Re: Why Index is not used

Chetan Suttraway wrote:
>
>
> On Fri, Mar 25, 2011 at 12:05 PM, Adarsh Sharma
> <adarsh(dot)sharma(at)orkash(dot)com <mailto:adarsh(dot)sharma(at)orkash(dot)com>> wrote:
>
> Dear all,
>
> Today I got to run a query internally from my application by more
> than 10 connections.
>
> But The query performed very badly. A the data size of tables are as :
>
> pdc_uima=# select pg_size_pretty(pg_total_relation_size('clause2'));
> pg_size_pretty
> ----------------
> 5858 MB
> (1 row)
>
> pdc_uima=# select pg_size_pretty(pg_total_relation_size('svo2'));
> pg_size_pretty
> ----------------
> 4719 MB
> (1 row)
>
>
> I explain the query as after making the indexes as :
>
> pdc_uima=# explain select c.clause, s.* from clause2 c, svo2 s
> where c.clause_id=s.clause_id and s.doc_id=c.source_id and c.
> pdc_uima-# sentence_id=s.sentence_id ;
> QUERY PLAN
>
> --------------------------------------------------------------------------------------------------------------
> Merge Join (cost=5673831.05..34033959.87 rows=167324179 width=2053)
> Merge Cond: ((s.clause_id = c.clause_id) AND (s.doc_id =
> c.source_id) AND (s.sentence_id = c.sentence_id))
> -> Index Scan using idx_svo2 on svo2 s (cost=0.00..24489343.65
> rows=27471560 width=1993)
> -> Materialize (cost=5673828.74..6071992.29 rows=31853084 width=72)
> -> Sort (cost=5673828.74..5753461.45 rows=31853084 width=72)
> Sort Key: c.clause_id, c.source_id, c.sentence_id
> -> Seq Scan on clause2 c (cost=0.00..770951.84
> rows=31853084 width=72)
>
>
>
> Indexes are :
>
> CREATE INDEX idx_clause ON clause2 USING btree (clause_id,
> source_id, sentence_id);
> CREATE INDEX idx_svo2 ON svo2 USING btree (clause_id, doc_id,
> sentence_id);
>
> I don't know why it not uses the index scan for clause2 table.
>
>
> In this case, there are no predicates or filters on individual table.
> (maybe something like c.source_id=10)
> so either of the 2 tables will have to go for simple scan.
>
> Are you expecting seq. scan on svo2 and index scan on clause2?
>

As per the size consideration and the number of rows, I think index scan
on clause2 is better.

Your constraint is valid but I need to perform this query faster.
What is the reason behind the seq scan of clause2.

Regards,
Adarsh
>
>