Re: Making CASE error handling less surprising

From: ilmari(at)ilmari(dot)org (Dagfinn Ilmari Mannsåker )
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: Making CASE error handling less surprising
Date: 2020-07-23 17:50:32
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Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> Every so often we get a complaint like [1] about how a CASE should have
> prevented a run-time error and didn't, because constant-folding tried
> to evaluate a subexpression that would not have been entered at run-time.
> It struck me that it would not be hard to improve this situation a great
> deal. If, within a CASE subexpression that isn't certain to be executed
> at runtime, we refuse to pre-evaluate *any* function (essentially, treat
> them all as volatile), then we should largely get the semantics that
> users expect. There's some potential for query slowdown if a CASE
> contains a constant subexpression that we formerly reduced at plan time
> and now do not, but that doesn't seem to me to be a very big deal.
> Thoughts?

Would it be feasible to set up an exception handler when constant-
folding cases that might not be reached, and leave the expression
unfolded only if an error was thrown, or does that have too much
overhead to be worthwhile?

- ilmari
"A disappointingly low fraction of the human race is,
at any given time, on fire." - Stig Sandbeck Mathisen

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