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* Slow query with planner row strange estimation
@ 2010-07-02 08:48 damien hostin <damien.hostin@axege.com>
0 siblings, 1 reply; 11+ messages in thread
From: damien hostin @ 2010-07-02 08:48 UTC (permalink / raw)
To: pgsql-performance
Hello,
I try to make a query run quicker but I don't really know how to give
hints to the planner.
We are using postgresql 8.4.3 64bit on ubuntu 9.10 server. The hardware
is a 10 SAS drive (15k) on a single RAID 10 array with 8Go RAM.
Queries come from J2EE application (OLAP cube), but running them in
pg_admin perform the same way.
I made a short example that shows what I think is the problem. The real
query is much longer but with only one join it already cause problems.
Here is the short example :
select rfoadv_8.rfoadvsup as c8,
sum(dwhinv.dwhinvqte) as m0
from
dwhinv as dwhinv,
rfoadv as rfoadv_8
where (dwhinv.dwhinv___rforefide = 'HPLUS'
and (dwhinv.dwhinv___rfodomide = 'PMSI' and
dwhinv.dwhinv___rfoindrvs = '1' and
dwhinv.dwhinv___rfoindide='recN3_BB_reel') )
and dwhinv.dwhinv_p2rfodstide = rfoadv_8.rfoadvinf
and rfoadv_8.rfoadvsup = 'ACTI'
group by rfoadv_8.rfoadvsup
dwhinv is a table with almost 6.000.000 records
rfoadv is a view with 800.000 records
rfoadv is based on rfoade which is 50.000 records
Here is the explain analyse :
GroupAggregate (cost=0.00..16.56 rows=1 width=13) (actual
time=2028.452..2028.453 rows=1 loops=1)
-> Nested Loop (cost=0.00..16.54 rows=1 width=13) (actual
time=0.391..1947.432 rows=42664 loops=1)
Join Filter: (((ade2.rfoadegch)::text >= (ade1.rfoadegch)::text)
AND ((ade2.rfoadedrt)::text <= (ade1.rfoadedrt)::text))
-> Nested Loop (cost=0.00..12.54 rows=1 width=214) (actual
time=0.304..533.281 rows=114350 loops=1)
-> Index Scan using dwhinv_rdi_idx on dwhinv
(cost=0.00..4.87 rows=1 width=12) (actual time=0.227..16.827 rows=6360
loops=1)
Index Cond: (((dwhinv___rforefide)::text =
'HPLUS'::text) AND ((dwhinv___rfodomide)::text = 'PMSI'::text) AND
((dwhinv___rfoindide)::text = 'recN3_BB_reel'::text) AND
(dwhinv___rfoindrvs = 1))
-> Index Scan using rfoade_dsi_idx on rfoade ade2
(cost=0.00..7.63 rows=3 width=213) (actual time=0.007..0.037 rows=18
loops=6360)
Index Cond: ((ade2.rfoade_i_rfodstide)::text =
(dwhinv.dwhinv_p2rfodstide)::text)
-> Index Scan using rfoade_pk on rfoade ade1 (cost=0.00..3.98
rows=1 width=213) (actual time=0.008..0.009 rows=0 loops=114350)
Index Cond: (((ade1.rfoade___rforefide)::text =
(ade2.rfoade___rforefide)::text) AND ((ade1.rfoade_i_rfodstide)::text =
'ACTI'::text) AND ((ade1.rfoade___rfovdeide)::text =
(ade2.rfoade___rfovdeide)::text) AND (ade1.rfoadervs = ade2.rfoadervs))
We can see that the planner think that accessing dwhinv with the
dwhinv_rdi_idx index will return 1 row, but in fact there are 6360. So
the nested loop is not done with 1 loop but 6360. With only one Join,
the query runs in about 1.5 sec which is not really long, but with 8
join, the same mistake is repeated 8 times, the query runs in 30-60 sec.
I try to disable nested loop, hash join and merge join are done instead
of nested loops, example query runs in 0.2 - 0.5 sec, and the real query
no more that 1 sec ! Which is great.
Here is the execution plan with nested loop off:
GroupAggregate (cost=12.56..2453.94 rows=1 width=13) (actual
time=817.306..817.307 rows=1 loops=1)
-> Hash Join (cost=12.56..2453.93 rows=1 width=13) (actual
time=42.583..720.746 rows=42664 loops=1)
Hash Cond: (((ade2.rfoade___rforefide)::text =
(ade1.rfoade___rforefide)::text) AND ((ade2.rfoade___rfovdeide)::text =
(ade1.rfoade___rfovdeide)::text) AND (ade2.rfoadervs = ade1.rfoadervs))
Join Filter: (((ade2.rfoadegch)::text >= (ade1.rfoadegch)::text)
AND ((ade2.rfoadedrt)::text <= (ade1.rfoadedrt)::text))
-> Hash Join (cost=4.88..2446.21 rows=1 width=214) (actual
time=42.168..411.962 rows=114350 loops=1)
Hash Cond: ((ade2.rfoade_i_rfodstide)::text =
(dwhinv.dwhinv_p2rfodstide)::text)
-> Seq Scan on rfoade ade2 (cost=0.00..2262.05
rows=47805 width=213) (actual time=0.057..78.988 rows=47805 loops=1)
-> Hash (cost=4.87..4.87 rows=1 width=12) (actual
time=41.632..41.632 rows=6360 loops=1)
-> Index Scan using dwhinv_rdi_idx on dwhinv
(cost=0.00..4.87 rows=1 width=12) (actual time=0.232..28.199 rows=6360
loops=1)
Index Cond: (((dwhinv___rforefide)::text =
'HPLUS'::text) AND ((dwhinv___rfodomide)::text = 'PMSI'::text) AND
((dwhinv___rfoindide)::text = 'recN3_BB_reel'::text) AND
(dwhinv___rfoindrvs = 1))
-> Hash (cost=7.63..7.63 rows=3 width=213) (actual
time=0.347..0.347 rows=11 loops=1)
-> Index Scan using rfoade_dsi_idx on rfoade ade1
(cost=0.00..7.63 rows=3 width=213) (actual time=0.095..0.307 rows=11
loops=1)
Index Cond: ((rfoade_i_rfodstide)::text = 'ACTI'::text)
Even if dwhinv row estimation is wrong, the query is quicker
So after looking at dwhinv_rdi_idx statistics, I found that
dwhinv___rfoindide related stats wasn't good, so I try "ALTER TABLE
dwhinv ALTER dwhinv_p2rfodstide SET STATISTICS 2000" and launch an
vaccum analyse to gather more impressive stats. Stats are better but
query plan is the same and query is not optimised. So I try reindex on
DWHINV as a last chance, but it changes nothing !
Maybe I'm wrong with the interpretation of the plan but I don't really
think so because with no nested loops this query is really fast ! I do
not plan to disable nested loop on the whole database because sometimes,
nested loops are greats !
Now I'm stuck ! I don't know how to make the planner understand there
are 6000 rows. Or maybe the 3 column index is a bad idea... ?!
Thanks
--
HOSTIN Damien - Equipe R&D
Société Axège
www.axege.com
^ permalink raw reply [nested|flat] 11+ messages in thread
* Re: Slow query with planner row strange estimation
@ 2010-07-05 07:28 damien hostin <damien.hostin@axege.com>
parent: damien hostin <damien.hostin@axege.com>
0 siblings, 1 reply; 11+ messages in thread
From: damien hostin @ 2010-07-05 07:28 UTC (permalink / raw)
To: pgsql-performance
Hello,
Before the week end I tried to change the index, but even with the
mono-column index on differents columns, the estimated number of rows
from dwhinv is 1.
Anyone have a suggestion, what can I check ?
thx
damien hostin a écrit :
> Hello,
>
> I try to make a query run quicker but I don't really know how to give
> hints to the planner.
>
> We are using postgresql 8.4.3 64bit on ubuntu 9.10 server. The
> hardware is a 10 SAS drive (15k) on a single RAID 10 array with 8Go RAM.
> Queries come from J2EE application (OLAP cube), but running them in
> pg_admin perform the same way.
>
> I made a short example that shows what I think is the problem. The
> real query is much longer but with only one join it already cause
> problems.
>
> Here is the short example :
>
> select rfoadv_8.rfoadvsup as c8,
> sum(dwhinv.dwhinvqte) as m0
> from
> dwhinv as dwhinv,
> rfoadv as rfoadv_8
> where (dwhinv.dwhinv___rforefide = 'HPLUS'
> and (dwhinv.dwhinv___rfodomide = 'PMSI' and
> dwhinv.dwhinv___rfoindrvs = '1' and
> dwhinv.dwhinv___rfoindide='recN3_BB_reel') )
> and dwhinv.dwhinv_p2rfodstide = rfoadv_8.rfoadvinf
> and rfoadv_8.rfoadvsup = 'ACTI'
> group by rfoadv_8.rfoadvsup
>
> dwhinv is a table with almost 6.000.000 records
> rfoadv is a view with 800.000 records
> rfoadv is based on rfoade which is 50.000 records
>
> Here is the explain analyse :
> GroupAggregate (cost=0.00..16.56 rows=1 width=13) (actual
> time=2028.452..2028.453 rows=1 loops=1)
> -> Nested Loop (cost=0.00..16.54 rows=1 width=13) (actual
> time=0.391..1947.432 rows=42664 loops=1)
> Join Filter: (((ade2.rfoadegch)::text >=
> (ade1.rfoadegch)::text) AND ((ade2.rfoadedrt)::text <=
> (ade1.rfoadedrt)::text))
> -> Nested Loop (cost=0.00..12.54 rows=1 width=214) (actual
> time=0.304..533.281 rows=114350 loops=1)
> -> Index Scan using dwhinv_rdi_idx on dwhinv
> (cost=0.00..4.87 rows=1 width=12) (actual time=0.227..16.827 rows=6360
> loops=1)
> Index Cond: (((dwhinv___rforefide)::text =
> 'HPLUS'::text) AND ((dwhinv___rfodomide)::text = 'PMSI'::text) AND
> ((dwhinv___rfoindide)::text = 'recN3_BB_reel'::text) AND
> (dwhinv___rfoindrvs = 1))
> -> Index Scan using rfoade_dsi_idx on rfoade ade2
> (cost=0.00..7.63 rows=3 width=213) (actual time=0.007..0.037 rows=18
> loops=6360)
> Index Cond: ((ade2.rfoade_i_rfodstide)::text =
> (dwhinv.dwhinv_p2rfodstide)::text)
> -> Index Scan using rfoade_pk on rfoade ade1 (cost=0.00..3.98
> rows=1 width=213) (actual time=0.008..0.009 rows=0 loops=114350)
> Index Cond: (((ade1.rfoade___rforefide)::text =
> (ade2.rfoade___rforefide)::text) AND ((ade1.rfoade_i_rfodstide)::text
> = 'ACTI'::text) AND ((ade1.rfoade___rfovdeide)::text =
> (ade2.rfoade___rfovdeide)::text) AND (ade1.rfoadervs = ade2.rfoadervs))
>
> We can see that the planner think that accessing dwhinv with the
> dwhinv_rdi_idx index will return 1 row, but in fact there are 6360. So
> the nested loop is not done with 1 loop but 6360. With only one Join,
> the query runs in about 1.5 sec which is not really long, but with 8
> join, the same mistake is repeated 8 times, the query runs in 30-60
> sec. I try to disable nested loop, hash join and merge join are done
> instead of nested loops, example query runs in 0.2 - 0.5 sec, and the
> real query no more that 1 sec ! Which is great.
>
> Here is the execution plan with nested loop off:
>
> GroupAggregate (cost=12.56..2453.94 rows=1 width=13) (actual
> time=817.306..817.307 rows=1 loops=1)
> -> Hash Join (cost=12.56..2453.93 rows=1 width=13) (actual
> time=42.583..720.746 rows=42664 loops=1)
> Hash Cond: (((ade2.rfoade___rforefide)::text =
> (ade1.rfoade___rforefide)::text) AND ((ade2.rfoade___rfovdeide)::text
> = (ade1.rfoade___rfovdeide)::text) AND (ade2.rfoadervs = ade1.rfoadervs))
> Join Filter: (((ade2.rfoadegch)::text >=
> (ade1.rfoadegch)::text) AND ((ade2.rfoadedrt)::text <=
> (ade1.rfoadedrt)::text))
> -> Hash Join (cost=4.88..2446.21 rows=1 width=214) (actual
> time=42.168..411.962 rows=114350 loops=1)
> Hash Cond: ((ade2.rfoade_i_rfodstide)::text =
> (dwhinv.dwhinv_p2rfodstide)::text)
> -> Seq Scan on rfoade ade2 (cost=0.00..2262.05
> rows=47805 width=213) (actual time=0.057..78.988 rows=47805 loops=1)
> -> Hash (cost=4.87..4.87 rows=1 width=12) (actual
> time=41.632..41.632 rows=6360 loops=1)
> -> Index Scan using dwhinv_rdi_idx on dwhinv
> (cost=0.00..4.87 rows=1 width=12) (actual time=0.232..28.199 rows=6360
> loops=1)
> Index Cond: (((dwhinv___rforefide)::text =
> 'HPLUS'::text) AND ((dwhinv___rfodomide)::text = 'PMSI'::text) AND
> ((dwhinv___rfoindide)::text = 'recN3_BB_reel'::text) AND
> (dwhinv___rfoindrvs = 1))
> -> Hash (cost=7.63..7.63 rows=3 width=213) (actual
> time=0.347..0.347 rows=11 loops=1)
> -> Index Scan using rfoade_dsi_idx on rfoade ade1
> (cost=0.00..7.63 rows=3 width=213) (actual time=0.095..0.307 rows=11
> loops=1)
> Index Cond: ((rfoade_i_rfodstide)::text =
> 'ACTI'::text)
>
> Even if dwhinv row estimation is wrong, the query is quicker
>
>
> So after looking at dwhinv_rdi_idx statistics, I found that
> dwhinv___rfoindide related stats wasn't good, so I try "ALTER TABLE
> dwhinv ALTER dwhinv_p2rfodstide SET STATISTICS 2000" and launch an
> vaccum analyse to gather more impressive stats. Stats are better but
> query plan is the same and query is not optimised. So I try reindex on
> DWHINV as a last chance, but it changes nothing !
>
> Maybe I'm wrong with the interpretation of the plan but I don't really
> think so because with no nested loops this query is really fast ! I do
> not plan to disable nested loop on the whole database because
> sometimes, nested loops are greats !
>
> Now I'm stuck ! I don't know how to make the planner understand there
> are 6000 rows. Or maybe the 3 column index is a bad idea... ?!
>
> Thanks
>
--
HOSTIN Damien - Equipe R&D
Tel:+33(0)4 63 05 95 40
Société Axège
23 rue Saint Simon
63000 Clermont Ferrand
www.axege.com
^ permalink raw reply [nested|flat] 11+ messages in thread
* Re: Slow query with planner row strange estimation
@ 2010-07-06 12:48 damien hostin <damien.hostin@axege.com>
parent: damien hostin <damien.hostin@axege.com>
0 siblings, 1 reply; 11+ messages in thread
From: damien hostin @ 2010-07-06 12:48 UTC (permalink / raw)
To: pgsql-performance
Hello,
Postgresql configuration was default. So I take a look at pgtune which
help me start a bit of tuning. I thought that the planner mistake could
come from the default low memory configuration. But after applying new
parameters, nothing has changed. The query is still low, the execution
plan is still using nested loops where hashjoin/hashmerge seems a lot
better.
Here are the postgresql.conf parameters I changed using pgtune advises,
all other are defaults.
(The hardware is a 10 SAS drive (15k) on a single RAID 10 array with 8Go
RAM, with 2 opteron dual core 64bit (I can't remember the exact model))
# generated for 100 connection and 6G RAM with datawarehouse type
#
default_statistics_target = 100
maintenance_work_mem = 768MB
#constraint_exclusion = on
#checkpoint_completion_target = 0.9
effective_cache_size = 4608MB
work_mem = 30MB
wal_buffers = 32MB
checkpoint_segments = 64
shared_buffers = 1536MB
Some information that I may have forgotten.
SELECT version();
"PostgreSQL 8.4.3 on x86_64-pc-linux-gnu, compiled by GCC gcc-4.4.real
(Ubuntu 4.4.1-4ubuntu8) 4.4.1, 64-bit"
and here is a link with the full request explain analyse
http://explain.depesz.com/s/Yx0
I will try the same query with the same data on another server, with
"PostgreSQL 8.3.11 on i486-pc-linux-gnu, compiled by GCC cc (GCC) 4.2.4
(Ubuntu 4.2.4-1ubuntu3)".
damien hostin a écrit :
> Hello,
>
> Before the week end I tried to change the index, but even with the
> mono-column index on differents columns, the estimated number of rows
> from dwhinv is 1.
>
> Anyone have a suggestion, what can I check ?
>
>
> thx
>
>
> damien hostin a écrit :
>> Hello,
>>
>> I try to make a query run quicker but I don't really know how to give
>> hints to the planner.
>>
>> We are using postgresql 8.4.3 64bit on ubuntu 9.10 server. The
>> hardware is a 10 SAS drive (15k) on a single RAID 10 array with 8Go RAM.
>> Queries come from J2EE application (OLAP cube), but running them in
>> pg_admin perform the same way.
>>
>> I made a short example that shows what I think is the problem. The
>> real query is much longer but with only one join it already cause
>> problems.
>>
>> Here is the short example :
>>
>> select rfoadv_8.rfoadvsup as c8,
>> sum(dwhinv.dwhinvqte) as m0
>> from
>> dwhinv as dwhinv,
>> rfoadv as rfoadv_8
>> where (dwhinv.dwhinv___rforefide = 'HPLUS'
>> and (dwhinv.dwhinv___rfodomide = 'PMSI' and
>> dwhinv.dwhinv___rfoindrvs = '1' and
>> dwhinv.dwhinv___rfoindide='recN3_BB_reel') )
>> and dwhinv.dwhinv_p2rfodstide = rfoadv_8.rfoadvinf
>> and rfoadv_8.rfoadvsup = 'ACTI'
>> group by rfoadv_8.rfoadvsup
>>
>> dwhinv is a table with almost 6.000.000 records
>> rfoadv is a view with 800.000 records
>> rfoadv is based on rfoade which is 50.000 records
>>
>> Here is the explain analyse :
>> GroupAggregate (cost=0.00..16.56 rows=1 width=13) (actual
>> time=2028.452..2028.453 rows=1 loops=1)
>> -> Nested Loop (cost=0.00..16.54 rows=1 width=13) (actual
>> time=0.391..1947.432 rows=42664 loops=1)
>> Join Filter: (((ade2.rfoadegch)::text >=
>> (ade1.rfoadegch)::text) AND ((ade2.rfoadedrt)::text <=
>> (ade1.rfoadedrt)::text))
>> -> Nested Loop (cost=0.00..12.54 rows=1 width=214) (actual
>> time=0.304..533.281 rows=114350 loops=1)
>> -> Index Scan using dwhinv_rdi_idx on dwhinv
>> (cost=0.00..4.87 rows=1 width=12) (actual time=0.227..16.827
>> rows=6360 loops=1)
>> Index Cond: (((dwhinv___rforefide)::text =
>> 'HPLUS'::text) AND ((dwhinv___rfodomide)::text = 'PMSI'::text) AND
>> ((dwhinv___rfoindide)::text = 'recN3_BB_reel'::text) AND
>> (dwhinv___rfoindrvs = 1))
>> -> Index Scan using rfoade_dsi_idx on rfoade ade2
>> (cost=0.00..7.63 rows=3 width=213) (actual time=0.007..0.037 rows=18
>> loops=6360)
>> Index Cond: ((ade2.rfoade_i_rfodstide)::text =
>> (dwhinv.dwhinv_p2rfodstide)::text)
>> -> Index Scan using rfoade_pk on rfoade ade1
>> (cost=0.00..3.98 rows=1 width=213) (actual time=0.008..0.009 rows=0
>> loops=114350)
>> Index Cond: (((ade1.rfoade___rforefide)::text =
>> (ade2.rfoade___rforefide)::text) AND ((ade1.rfoade_i_rfodstide)::text
>> = 'ACTI'::text) AND ((ade1.rfoade___rfovdeide)::text =
>> (ade2.rfoade___rfovdeide)::text) AND (ade1.rfoadervs = ade2.rfoadervs))
>>
>> We can see that the planner think that accessing dwhinv with the
>> dwhinv_rdi_idx index will return 1 row, but in fact there are 6360.
>> So the nested loop is not done with 1 loop but 6360. With only one
>> Join, the query runs in about 1.5 sec which is not really long, but
>> with 8 join, the same mistake is repeated 8 times, the query runs in
>> 30-60 sec. I try to disable nested loop, hash join and merge join are
>> done instead of nested loops, example query runs in 0.2 - 0.5 sec,
>> and the real query no more that 1 sec ! Which is great.
>>
>> Here is the execution plan with nested loop off:
>>
>> GroupAggregate (cost=12.56..2453.94 rows=1 width=13) (actual
>> time=817.306..817.307 rows=1 loops=1)
>> -> Hash Join (cost=12.56..2453.93 rows=1 width=13) (actual
>> time=42.583..720.746 rows=42664 loops=1)
>> Hash Cond: (((ade2.rfoade___rforefide)::text =
>> (ade1.rfoade___rforefide)::text) AND ((ade2.rfoade___rfovdeide)::text
>> = (ade1.rfoade___rfovdeide)::text) AND (ade2.rfoadervs =
>> ade1.rfoadervs))
>> Join Filter: (((ade2.rfoadegch)::text >=
>> (ade1.rfoadegch)::text) AND ((ade2.rfoadedrt)::text <=
>> (ade1.rfoadedrt)::text))
>> -> Hash Join (cost=4.88..2446.21 rows=1 width=214) (actual
>> time=42.168..411.962 rows=114350 loops=1)
>> Hash Cond: ((ade2.rfoade_i_rfodstide)::text =
>> (dwhinv.dwhinv_p2rfodstide)::text)
>> -> Seq Scan on rfoade ade2 (cost=0.00..2262.05
>> rows=47805 width=213) (actual time=0.057..78.988 rows=47805 loops=1)
>> -> Hash (cost=4.87..4.87 rows=1 width=12) (actual
>> time=41.632..41.632 rows=6360 loops=1)
>> -> Index Scan using dwhinv_rdi_idx on dwhinv
>> (cost=0.00..4.87 rows=1 width=12) (actual time=0.232..28.199
>> rows=6360 loops=1)
>> Index Cond: (((dwhinv___rforefide)::text =
>> 'HPLUS'::text) AND ((dwhinv___rfodomide)::text = 'PMSI'::text) AND
>> ((dwhinv___rfoindide)::text = 'recN3_BB_reel'::text) AND
>> (dwhinv___rfoindrvs = 1))
>> -> Hash (cost=7.63..7.63 rows=3 width=213) (actual
>> time=0.347..0.347 rows=11 loops=1)
>> -> Index Scan using rfoade_dsi_idx on rfoade ade1
>> (cost=0.00..7.63 rows=3 width=213) (actual time=0.095..0.307 rows=11
>> loops=1)
>> Index Cond: ((rfoade_i_rfodstide)::text =
>> 'ACTI'::text)
>>
>> Even if dwhinv row estimation is wrong, the query is quicker
>>
>>
>> So after looking at dwhinv_rdi_idx statistics, I found that
>> dwhinv___rfoindide related stats wasn't good, so I try "ALTER TABLE
>> dwhinv ALTER dwhinv_p2rfodstide SET STATISTICS 2000" and launch an
>> vaccum analyse to gather more impressive stats. Stats are better but
>> query plan is the same and query is not optimised. So I try reindex
>> on DWHINV as a last chance, but it changes nothing !
>>
>> Maybe I'm wrong with the interpretation of the plan but I don't
>> really think so because with no nested loops this query is really
>> fast ! I do not plan to disable nested loop on the whole database
>> because sometimes, nested loops are greats !
>>
>> Now I'm stuck ! I don't know how to make the planner understand there
>> are 6000 rows. Or maybe the 3 column index is a bad idea... ?!
>>
>> Thanks
>>
>
>
--
HOSTIN Damien - Equipe R&D
Tel:+33(0)4 63 05 95 40
Société Axège
23 rue Saint Simon
63000 Clermont Ferrand
www.axege.com
^ permalink raw reply [nested|flat] 11+ messages in thread
* Re: Slow query with planner row strange estimation
@ 2010-07-07 14:39 damien hostin <damien.hostin@axege.com>
parent: damien hostin <damien.hostin@axege.com>
0 siblings, 1 reply; 11+ messages in thread
From: damien hostin @ 2010-07-07 14:39 UTC (permalink / raw)
To: pgsql-performance
Hello again,
At last, I check the same query with the same data on my desktop
computer. Just after loading the data, the queries were slow, I launch a
vaccum analyse which collect good stats on the main table, the query
became quick (~200ms). Now 1classic sata disk computer is faster than
our little monster server !!
I compare the volume between the two database. On my desktop computer,
the table dwinv has 12000 row with 6000 implicated in my query. The dev
server has 6000000 rows with only 6000 implicated in the query. I check
the repartition of the column I am using in this query and actually,
only the 6000 rows implicated in the query are using column with non
null values. I put statistics target on this columns at 10000 which make
the analyse take half the table as sample for stats. This way I get some
values for these columns. But the execution plan is still mistaking.
(plan : http://explain.depesz.com/s/LKW)
I try to compare with desktop plan, but it seems to have nothing
comparable. I though I would find something like "access on dwhinv with
6000 estimated rows", but it does the following :
http://explain.depesz.com/s/kbn
I don't understand "rows=0" in :
Index Scan using dwhinv_dig_idx on dwhinv (cost=0.00..25.91 rows=1
width=80) (actual time=0.009..0.010 rows=0 loops=120)
* Index Cond: ((dwhinv.dwhinv___rsadigide)::text =
(adi2.rsaadi_i_rsadigide)::text)
* Filter: (((dwhinv.dwhinv___rforefide)::text = 'HPLUS'::text) AND
(dwhinv.dwhinv___rfoindrvs = 1) AND
((dwhinv.dwhinv___rfodomide)::text = 'PMSI'::text) AND
((dwhinv.dwhinv___rfoindide)::text = 'recN3_BB_reel'::text))
I also managed to make the query run 10x faster with SQL92 join syntax
instead of old "from table1, table where table1.col1=table2.col1". This
way the query takes 3sec instead of 30sec. But again, without nested
loops, 200ms !
I will try later with new mondrian release and a better balanced fact
table.
Thanks anyway__
damien hostin a écrit :
> Hello,
>
> Postgresql configuration was default. So I take a look at pgtune which
> help me start a bit of tuning. I thought that the planner mistake
> could come from the default low memory configuration. But after
> applying new parameters, nothing has changed. The query is still low,
> the execution plan is still using nested loops where
> hashjoin/hashmerge seems a lot better.
>
> Here are the postgresql.conf parameters I changed using pgtune
> advises, all other are defaults.
> (The hardware is a 10 SAS drive (15k) on a single RAID 10 array with
> 8Go RAM, with 2 opteron dual core 64bit (I can't remember the exact
> model))
>
> # generated for 100 connection and 6G RAM with datawarehouse type
> #
> default_statistics_target = 100
> maintenance_work_mem = 768MB
> #constraint_exclusion = on
> #checkpoint_completion_target = 0.9
> effective_cache_size = 4608MB
> work_mem = 30MB
> wal_buffers = 32MB
> checkpoint_segments = 64
> shared_buffers = 1536MB
>
> Some information that I may have forgotten.
> SELECT version();
> "PostgreSQL 8.4.3 on x86_64-pc-linux-gnu, compiled by GCC gcc-4.4.real
> (Ubuntu 4.4.1-4ubuntu8) 4.4.1, 64-bit"
>
>
> and here is a link with the full request explain analyse
> http://explain.depesz.com/s/Yx0
>
>
> I will try the same query with the same data on another server, with
> "PostgreSQL 8.3.11 on i486-pc-linux-gnu, compiled by GCC cc (GCC)
> 4.2.4 (Ubuntu 4.2.4-1ubuntu3)".
>
>
> damien hostin a écrit :
>> Hello,
>>
>> Before the week end I tried to change the index, but even with the
>> mono-column index on differents columns, the estimated number of rows
>> from dwhinv is 1.
>>
>> Anyone have a suggestion, what can I check ?
>>
>>
>> thx
>>
>>
>> damien hostin a écrit :
>>> Hello,
>>>
>>> I try to make a query run quicker but I don't really know how to
>>> give hints to the planner.
>>>
>>> We are using postgresql 8.4.3 64bit on ubuntu 9.10 server. The
>>> hardware is a 10 SAS drive (15k) on a single RAID 10 array with 8Go
>>> RAM.
>>> Queries come from J2EE application (OLAP cube), but running them in
>>> pg_admin perform the same way.
>>>
>>> I made a short example that shows what I think is the problem. The
>>> real query is much longer but with only one join it already cause
>>> problems.
>>>
>>> Here is the short example :
>>>
>>> select rfoadv_8.rfoadvsup as c8,
>>> sum(dwhinv.dwhinvqte) as m0
>>> from
>>> dwhinv as dwhinv,
>>> rfoadv as rfoadv_8
>>> where (dwhinv.dwhinv___rforefide = 'HPLUS'
>>> and (dwhinv.dwhinv___rfodomide = 'PMSI' and
>>> dwhinv.dwhinv___rfoindrvs = '1' and
>>> dwhinv.dwhinv___rfoindide='recN3_BB_reel') )
>>> and dwhinv.dwhinv_p2rfodstide = rfoadv_8.rfoadvinf
>>> and rfoadv_8.rfoadvsup = 'ACTI'
>>> group by rfoadv_8.rfoadvsup
>>>
>>> dwhinv is a table with almost 6.000.000 records
>>> rfoadv is a view with 800.000 records
>>> rfoadv is based on rfoade which is 50.000 records
>>>
>>> Here is the explain analyse :
>>> GroupAggregate (cost=0.00..16.56 rows=1 width=13) (actual
>>> time=2028.452..2028.453 rows=1 loops=1)
>>> -> Nested Loop (cost=0.00..16.54 rows=1 width=13) (actual
>>> time=0.391..1947.432 rows=42664 loops=1)
>>> Join Filter: (((ade2.rfoadegch)::text >=
>>> (ade1.rfoadegch)::text) AND ((ade2.rfoadedrt)::text <=
>>> (ade1.rfoadedrt)::text))
>>> -> Nested Loop (cost=0.00..12.54 rows=1 width=214) (actual
>>> time=0.304..533.281 rows=114350 loops=1)
>>> -> Index Scan using dwhinv_rdi_idx on dwhinv
>>> (cost=0.00..4.87 rows=1 width=12) (actual time=0.227..16.827
>>> rows=6360 loops=1)
>>> Index Cond: (((dwhinv___rforefide)::text =
>>> 'HPLUS'::text) AND ((dwhinv___rfodomide)::text = 'PMSI'::text) AND
>>> ((dwhinv___rfoindide)::text = 'recN3_BB_reel'::text) AND
>>> (dwhinv___rfoindrvs = 1))
>>> -> Index Scan using rfoade_dsi_idx on rfoade ade2
>>> (cost=0.00..7.63 rows=3 width=213) (actual time=0.007..0.037 rows=18
>>> loops=6360)
>>> Index Cond: ((ade2.rfoade_i_rfodstide)::text =
>>> (dwhinv.dwhinv_p2rfodstide)::text)
>>> -> Index Scan using rfoade_pk on rfoade ade1
>>> (cost=0.00..3.98 rows=1 width=213) (actual time=0.008..0.009 rows=0
>>> loops=114350)
>>> Index Cond: (((ade1.rfoade___rforefide)::text =
>>> (ade2.rfoade___rforefide)::text) AND
>>> ((ade1.rfoade_i_rfodstide)::text = 'ACTI'::text) AND
>>> ((ade1.rfoade___rfovdeide)::text = (ade2.rfoade___rfovdeide)::text)
>>> AND (ade1.rfoadervs = ade2.rfoadervs))
>>>
>>> We can see that the planner think that accessing dwhinv with the
>>> dwhinv_rdi_idx index will return 1 row, but in fact there are 6360.
>>> So the nested loop is not done with 1 loop but 6360. With only one
>>> Join, the query runs in about 1.5 sec which is not really long, but
>>> with 8 join, the same mistake is repeated 8 times, the query runs in
>>> 30-60 sec. I try to disable nested loop, hash join and merge join
>>> are done instead of nested loops, example query runs in 0.2 - 0.5
>>> sec, and the real query no more that 1 sec ! Which is great.
>>>
>>> Here is the execution plan with nested loop off:
>>>
>>> GroupAggregate (cost=12.56..2453.94 rows=1 width=13) (actual
>>> time=817.306..817.307 rows=1 loops=1)
>>> -> Hash Join (cost=12.56..2453.93 rows=1 width=13) (actual
>>> time=42.583..720.746 rows=42664 loops=1)
>>> Hash Cond: (((ade2.rfoade___rforefide)::text =
>>> (ade1.rfoade___rforefide)::text) AND
>>> ((ade2.rfoade___rfovdeide)::text = (ade1.rfoade___rfovdeide)::text)
>>> AND (ade2.rfoadervs = ade1.rfoadervs))
>>> Join Filter: (((ade2.rfoadegch)::text >=
>>> (ade1.rfoadegch)::text) AND ((ade2.rfoadedrt)::text <=
>>> (ade1.rfoadedrt)::text))
>>> -> Hash Join (cost=4.88..2446.21 rows=1 width=214) (actual
>>> time=42.168..411.962 rows=114350 loops=1)
>>> Hash Cond: ((ade2.rfoade_i_rfodstide)::text =
>>> (dwhinv.dwhinv_p2rfodstide)::text)
>>> -> Seq Scan on rfoade ade2 (cost=0.00..2262.05
>>> rows=47805 width=213) (actual time=0.057..78.988 rows=47805 loops=1)
>>> -> Hash (cost=4.87..4.87 rows=1 width=12) (actual
>>> time=41.632..41.632 rows=6360 loops=1)
>>> -> Index Scan using dwhinv_rdi_idx on dwhinv
>>> (cost=0.00..4.87 rows=1 width=12) (actual time=0.232..28.199
>>> rows=6360 loops=1)
>>> Index Cond: (((dwhinv___rforefide)::text =
>>> 'HPLUS'::text) AND ((dwhinv___rfodomide)::text = 'PMSI'::text) AND
>>> ((dwhinv___rfoindide)::text = 'recN3_BB_reel'::text) AND
>>> (dwhinv___rfoindrvs = 1))
>>> -> Hash (cost=7.63..7.63 rows=3 width=213) (actual
>>> time=0.347..0.347 rows=11 loops=1)
>>> -> Index Scan using rfoade_dsi_idx on rfoade ade1
>>> (cost=0.00..7.63 rows=3 width=213) (actual time=0.095..0.307 rows=11
>>> loops=1)
>>> Index Cond: ((rfoade_i_rfodstide)::text =
>>> 'ACTI'::text)
>>>
>>> Even if dwhinv row estimation is wrong, the query is quicker
>>>
>>>
>>> So after looking at dwhinv_rdi_idx statistics, I found that
>>> dwhinv___rfoindide related stats wasn't good, so I try "ALTER TABLE
>>> dwhinv ALTER dwhinv_p2rfodstide SET STATISTICS 2000" and launch an
>>> vaccum analyse to gather more impressive stats. Stats are better but
>>> query plan is the same and query is not optimised. So I try reindex
>>> on DWHINV as a last chance, but it changes nothing !
>>>
>>> Maybe I'm wrong with the interpretation of the plan but I don't
>>> really think so because with no nested loops this query is really
>>> fast ! I do not plan to disable nested loop on the whole database
>>> because sometimes, nested loops are greats !
>>>
>>> Now I'm stuck ! I don't know how to make the planner understand
>>> there are 6000 rows. Or maybe the 3 column index is a bad idea... ?!
>>>
>>> Thanks
>>>
>>
>>
>
>
--
HOSTIN Damien - Equipe R&D
Tel:+33(0)4 63 05 95 40
Société Axège
23 rue Saint Simon
63000 Clermont Ferrand
www.axege.com
^ permalink raw reply [nested|flat] 11+ messages in thread
* Re: Slow query with planner row strange estimation
@ 2010-07-08 18:42 Robert Haas <robertmhaas@gmail.com>
parent: damien hostin <damien.hostin@axege.com>
0 siblings, 1 reply; 11+ messages in thread
From: Robert Haas @ 2010-07-08 18:42 UTC (permalink / raw)
To: damien hostin <damien.hostin@axege.com>; +Cc: pgsql-performance
On Wed, Jul 7, 2010 at 10:39 AM, damien hostin <damien.hostin@axege.com> wrote:
> Hello again,
>
> At last, I check the same query with the same data on my desktop computer.
> Just after loading the data, the queries were slow, I launch a vaccum
> analyse which collect good stats on the main table, the query became quick
> (~200ms). Now 1classic sata disk computer is faster than our little monster
> server !!
Have you tried running ANALYZE on the production server?
You might also want to try ALTER TABLE ... SET STATISTICS to a large
value on some of the join columns involved in the query.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company
^ permalink raw reply [nested|flat] 11+ messages in thread
* Re: Slow query with planner row strange estimation
@ 2010-07-09 10:13 damien hostin <damien.hostin@axege.com>
parent: Robert Haas <robertmhaas@gmail.com>
0 siblings, 1 reply; 11+ messages in thread
From: damien hostin @ 2010-07-09 10:13 UTC (permalink / raw)
To: Robert Haas <robertmhaas@gmail.com>; +Cc: pgsql-performance
Robert Haas a écrit :
> On Wed, Jul 7, 2010 at 10:39 AM, damien hostin <damien.hostin@axege.com> wrote:
>
>> Hello again,
>>
>> At last, I check the same query with the same data on my desktop computer.
>> Just after loading the data, the queries were slow, I launch a vaccum
>> analyse which collect good stats on the main table, the query became quick
>> (~200ms). Now 1classic sata disk computer is faster than our little monster
>> server !!
>>
>
> Have you tried running ANALYZE on the production server?
>
> You might also want to try ALTER TABLE ... SET STATISTICS to a large
> value on some of the join columns involved in the query.
>
>
Hello,
Before comparing the test case on the two machines, I run analyse on the
whole and look at pg_stats table to see if change occurs for the
columns. but on the production server the stats never became as good as
on the desktop computer. I set statistic at 10000 on column used by the
join, run analyse which take a 3000000 row sample then look at the
stats. The stats are not as good as on the desktop. Row number is nearly
the same but only 1 or 2 values are found.
The data are not balanced the same way on the two computer :
- Desktop is 12000 rows with 6000 implicated in the query (50%),
- "Production" (actually a dev/test server) is 6 million rows with 6000
implicated in the query (0,1%).
Columns used in the query are nullable, and in the 5994000 other rows
that are not implicated in the query these columns are null.
I don't know if the statistic target is a % or a number of value to
obtain, but event set at max (10000), it didn't managed to collect good
stats (for this particular query).
As I don't know what more to do, my conclusion is that the data need to
be better balanced to allow the analyse gather better stats. But if
there is a way to improve the stats/query with this ugly balanced data,
I'm open to it !
I hope that in real production, data will never be loaded this way. If
this appened we will maybe set enable_nestloop to off, but I don't think
it's a good solution, other query have a chance to get slower.
Thanks for helping
--
HOSTIN Damien - Equipe R&D
Tel:+33(0)4 63 05 95 40
Société Axège
23 rue Saint Simon
63000 Clermont Ferrand
www.axege.com
^ permalink raw reply [nested|flat] 11+ messages in thread
* Re: Slow query with planner row strange estimation
@ 2010-07-09 20:25 Robert Haas <robertmhaas@gmail.com>
parent: damien hostin <damien.hostin@axege.com>
0 siblings, 1 reply; 11+ messages in thread
From: Robert Haas @ 2010-07-09 20:25 UTC (permalink / raw)
To: damien hostin <damien.hostin@axege.com>; +Cc: pgsql-performance
On Fri, Jul 9, 2010 at 6:13 AM, damien hostin <damien.hostin@axege.com> wrote:
>> Have you tried running ANALYZE on the production server?
>>
>> You might also want to try ALTER TABLE ... SET STATISTICS to a large
>> value on some of the join columns involved in the query.
>
> Hello,
>
> Before comparing the test case on the two machines, I run analyse on the
> whole and look at pg_stats table to see if change occurs for the columns.
> but on the production server the stats never became as good as on the
> desktop computer. I set statistic at 10000 on column used by the join, run
> analyse which take a 3000000 row sample then look at the stats. The stats
> are not as good as on the desktop. Row number is nearly the same but only 1
> or 2 values are found.
>
> The data are not balanced the same way on the two computer :
> - Desktop is 12000 rows with 6000 implicated in the query (50%),
> - "Production" (actually a dev/test server) is 6 million rows with 6000
> implicated in the query (0,1%).
> Columns used in the query are nullable, and in the 5994000 other rows that
> are not implicated in the query these columns are null.
>
> I don't know if the statistic target is a % or a number of value to obtain,
It's a number of values to obtain.
> but event set at max (10000), it didn't managed to collect good stats (for
> this particular query).
I think there's a cutoff where it won't collect values unless they
occur significantly more often than the average frequency. I wonder
if that might be biting you here: without the actual values in the MCV
table, the join selectivity estimates probably aren't too good.
> As I don't know what more to do, my conclusion is that the data need to be
> better balanced to allow the analyse gather better stats. But if there is a
> way to improve the stats/query with this ugly balanced data, I'm open to it
> !
>
> I hope that in real production, data will never be loaded this way. If this
> appened we will maybe set enable_nestloop to off, but I don't think it's a
> good solution, other query have a chance to get slower.
Yeah, that usually works out poorly.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company
^ permalink raw reply [nested|flat] 11+ messages in thread
* Re: Slow query with planner row strange estimation
@ 2010-07-12 07:17 Dimitri <dimitrik.fr@gmail.com>
parent: Robert Haas <robertmhaas@gmail.com>
0 siblings, 1 reply; 11+ messages in thread
From: Dimitri @ 2010-07-12 07:17 UTC (permalink / raw)
To: Robert Haas <robertmhaas@gmail.com>; +Cc: damien hostin <damien.hostin@axege.com>; pgsql-performance
It's probably one of the cases when having HINTS in PostgreSQL may be
very helpful..
SELECT /*+ enable_nestloop=off */ ... FROM ...
will just fix this query without impacting other queries and without
adding any additional instructions into the application code..
So, why there is a such resistance to implement hints withing SQL
queries in PG?..
Rgds,
-Dimitri
On 7/9/10, Robert Haas <robertmhaas@gmail.com> wrote:
> On Fri, Jul 9, 2010 at 6:13 AM, damien hostin <damien.hostin@axege.com>
> wrote:
>>> Have you tried running ANALYZE on the production server?
>>>
>>> You might also want to try ALTER TABLE ... SET STATISTICS to a large
>>> value on some of the join columns involved in the query.
>>
>> Hello,
>>
>> Before comparing the test case on the two machines, I run analyse on the
>> whole and look at pg_stats table to see if change occurs for the columns.
>> but on the production server the stats never became as good as on the
>> desktop computer. I set statistic at 10000 on column used by the join, run
>> analyse which take a 3000000 row sample then look at the stats. The stats
>> are not as good as on the desktop. Row number is nearly the same but only
>> 1
>> or 2 values are found.
>>
>> The data are not balanced the same way on the two computer :
>> - Desktop is 12000 rows with 6000 implicated in the query (50%),
>> - "Production" (actually a dev/test server) is 6 million rows with 6000
>> implicated in the query (0,1%).
>> Columns used in the query are nullable, and in the 5994000 other rows that
>> are not implicated in the query these columns are null.
>>
>> I don't know if the statistic target is a % or a number of value to
>> obtain,
>
> It's a number of values to obtain.
>
>> but event set at max (10000), it didn't managed to collect good stats (for
>> this particular query).
>
> I think there's a cutoff where it won't collect values unless they
> occur significantly more often than the average frequency. I wonder
> if that might be biting you here: without the actual values in the MCV
> table, the join selectivity estimates probably aren't too good.
>
>> As I don't know what more to do, my conclusion is that the data need to be
>> better balanced to allow the analyse gather better stats. But if there is
>> a
>> way to improve the stats/query with this ugly balanced data, I'm open to
>> it
>> !
>>
>> I hope that in real production, data will never be loaded this way. If
>> this
>> appened we will maybe set enable_nestloop to off, but I don't think it's a
>> good solution, other query have a chance to get slower.
>
> Yeah, that usually works out poorly.
>
> --
> Robert Haas
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise Postgres Company
>
> --
> Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-performance
>
^ permalink raw reply [nested|flat] 11+ messages in thread
* Re: Slow query with planner row strange estimation
@ 2010-07-12 20:33 phb07 <phb07@apra.asso.fr>
parent: Dimitri <dimitrik.fr@gmail.com>
0 siblings, 2 replies; 11+ messages in thread
From: phb07 @ 2010-07-12 20:33 UTC (permalink / raw)
To: Dimitri <dimitrik.fr@gmail.com>; +Cc: Robert Haas <robertmhaas@gmail.com>; damien hostin <damien.hostin@axege.com>; pgsql-performance
Dimitri a écrit :
> It's probably one of the cases when having HINTS in PostgreSQL may be
> very helpful..
>
> SELECT /*+ enable_nestloop=off */ ... FROM ...
>
> will just fix this query without impacting other queries and without
> adding any additional instructions into the application code..
>
> So, why there is a such resistance to implement hints withing SQL
> queries in PG?..
>
> Rgds,
> -Dimitri
>
>
+1.
Another typical case when it would be helpful is with setting the
cursor_tuple_fraction GUC variable for a specific statement, without
being obliged to issue 2 SET statements, one before the SELECT and the
other after.
> On 7/9/10, Robert Haas <robertmhaas@gmail.com> wrote:
>
>> On Fri, Jul 9, 2010 at 6:13 AM, damien hostin <damien.hostin@axege.com>
>> wrote:
>>
>>>> Have you tried running ANALYZE on the production server?
>>>>
>>>> You might also want to try ALTER TABLE ... SET STATISTICS to a large
>>>> value on some of the join columns involved in the query.
>>>>
>>> Hello,
>>>
>>> Before comparing the test case on the two machines, I run analyse on the
>>> whole and look at pg_stats table to see if change occurs for the columns.
>>> but on the production server the stats never became as good as on the
>>> desktop computer. I set statistic at 10000 on column used by the join, run
>>> analyse which take a 3000000 row sample then look at the stats. The stats
>>> are not as good as on the desktop. Row number is nearly the same but only
>>> 1
>>> or 2 values are found.
>>>
>>> The data are not balanced the same way on the two computer :
>>> - Desktop is 12000 rows with 6000 implicated in the query (50%),
>>> - "Production" (actually a dev/test server) is 6 million rows with 6000
>>> implicated in the query (0,1%).
>>> Columns used in the query are nullable, and in the 5994000 other rows that
>>> are not implicated in the query these columns are null.
>>>
>>> I don't know if the statistic target is a % or a number of value to
>>> obtain,
>>>
>> It's a number of values to obtain.
>>
>>
>>> but event set at max (10000), it didn't managed to collect good stats (for
>>> this particular query).
>>>
>> I think there's a cutoff where it won't collect values unless they
>> occur significantly more often than the average frequency. I wonder
>> if that might be biting you here: without the actual values in the MCV
>> table, the join selectivity estimates probably aren't too good.
>>
>>
>>> As I don't know what more to do, my conclusion is that the data need to be
>>> better balanced to allow the analyse gather better stats. But if there is
>>> a
>>> way to improve the stats/query with this ugly balanced data, I'm open to
>>> it
>>> !
>>>
>>> I hope that in real production, data will never be loaded this way. If
>>> this
>>> appened we will maybe set enable_nestloop to off, but I don't think it's a
>>> good solution, other query have a chance to get slower.
>>>
>> Yeah, that usually works out poorly.
>>
>> --
>> Robert Haas
>> EnterpriseDB: http://www.enterprisedb.com
>> The Enterprise Postgres Company
>>
>> --
>> Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
>> To make changes to your subscription:
>> http://www.postgresql.org/mailpref/pgsql-performance
>>
>>
Regards.
Philippe Beaudoin.
^ permalink raw reply [nested|flat] 11+ messages in thread
* Re: Slow query with planner row strange estimation
@ 2010-07-13 08:51 damien hostin <damien.hostin@axege.com>
parent: phb07 <phb07@apra.asso.fr>
1 sibling, 0 replies; 11+ messages in thread
From: damien hostin @ 2010-07-13 08:51 UTC (permalink / raw)
To: phb07 <phb07@apra.asso.fr>; +Cc: pgsql-performance
phb07 a écrit :
>
> Dimitri a écrit :
>> It's probably one of the cases when having HINTS in PostgreSQL may be
>> very helpful..
>>
>> SELECT /*+ enable_nestloop=off */ ... FROM ...
>>
>> will just fix this query without impacting other queries and without
>> adding any additional instructions into the application code..
>>
>> So, why there is a such resistance to implement hints withing SQL
>> queries in PG?..
>>
>> Rgds,
>> -Dimitri
>>
>>
> +1.
> Another typical case when it would be helpful is with setting the
> cursor_tuple_fraction GUC variable for a specific statement, without
> being obliged to issue 2 SET statements, one before the SELECT and the
> other after.
>
>
I remember that the "dimension" columns of the fact table have indexes
like with "WHERE IS NOT NULL" on the column indexed. Example:
CREATE INDEX dwhinv_pd2_idx
ON dwhinv
USING btree
(dwhinv_p2rfodstide)
TABLESPACE tb_index
WHERE dwhinv_p2rfodstide IS NOT NULL;
Is the where clause being used to select the sample rows on which the
stats will be calculated or just used to exclude values after collecting
stat ? As I am writing I realize there's must be no link between a table
column stats and an index a the same column. (By the way, If I used is
not null on each column with such an index, it changes nothing)
About the oracle-like hints, it does not really help, because the query
is generated in an external jar that I should fork to include the
modification. I would prefer forcing a plan based on the query hashcode,
but this does not fix what make the planner goes wrong.
--
HOSTIN Damien - Equipe R&D
Tel:+33(0)4 63 05 95 40
Société Axège
23 rue Saint Simon
63000 Clermont Ferrand
www.axege.com
^ permalink raw reply [nested|flat] 11+ messages in thread
* Re: Slow query with planner row strange estimation
@ 2010-07-22 18:08 Robert Haas <robertmhaas@gmail.com>
parent: phb07 <phb07@apra.asso.fr>
1 sibling, 0 replies; 11+ messages in thread
From: Robert Haas @ 2010-07-22 18:08 UTC (permalink / raw)
To: phb07 <phb07@apra.asso.fr>; +Cc: Dimitri <dimitrik.fr@gmail.com>; damien hostin <damien.hostin@axege.com>; pgsql-performance
On Mon, Jul 12, 2010 at 4:33 PM, phb07 <phb07@apra.asso.fr> wrote:
>
> Dimitri a écrit :
>>
>> It's probably one of the cases when having HINTS in PostgreSQL may be
>> very helpful..
>>
>> SELECT /*+ enable_nestloop=off */ ... FROM ...
>>
>> will just fix this query without impacting other queries and without
>> adding any additional instructions into the application code..
>>
>> So, why there is a such resistance to implement hints withing SQL
>> queries in PG?..
>>
>
> +1.
> Another typical case when it would be helpful is with setting the
> cursor_tuple_fraction GUC variable for a specific statement, without being
> obliged to issue 2 SET statements, one before the SELECT and the other
> after.
We've previously discussed adding a command something like:
LET (variable = value, variable = value, ...) command
...which would set those variables just for that one command. But
honestly I'm not sure how much it'll help with query planner problems.
Disabling nestloops altogether, even for one particular query, is
often going to be a sledgehammer where you need a scalpel. But then
again, a sledgehammer is better than no hammer.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company
^ permalink raw reply [nested|flat] 11+ messages in thread
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Thread overview: 11+ messages (download: mbox mbox.gz follow: Atom feed)
-- links below jump to the message on this page --
2010-07-02 08:48 Slow query with planner row strange estimation damien hostin <damien.hostin@axege.com>
2010-07-05 07:28 ` damien hostin <damien.hostin@axege.com>
2010-07-06 12:48 ` damien hostin <damien.hostin@axege.com>
2010-07-07 14:39 ` damien hostin <damien.hostin@axege.com>
2010-07-08 18:42 ` Robert Haas <robertmhaas@gmail.com>
2010-07-09 10:13 ` damien hostin <damien.hostin@axege.com>
2010-07-09 20:25 ` Robert Haas <robertmhaas@gmail.com>
2010-07-12 07:17 ` Dimitri <dimitrik.fr@gmail.com>
2010-07-12 20:33 ` phb07 <phb07@apra.asso.fr>
2010-07-13 08:51 ` damien hostin <damien.hostin@axege.com>
2010-07-22 18:08 ` Robert Haas <robertmhaas@gmail.com>
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