On 07/06/2012 02:34 PM, Perry Smith wrote:
Hi Guys,
This isn't a PostgreSQL specific question but just a SQL question. If this is
not an appropriate question for this list, please let me know.
It is also, perhaps, a really silly question.
This query (without the 'explain' keyword) , when executed takes forever and a
day:
condor_development=> explain select id from filesets where id not in ( select
fileset_id from service_pack_fileset_maps );
QUERY PLAN
----------------------------------------------------------------------------------------------
Seq Scan on filesets (cost=0.00..71937742.00 rows=26088 width=4)
Filter: (NOT (SubPlan 1))
SubPlan 1
-> Materialize (cost=0.00..2517.78 rows=95852 width=4)
-> Seq Scan on service_pack_fileset_maps (cost=0.00..1663.52
rows=95852 width=4)
(5 rows)
This query returns within a second:
condor_development=> explain select id from filesets where id not in ( select
distinct fileset_id from service_pack_fileset_maps );
QUERY PLAN
----------------------------------------------------------------------------------------------
Seq Scan on filesets (cost=2102.31..3153.53 rows=26088 width=4)
Filter: (NOT (hashed SubPlan 1))
SubPlan 1
-> HashAggregate (cost=1903.15..2062.48 rows=15933 width=4)
-> Seq Scan on service_pack_fileset_maps (cost=0.00..1663.52
rows=95852 width=4)
(5 rows)
The difference is the "distinct" keyword in the inner select.
What I'm confused about is why isn't the "distinct" implicit? I thought the construct "blah
in ( select ... )" was using "sets" and an item (I thought) can not be in a set more than once.
Perhaps my question is the opposite really? Why would you not always use "distinct" in the inner
select when the operator is "in" or "not in" ?
And if I can throw in another question on top: is there a different method other than
"not in" that would work better?
Actually it is *very* PostgreSQL specific. In fact, it may even be
PostgreSQL *version* specific as you are delving into how the planner
decides how to handle a query.
It appears that the planner is assuming, based on collected stats and
available indexes, that there will be roughly 1/6 the records returned
by the "distinct" query and thus chose a different method to join the
records. One useful piece of information would be the indexes on the two
tables.
As to other methods, you can use:
... where not exists (select 1 from service_pack_fileset_maps where
fileset_id = filesets.id)...
(Note: as alluded to above, ...not in... works better in some releases
and ...not exists... better in others due to improvements over time.)
Still another method:
select id from filesets except select fileset_id from
service_pack_fileset_maps;
Cheers,
Steve
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