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Re: Query in 9.0.2 not using index in 9.0.0 works fine - Mailing list pgsql-performance

From Samuel Gendler
Subject Re: Query in 9.0.2 not using index in 9.0.0 works fine
Date
Msg-id CAEV0TzAaYAHPO5x0935FYdncU3_w6DbZo5HHMT6nKejE8pwz6w@mail.gmail.com
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In response to Query in 9.0.2 not using index in 9.0.0 works fine  ("Matthias Howell" <Matthias.Howell@voxco.com>)
Responses Re: Query in 9.0.2 not using index in 9.0.0 works fine
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On Tue, Jul 5, 2011 at 1:50 PM, Matthias Howell <Matthias.Howell@voxco.com> wrote:

I've just copied a database from one linux machine to another.

"Fast" machine is CentOS 5.5, running postgres 9.0.0 64 bit

 

"Slow" machine is Red Hat 5.5 running postgres 9.0.2 64 bit.

 

Here's the query:

explain analyze select sentenceid from sentences where sentenceid = any ( array(select sentenceid from sentences where docid = any(array[696374,696377])))

 

on the fast machine this is the explain:

"Bitmap Heap Scan on sentences  (cost=924.41..964.47 rows=10 width=8) (actual time=0.748..0.800 rows=41 loops=1)"

"  Recheck Cond: (sentenceid = ANY ($0))"

"  InitPlan 1 (returns $0)"

"    ->  Bitmap Heap Scan on sentences  (cost=12.93..879.27 rows=220 width=8) (actual time=0.199..0.446 rows=41 loops=1)"

"          Recheck Cond: (docid = ANY ('{696374,696377}'::bigint[]))"

"          ->  Bitmap Index Scan on sentdocs  (cost=0.00..12.87 rows=220 width=0) (actual time=0.134..0.134 rows=41 loops=1)"

"                Index Cond: (docid = ANY ('{696374,696377}'::bigint[]))"

"  ->  Bitmap Index Scan on pk_sentences  (cost=0.00..45.14 rows=10 width=0) (actual time=0.741..0.741 rows=41 loops=1)"

"        Index Cond: (sentenceid = ANY ($0))"

"Total runtime: 0.925 ms"

 

And on the slow machine:

"Seq Scan on sentences  (cost=10000000608.90..10000445893.60 rows=10 width=8) (actual time=2679.412..6372.393 rows=41 loops=1)"

"  Filter: (sentenceid = ANY ($0))"

"  InitPlan 1 (returns $0)"

"    ->  Bitmap Heap Scan on sentences  (cost=10.73..608.90 rows=152 width=8) (actual time=0.044..0.076 rows=41 loops=1)"

"          Recheck Cond: (docid = ANY ('{696374,696377}'::integer[]))"

"          ->  Bitmap Index Scan on sentdocs  (cost=0.00..10.69 rows=152 width=0) (actual time=0.037..0.037 rows=41 loops=1)"

"                Index Cond: (docid = ANY ('{696374,696377}'::integer[]))"

"Total runtime: 6372.468 ms"

 

The configurations were identical initially, I've changed those on the slow machine but to no avail.

 

there is an index on sentences on the docid in both systems.

 

I'm at quite a loss as to how/why this is occurring and what to do about it.

 

I tried disabling seqscan on the slow machine but that also made no difference.

 

Any help/ideas much appreciated.


Have you done a vacuum analyze since loading the data on the slow db?  Are statistics settings the same between the two hosts?  It's interesting that one version coerces the docid values to bigint and the other coerces to integer, but that shouldn't impact the sentenceid comparison, which have to be a consistent type since it is comparing sentenceid to sentenceid. Any reason why this isn't collapsed down to 'select distinct sentenceid from sentences where docid = any(array[696374,696377])' - is there a benefit to the more complex structure?  For that matter, why not 'where docid in (696374,696377)'

I didn't see anything in the docs about distinct or any(array) that would indicate that that form should be preferred over IN ()

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