Message-ID: <1521954043.2350.15.camel@cybertec.at>
Subject: Re: functions: VOLATILE performs better than STABLE
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Peter <pmc@citylink.dinoex.sub.org>,
	pgsql-performance@lists.postgresql.org
Date: Sun, 25 Mar 2018 07:00:43 +0200
In-Reply-To: <20180324012746.GA14631@gate.oper.dinoex.org>
References: <20180324012746.GA14631@gate.oper.dinoex.org>
Organization: Cybertec
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Peter wrote:
> Given an arbitrary function fn(x) returning numeric.
> 
> Question: how often is the function executed?
> [...]
> C.
> select v.v,v.v from (select fn('const') as v) as v;
> 
> Answer:
> Once if declared VOLATILE.
> Twice if declared STABLE.
> 
> Now this IS a surprize. It is clear that the system is not allowed to
> execute the function twice when declared VOLATILE. It IS ALLOWED to
> execute it twice when STABLE - but to what point, except prolonging
> execution time?
> 
> Over all, VOLATILE performs better than STABLE.

The reason is that the subquery with the VOLATILE function can be
flattened; see the EXPLAIN (VERBOSE) output.

There is not guarantee that less volatility means better performance.

Yours,
Laurenz Albe
-- 
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