja@netlight.se>
2005-06-27 11:40 ` Jacques Caron <jc@directinfos.com>
2005-06-27 11:50 ` Praveen Raja <praveen.raja@netlight.se>
2005-06-27 12:05 ` Jacques Caron <jc@directinfos.com>
2005-06-28 09:50 ` Praveen Raja <praveen.raja@netlight.se>
2005-06-28 10:43 ` Jacques Caron <jc@directinfos.com>
2005-06-28 14:25 ` Tom Lane <tgl@sss.pgh.pa.us>
2005-06-27 11:30 Re: Insert performance vs Table size 李江华 <ljh1469@alibaba-inc.com>
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