Обсуждение: Approximate count(*)

Folks,

Please find enclosed a patch that shows how to get a quick
approximation of count(*) on a table.

Cheers,
D
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David Fetter david@fetter.org http://fetter.org/
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David Fetter <david@fetter.org> writes:
> Please find enclosed a patch that shows how to get a quick
> approximation of count(*) on a table.

I'm not sure we should be encouraging people to look at reltuples...
for one thing, it's deliberately a moving average under 8.0.

            regards, tom lane

On Thu, Mar 24, 2005 at 12:34:51PM -0500, Tom Lane wrote:
> David Fetter <david@fetter.org> writes:
> > Please find enclosed a patch that shows how to get a quick
> > approximation of count(*) on a table.
>
> I'm not sure we should be encouraging people to look at reltuples...
> for one thing, it's deliberately a moving average under 8.0.

Should there be more caveats?

Cheers,
D
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David Fetter david@fetter.org http://fetter.org/
phone: +1 510 893 6100   mobile: +1 415 235 3778

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David Fetter wrote:
> Please find enclosed a patch that shows how to get a quick
> approximation of count(*) on a table.

You should mention that ANALYZE will also populate reltuples.

To be real anal, you should say pg_catalog.pg_class too. :)

Tom Lane asked:
>> I'm not sure we should be encouraging people to look at reltuples...
>> for one thing, it's deliberately a moving average under 8.0.

> Should there be more caveats?

Well, it already says "approximate", the value-laden word is "good".
Perhaps if that went away...

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David,

If Jim and I finish our work for 8.1, then you'll be able to do:

SELECT approx_records FROM pg_sysviews.pg_tables WHERE schema_name = 'schema'
AND table_name = 'table'

But the same caveats will apply.

--
Josh Berkus
Aglio Database Solutions
San Francisco

Josh, David-

Do you think it's worth adding a function that would do the select for
you? IE: tuplecount('schema', 'tablename')?

On Thu, Mar 24, 2005 at 07:19:00PM -0800, Josh Berkus wrote:
> David,
>
> If Jim and I finish our work for 8.1, then you'll be able to do:
>
> SELECT approx_records FROM pg_sysviews.pg_tables WHERE schema_name = 'schema'
> AND table_name = 'table'
>
> But the same caveats will apply.
>
> --
> Josh Berkus
> Aglio Database Solutions
> San Francisco
>
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