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From: Samuel Gendler
[mailto:sgendler@xxxxxxxxxxxxxxxx] On Tue, Jul 5, 2011 at 1:50 PM, Matthias Howell <Matthias.Howell@xxxxxxxxx>
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 () ______ I ran vacuum analyze, and I even dropped and recreated the
index. The differences between postgresql.conf are: FAST: shared_buffers = 2GB SLOW: shared_buffers = 4GB The reason for using the any=array is that the array of
docids is passed in as a parameter. This query is a subquery of a larger
query. I am trying to solve the problem for the smaller query. The
difference in the explain in the big query is essentially the seq scan on
sentences. This query is a sub query that is performed - with variations
- 3 times in the larger query. The the fast instance the larger query
takes 950 milliseconds, on the slow instance, over 30 seconds. However, in the end, it was user brain damage. It does use the doc id index for the subquery, but for some
reason, the primary key on sentences - the sentenceid - was not set. So
in fact, there is no index. Machines vindicated once again. |
