Message-ID: <46a455f6-16f8-1c98-955f-140449004d72@postgrespro.ru>
Date: Thu, 15 Jun 2023 17:06:44 +0300
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Subject: Re: When IMMUTABLE is not.
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To: chap@anastigmatix.net, Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@lists.postgresql.org
References: <389c986d-fbb4-c644-9280-db7836af7ca9@postgrespro.ru>
 <2111925.1686835298@sss.pgh.pa.us>
 <69234e25e34e68425f49630f79ef9991@anastigmatix.net>
From: Yura Sokolov <y.sokolov@postgrespro.ru>
In-Reply-To: <69234e25e34e68425f49630f79ef9991@anastigmatix.net>
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15.06.2023 16:58, chap@anastigmatix.net пишет:
> On 2023-06-15 09:21, Tom Lane wrote:
>> Yura Sokolov <y.sokolov@postgrespro.ru> writes:
>>> not enough to be sure function doesn't manipulate data.
>>
>> Of course not.  It is the user's responsibility to mark functions
>> properly.
>
> And also, isn't it the case that IMMUTABLE should mark a function,
> not merely that "doesn't manipulate data", but whose return value
> doesn't depend in any way on data (outside its own arguments)?
>
> The practice among PLs of choosing an SPI readonly flag based on
> the IMMUTABLE/STABLE/VOLATILE declaration seems to be a sort of
> peculiar heuristic, not something inherent in what that declaration
> means to the optimizer. (And also influences what snapshot the
> function is looking at, and therefore what it can see, which has
> also struck me more as a tacked-on effect than something inherent
> in the declaration's meaning.)

Documentation disagrees:

https://www.postgresql.org/docs/current/sql-createfunction.html#:~:text=IMMUTABLE%0ASTABLE%0AVOLATILE

 > |IMMUTABLE|indicates that the function cannot modify the database and 
always returns the same result when given the same argument values

 > |STABLE|indicates that the function cannot modify the database, and 
that within a single table scan it will consistently return the same 
result for the same argument values, but that its result could change 
across SQL statements.

 > |VOLATILE|indicates that the function value can change even within a 
single table scan, so no optimizations can be made... But note that any 
function that has side-effects must be classified volatile, even if its 
result is quite predictable, to prevent calls from being optimized away