RE: [HACKERS] What does explain show ?

> -----Original Message-----
> From: Bruce Momjian [mailto:maillist@candle.pha.pa.us]
> Sent: Wednesday, July 14, 1999 11:57 AM
> To: Hiroshi Inoue
> Cc: pgsql-hackers
> Subject: Re: [HACKERS] What does explain show ?
>
>
> > Table a has 15905 rows and table b has 25905 rows.
> >
> >
> > For the following query
> >
> >   select a.pkey, b.key2 from a, b
> >   where b.key1 = 1369
> >   and     a.pkey = b.key1;
> >
> > "explain" shows
> >
> > NOTICE:  QUERY PLAN:
> >
> > Nested Loop  (cost=6.19 rows=3 width=10)
> >   ->  Index Scan using b_pkey on b on b  (cost=2.09 rows=2 width=6)
> >   ->  Index Scan using a_pkey on a on a  (cost=2.05 rows=15905 width=4)
> >
> > What does "rows=15905" of InnerPlan mean ?
> > Is "rows=3" of Nested Loop irrelevant to "rows=15905" ?
>
> It means it thinks it is going to access X rows in that pass, but end
> up with 3 joined rows as a result of the nested loop.  It is only an
> estimate, based on table size and column uniqueness from vacuum analyze.
>

Hmmm,I couldn't understand where does "rows=15905" come from.
Shouldn't "rows" of InnerPlan be 1 ?
Is the caluculation "rows of Nested loop = rows of OuterPlan * rows of
InnerPlan"
wrong ?

Regards.

Hiroshi Inoue
Inoue@tpf.co.jp