"David G. Johnston" <david.g.johnston@gmail.com> writes:
>> The SQL standard (Section 7.4 general rule 1) mandates that WHERE should be
>> applied to the result of FROM so in the case above pushing filters below
>> the join seems to violate the standard.
> The failure to document such a deviation from the standard can be
> considered a bug but not the deviation itself. That is intentional. In
> terms of trade-offs the current behavior seems reasonable.
Enforcing such a restriction would be absolutely disastrous for the
performance of a large number of real-world queries. I doubt that
any other RDBMS takes that spec wording literally either.
I'd suggest something like
CASE WHEN e.empno = 0 THEN NULL ELSE 10 / e.empno END
if you need to program around the lack of guarantees about the
order of evaluation. A WITH clause can also be used as an
optimization fence when you need one.
BTW, this *is* documented, see for example
https://www.postgresql.org/docs/current/sql-expressions.html#SYNTAX-EXPRESS-EVAL
regards, tom lane
