Gregory Stark wrote:
> There are a few things going on here.
>
> 1) The optimizer can't build a plan which ignores those partitions
> because the statistics are just approximations. You could insert into
> one of them at any time and the statistics won't update immediately. If
> you have a partition which is empty of some type of data you can put a
> constraint on it to promise the optimizer that that condition will stay
> true.
I actually do have constraints on all the partitions, e.g. for week 34:
Check constraints [for email_2007_week34]:
"email_2007_week34_ts_check" CHECK (ts >= '2007-08-20 00:00:00'::timestamp without time zone AND ts < '2007-08-27 00:00:00'::timestamp without time zone)
Check constraints [for email_2007_week34_extras]:
"email_2007_week34_extras_ts_check" CHECK (ts >= '2007-08-20 00:00:00'::timestamp without time zone AND ts < '2007-08-27 00:00:00'::timestamp without time zone)
Shouldn't this be enough to give the query planner a clue that it only
has to join the "email" and "email_extras" tables' partitions pair-wise,
as opposed to cross-joining them?
For the record, I also have indexes:
Indexes [for email_2007_week34]:
"email_2007_week34_pkey" PRIMARY KEY, btree (id)
"index_email_2007_week34_on_helo" btree (helo)
"index_email_2007_week34_on_ip" btree (ip)
"index_email_2007_week34_on_load_id" btree (load_id)
"index_email_2007_week34_on_mfrom" btree (mfrom)
"index_email_2007_week34_on_ts" btree (ts)
Indexes [for for email_2007_week34_extras]:
"email_2007_week34_extras_pkey" PRIMARY KEY, btree (id)
> 2) The optimizer is assuming that empty tables have a default 1,000
> records in them with no idea about their statistics. Otherwise you get
> terrible plans on tables which have just been created or never
> analyzed. In this case that's causing it to think there will be tons of
> matches on what is apparently a very selective criterion.
I see. But this shouldn't matter under the assumption that constraint
exclusion works correctly, right?
> 3) The optimizer is a bit dumb about partitioned tables. But I'm not
> sure if that's actually the fault here.
>
> Try adding one record of data to each of those partitions or putting a
> constraint on them which will allow constraint_exclusion (I assume you
> have that enabled?) to kick in. You'll still be bitten by the parent
> table but hopefully that's not enough to cause a problem.
The parent table is empty. How will adding one record to each of the
partitions make a difference given the above constraints?
Julian.
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