From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jon Nelson <jnelson+pgsql@jamponi.net>
Cc: pgsql-performance@postgresql.org
Subject: Re: select distinct uses index scan vs full table scan
Date: Tue, 13 Dec 2011 14:57:57 -0500
Message-ID: <9562.1323806277@sss.pgh.pa.us> (raw)
In-Reply-To: <CAKuK5J12QokFh88tQz-oJMSiBg2QyjM7K7HLnbYi3Ze+Y5BtWQ@mail.gmail.com>
References: <CAKuK5J12QokFh88tQz-oJMSiBg2QyjM7K7HLnbYi3Ze+Y5BtWQ@mail.gmail.com>
Jon Nelson <jnelson+pgsql@jamponi.net> writes:
> I've got a 5GB table with about 12 million rows.
> Recently, I had to select the distinct values from just one column.
> The planner chose an index scan. The query took almost an hour.
> When I forced index scan off, the query took 90 seconds (full table scan).
Usually, we hear complaints about the opposite. Are you using
nondefault cost settings?
> The planner estimated 70,000 unique values when, in fact, there are 12
> million (the value for this row is *almost* but not quite unique).
> What's more, despite bumping the statistics on that column up to 1000
> and re-analyzing, the planner now thinks that there are 300,000 unique
> values.
Accurate ndistinct estimates are hard, but that wouldn't have much of
anything to do with this particular choice, AFAICS.
> How can I tell the planner that a given column is much more unique
> than, apparently, it thinks it is?
9.0 and up have ALTER TABLE ... ALTER COLUMN ... SET n_distinct.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, jnelson+pgsql@jamponi.net
Subject: Re: select distinct uses index scan vs full table scan
In-Reply-To: <9562.1323806277@sss.pgh.pa.us>
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