Re: strange explain - Mailing list pgsql-hackers

Tom,

one more question.

What's the difference for planner between 2 queries ?
For the first query I have plain index scan, but multiple
index scan for second.

tour=# explain analyze  select * from tours  where             ( operator_id in (2,3,4,5,7) and type_id = 2 );
NOTICE:  QUERY PLAN:

Index Scan using type_idx on tours  (cost=0.00..2.03 rows=1 width=1091) (actual time=0.03..0.03 rows=0 loops=1)
Total runtime: 0.16 msec

EXPLAIN
tour=# explain analyze  select * from tours  where          ( operator_id in (2,3,4,5,7) and type_id = 4 ) or
(operator_id = 8 and type_id = 3);
 
NOTICE:  QUERY PLAN:

Index Scan using type_idx, type_idx, type_idx, type_idx, type_idx, type_idx on tours  (cost=0.00..12.25 rows=1
width=1091)(actual time=0.27..0.27 rows=0 loops=1)
 
Total runtime: 0.44 msec

EXPLAIN




On Mon, 13 May 2002, Tom Lane wrote:

> Oleg Bartunov <oleg@sai.msu.su> writes:
> > tour=# explain analyze  select * from tours  where
> >              ( operator_id in (2,3,4,5,7) and type_id = 2 )  or
> >              ( operator_id = 8 and type_id=4 );
>
> > Index Scan using type_idx, type_idx, type_idx, type_idx, type_idx, type_idx on tours  (cost=0.00..12.25 rows=1
width=1091)(actual time=0.26..0.26 rows=0 loops=1)
 
>
> > What does many 'type_idx' means ?
>
> Multiple indexscans.
>
> It looks to me like your WHERE clause is being flattened into
>
>              ( operator_id = 2 and type_id=2 ) or
>              ( operator_id = 3 and type_id=2 ) or
>              ( operator_id = 4 and type_id=2 ) or
>              ( operator_id = 5 and type_id=2 ) or
>              ( operator_id = 7 and type_id=2 ) or
>              ( operator_id = 8 and type_id=4 )
>
> and then it has a choice of repeated indexscans on operator_id or
> type_id.  Depending on the selectivity stats it might pick either.
> You might find that a 2-column index on both would be a win.
>
>             regards, tom lane
>
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Regards,    Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83