From: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
---|---|
To: | Michael Paquier <michael(dot)paquier(at)gmail(dot)com> |
Cc: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Petr Jelinek <petr(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Jaime Casanova <jaime(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Tomas Vondra <tv(at)fuzzy(dot)cz> |
Subject: | Re: TABLESAMPLE patch |
Date: | 2015-04-09 09:37:19 |
Message-ID: | CA+U5nMJCTrmJ+pJ=dS3RaBfKPzu8dGzZfbKeBWxs9h9N2ffTjQ@mail.gmail.com |
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Lists: | pgsql-hackers |
On 9 April 2015 at 04:52, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> TABLESAMPLE BERNOULLI could work in this case, or any other non-block
> based sampling mechanism. Whether it does work yet is another matter.
>
> This query should be part of the test suite and should generate a
> useful message or work correctly.
The SQL Standard does allow the WITH query given. It makes no mention
of the obvious point that SYSTEM-defined mechanisms might not work,
but that is for the implementation to define, AFAICS.
The SQL Standard goes on to talk about "possibly non-deterministic"
issues. Which in Postgres relates to the point that the results of a
SampleScan will never be IMMUTABLE. That raises the possibility of
planner issues. We must, for example, never do inner join removal on a
sampled relation - we don't do that yet, but something to watch for.
On balance, in this release, I would be happier to exclude sampled
results from queries, and only allow sampling against base tables.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, RemoteDBA, Training & Services
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