*very* inefficient choice made by the planner (regarding IN(...))

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

--
Best,

Frank.