Re: how does the planer to estimate row when i use order by and group by

On Tue, Aug 12, 2014 at 5:59 AM, 楊新波 <silent0608(at)gmail(dot)com> wrote:
> why does the planer estimate 200 rows when i use order by and group by .
> evn:postgresql 8.4 and 9.3

> Can anybody suggest something or explain this behavior?

Because the table is empty, analyze doesn't store any stats for the
table, so the planner uses some default guesses.

This is actually beneficial for cases where you have done some inserts
to a new table, and autovacuum hasn't gotten around to analyzing it
yet. And it rarely hurts because any query plan will be fast when
there's no data.

Regards,
Marti