From: Tom Lane <tgl@sss.pgh.pa.us>
To: Mark Rostron <mrostron@ql2.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: what does "initplan" operation in explain output mean?
Date: Sun, 01 Aug 2010 10:08:26 -0400
Message-ID: <4572.1280671706@sss.pgh.pa.us> (raw)
In-Reply-To: <FD020D3E50E7FA479567872E5F5F31E30459D622B0@ex01.corp.ql2.com>
References: <FD020D3E50E7FA479567872E5F5F31E30459D622B0@ex01.corp.ql2.com>
Mark Rostron <mrostron@ql2.com> writes:
> This message is a request for information about the "initplan" operation in explain plan.
An initplan is a sub-SELECT that only needs to be executed once because it
has no dependency on the immediately surrounding query level. The cases
you show here are from sub-SELECTs like this:
(select min(wu_id) from work_active limit 1)
which yields a value that's independent of anything in the outer query.
If there were an outer reference in there, you'd get a SubPlan instead,
because the subquery would need to be done over again for each row of
the outer query.
BTW, adding LIMIT 1 to an aggregate query is pretty pointless.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, mrostron@ql2.com
Subject: Re: what does "initplan" operation in explain output mean?
In-Reply-To: <4572.1280671706@sss.pgh.pa.us>
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