Re: Query does not use index

Hi,

Am 30.04.2004 um 01:32 schrieb Tom Lane:

> Martin Hampl <Martin(dot)Hampl(at)gmx(dot)de> writes:
>> It would be very nice, if someone could have a look at the query I'm
>> trying to optimize. At the moment, I don't understand PostgreSQL's
>> behaviour and are stuck. Thanks a lot in advance.
>
> Did you ANALYZE these tables?

I did.

> Also, please post EXPLAIN ANALYZE not
> just EXPLAIN when complaining about bad plans. Since the essence of
> your complaint is that the planner's estimates are wrong, showing us
> only estimates and not reality makes it hard to give constructive
> suggestions ...

OK.

bnc23Mio=# EXPLAIN ANALYZE select * from
bnc23Mio-# token,
bnc23Mio-# s
bnc23Mio-# where
bnc23Mio-# token.word = 'FACTSHEET' and
bnc23Mio-# s.text_id = token.text_id and
bnc23Mio-# s.start = token.position;
QUERY PLAN
------------------------------------------------------------------------
------------------------------------------------------------
Nested Loop (cost=0.00..39120.95 rows=1 width=32) (actual
time=102.263..692248.553 rows=3 loops=1)
-> Index Scan using s_pkey on s (cost=0.00..24698.44 rows=3367
width=16) (actual time=75.933..589743.642 rows=1111220 loops=1)
-> Index Scan using token_pkey on token (cost=0.00..4.27 rows=1
width=16) (actual time=0.086..0.086 rows=0 loops=1111220)
Index Cond: (("outer".text_id = token.text_id) AND
("outer"."start" = token."position"))
Filter: ((word)::text = 'FACTSHEET'::text)
Total runtime: 692249.314 ms

bnc23Mio=# EXPLAIN ANALYZE select * from (select text_id, position from
token where word = 'FACTSHEET') t left join s on (s.text_id = t.text_id
and s.start = t.position );
QUERY
PLAN
------------------------------------------------------------------------
-------------------------------------------------------------------
Hash Left Join (cost=24715.28..56630.78 rows=7892 width=24) (actual
time=255329.976..255355.967 rows=5 loops=1)
Hash Cond: (("outer".text_id = "inner".text_id) AND
("outer"."position" = "inner"."start"))
-> Index Scan using word_idx on token (cost=0.00..31402.51
rows=7892 width=8) (actual time=91.010..109.394 rows=5 loops=1)
Index Cond: ((word)::text = 'FACTSHEET'::text)
-> Hash (cost=24698.44..24698.44 rows=3367 width=16) (actual
time=255236.914..255236.914 rows=0 loops=1)
-> Index Scan using s_pkey on s (cost=0.00..24698.44
rows=3367 width=16) (actual time=105.100..247798.661 rows=1111220
loops=1)
Total runtime: 255502.736 ms

Maybe that *is* what i wanted it to do? However, searching just for
'FACTSHEET' is very quick (I rebooted before this query to clear any
cache---is there a better way to do this?):

bnc23Mio=# EXPLAIN ANALYZE select * from token where word = 'FACTSHEET';
QUERY PLAN
------------------------------------------------------------------------
----------------------------------------------------
Index Scan using word_idx on token (cost=0.00..31402.51 rows=7892
width=16) (actual time=102.350..125.032 rows=5 loops=1)
Index Cond: ((word)::text = 'FACTSHEET'::text)
Total runtime: 125.289 ms

and I would have thought that the results of this query could have been
used to search for the respective records in s (using on of the
indexes)?

Regards,
Martin.