| From: | Marti Raudsepp <marti(at)juffo(dot)org> |
|---|---|
| To: | ach <alanchines(at)gmail(dot)com> |
| Cc: | "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: statistics target for columns in unique constraint? |
| Date: | 2013-05-13 22:10:06 |
| Message-ID: | CABRT9RBwmDxjq-tzLMh6rFX-suJYVQMiGSQAADsueg07rvB1dg@mail.gmail.com |
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| Lists: | pgsql-performance |
On Mon, May 13, 2013 at 6:01 PM, ach <alanchines(at)gmail(dot)com> wrote:
> what I'm wondering is, since
> the unique constraint already covers the whole table and all rows in
> entirety, is it really necessary for statistics to be set that high on
> those?
AFAIK if there are exact-matching unique constraints/indexes for a
query's WHERE clause, the planner will deduce that the query only
returns 1 row and won't consult statistics at all.
> Or does that only serve to slow down inserts to that table?
It doesn't slow down inserts directly. Tables are analyzed in the
background by autovacuum. However, I/O traffic from autovacuum analyze
may slow down inserts running concurrently.
Regards,
Marti
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