*very* inefficient choice made by the planner (regarding IN(...)) - Mailing list pgsql-performance

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.