Hello.
I have a problem concerning multi-column indexes.
I have a table containing some 250k lines.
Table "public.descriptionprodftdiclnk"
Column | Type | Modifiers
-------------+---------+-----------
idword | integer | not null
idqualifier | integer | not null
Indexes:
"descriptionprodftdiclnk_pkey" primary key, btree (idword, idqualifier)
"ix_descriptionprodftdiclnk_idqualif" btree (idqualifier)
When analyzing a simple query on the idword column the query planner displays:
explain analyze select * from descriptionprodftdiclnk where idword=44;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------
Seq Scan on descriptionprodftdiclnk (cost=0.00..4788.14 rows=44388 width=8) (actual time=87.582..168.041 rows=43792 loops=1)
Filter: (idword = 44)
Total runtime: 195.339 ms
(3 rows)
I don't understand why the query planner would not use the default created multi-column index
on the primary key. According to the Postgres online documentation it should. By setting the
"enable_seqscan" parameter to off, i can force the planner to use the index:
explain analyze select * from descriptionprodftdiclnk where idword=44;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
Index Scan using descriptionprodftdiclnk_pkey on descriptionprodftdiclnk (cost=0.00..36720.39 rows=44388 width=8) (actual time=0.205..73.489 rows=43792 loops=1)
Index Cond: (idword = 44)
Total runtime: 100.564 ms
(3 rows)
On the other hand, by defining a new index on the idword column (and "enable_seqscan" set to on),
the query uses the index:
create index ix_tempIndex on descriptionprodftdiclnk(idword);
CREATE INDEX
explain analyze select * from descriptionprodftdiclnk where idword=44;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------
Index Scan using ix_tempindex on descriptionprodftdiclnk (cost=0.00..916.24 rows=44388 width=8) (actual time=0.021..79.879 rows=43792 loops=1)
Index Cond: (idword = 44)
Total runtime: 107.081 ms
(3 rows)
Could someone provide an explanation for the planner's behaviour?
Thanks for your help, Daniel
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