Re: *very* inefficient choice made by the planner (regarding

The real question is:

If the two statments are functionally equivalent, why can't PG rewrite
the "NOT IN" version into the more efficient "NOT EXISTS"?

Frank van Vugt wrote:

> L.S.
>
> Could anybody explain why the planner is doing what it is doing?
>
> What could I do to make it easier to choose a better plan?
>
>
>
> *********
> Summary
> *********
> On a freshly vacuum/analysed pair of tables with 7389 and 64333 records, this:
>
> select id from location where id not in (select location_id from
> location_carrier);
>
> takes 581546,497 ms
>
>
> While a variant like:
>
> select id from location where not exists (select 1 from location_carrier where
> location_id = location.id);
>
> takes only 124,625 ms
>
>
> *********
> Details
> *********
> =# select version();
> version
> ---------------------------------------------------------------------
> PostgreSQL 7.4.2 on i686-pc-linux-gnu, compiled by GCC egcs-2.91.66
> (1 row)
>
>
> =# \d location
> Table "public.location"
> Column | Type | Modifiers
> ------------+-----------------------------+-----------
> id | integer | not null
> Indexes:
> "location_pkey" primary key, btree (id)
>
>
> =# select count(*) from location;
> count
> -------
> 7389
> (1 row)
>
>
> =# \d location_carrier
> Table "public.location_carrier"
> Column | Type | Modifiers
> ---------------------+-----------------------------+-----------
> location_id | integer | not null
> carrier_id | integer | not null
> Indexes:
> "location_carrier_pkey" primary key, btree (location_id, carrier_id)
>
>
> =# select count(*) from location_carrier;
> count
> -------
> 64333
> (1 row)
>
>
> =# explain select id from location where id not in (select location_id from
> location_carrier);
> QUERY PLAN
> -------------------------------------------------------------------------------
> Seq Scan on "location" (cost=0.00..5077093.72 rows=3695 width=4)
> Filter: (NOT (subplan))
> SubPlan
> -> Seq Scan on location_carrier (cost=0.00..1213.33 rows=64333 width=4)
> (4 rows)
>
>
> =# explain analyse select id from location where id not in (select location_id
> from location_carrier);
> QUERY PLAN
> ---------------------------------------------------------------------------------------------------------------------------------
> Seq Scan on "location" (cost=0.00..5077093.72 rows=3695 width=4) (actual
> time=248.310..581541.483 rows=240 loops=1)
> Filter: (NOT (subplan))
> SubPlan
> -> Seq Scan on location_carrier (cost=0.00..1213.33 rows=64333 width=4)
> (actual time=0.007..48.517 rows=19364 loops=7389)
> Total runtime: 581542.560 ms
> (5 rows)
>
> Time: 581546,497 ms
>
>
> =# explain analyse select id from location l left outer join location_carrier
> lc on l.id = lc.location_id where lc.location_id is null;
> QUERY
> PLAN
> --------------------------------------------------------------------------------------------------------------------------------------------------------------
> Merge Left Join (cost=0.00..3022.51 rows=7389 width=4) (actual
> time=0.083..435.841 rows=240 loops=1)
> Merge Cond: ("outer".id = "inner".location_id)
> Filter: ("inner".location_id IS NULL)
> -> Index Scan using location_pkey on "location" l (cost=0.00..258.85
> rows=7389 width=4) (actual time=0.041..26.211 rows=7389 loops=1)
> -> Index Scan using location_carrier_pkey on location_carrier lc
> (cost=0.00..1941.22 rows=64333 width=4) (actual time=0.015..238.305
> rows=64333 loops=1)
> Total runtime: 436.213 ms
> (6 rows)
>
> Time: 440,787 ms
>
>
> megafox=# explain analyse select id from location where not exists (select 1
> from location_carrier where location_id = location.id);
> QUERY
> PLAN
> -----------------------------------------------------------------------------------------------------------------------------------------------------
> Seq Scan on "location" (cost=0.00..13242.14 rows=3695 width=4) (actual
> time=0.078..120.785 rows=240 loops=1)
> Filter: (NOT (subplan))
> SubPlan
> -> Index Scan using location_carrier_pkey on location_carrier
> (cost=0.00..17.61 rows=10 width=0) (actual time=0.011..0.011 rows=1
> loops=7389)
> Index Cond: (location_id = $0)
> Total runtime: 121.165 ms
> (6 rows)
>
> Time: 124,625 ms
>
>
>
>
>
>
>