| From: | "Hiroshi Inoue" <Inoue(at)tpf(dot)co(dot)jp> |
|---|---|
| To: | "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | "Bruce Momjian" <maillist(at)candle(dot)pha(dot)pa(dot)us>, "pgsql-hackers" <pgsql-hackers(at)postgreSQL(dot)org> |
| Subject: | RE: [HACKERS] What does explain show ? |
| Date: | 1999-07-15 08:33:01 |
| Message-ID: | 001c01bece9c$a6b43300$2801007e@cadzone.tpf.co.jp |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-hackers |
>
> > Is the caluculation "rows of Nested loop = rows of OuterPlan * rows of
> > InnerPlan" wrong ?
>
> Careful --- rows produced and cost are quite different things. The
> cost estimate for a nestloop is "cost of outerplan + rows of outerplan *
> cost of innerplan", but we don't necessarily expect to get as many rows
> out as the product of the row counts. Typically, it'd be lower due to
> join selectivity. Above you see only 3 rows out, which is not too bad
> a guess, certainly better than 2*15905 would be.
>
I see. rows of Join = rows of outerplan * rows of innerplan * "join
selectity".
and "join selectivity" is calcutated by eqjoinsel() etc.
Thanks.
Hiroshi Inoue
Inoue(at)tpf(dot)co(dot)jp
| From | Date | Subject | |
|---|---|---|---|
| Next Message | Jan Wieck | 1999-07-15 08:36:16 | Re: [HACKERS] MAX Query length |
| Previous Message | Zeugswetter Andreas IZ5 | 1999-07-15 07:35:17 | Re: [HACKERS] Re: Arrays versus 'type constant' syntax |