From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | David Rowley <dgrowleyml(at)gmail(dot)com> |
Cc: | PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: [PATCH] Negative Transition Aggregate Functions (WIP) |
Date: | 2013-12-16 12:18:49 |
Message-ID: | 11808.1387196329@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
David Rowley <dgrowleyml(at)gmail(dot)com> writes:
>> One thing that is currently on my mind is what to do when passing volatile
>> functions to the aggregate. Since the number of times we execute a volatile
>> function will much depend on the window frame options, I think we should
>> include some sort of warning in the documentation that "The number of times
>> that the expression is evaluated within the aggregate function when used in
>> the context of a WINDOW is undefined". The other option would be to disable
>> this optimisation if the aggregate expression contained a volatile
>> function, but doing this, to me seems a bit weird as is anyone actually
>> going to be depending on a volatile function being executed so many times?
Once again: this patch has no business changing any user-visible behavior.
That would include not changing the number of evaluations of volatile
functions. The planner is full of places where optimizations are disabled
for volatile subexpressions, and I don't see why this should be different.
regards, tom lane
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