Is exec_simple_check_node still doing anything?
I'm a little mystified by exec_simple_check_node(). The regression
tests seem not to exercise it. It can only be reached when
exec_simple_recheck_plan() finds no other reason to reject the plan,
and the only case it seems to reject is the one where there's a
set-returning function buried in there someplace. But then it seems
like hasTargetSRFs would have been true and we would have given up
before making a plan in the first place. Of course, that only
protects us when originally forming the plan; they don't account for
later changes -- and the code comments claim that an expression which
was originally simple can become non-simple:
* It is possible though unlikely for a simple expression to become non-simple
* (consider for example redefining a trivial view).
But I can't quite figure that one out. If we're selecting from a
trivial view, then the range table won't be empty and the expression
won't be simple in the first place. The check for a non-empty range
table didn't exist when this comment was originally added
(95f6d2d20921b7c2dbec29bf2706fd9448208aa6, 2007); it was added in a
subsequent redesign (e6faf910d75027bdce7cd0f2033db4e912592bcc; 2011).
Did that, possibly, remove the last way in which a simple expression
could be could become non-simple? If so, between that and the new
hasTargetSRFs test, it might now be impossible for
exec_simple_check_node() to fail.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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Robert Haas <robertmhaas@gmail.com> writes:
I'm a little mystified by exec_simple_check_node().
...
Did that, possibly, remove the last way in which a simple expression
could be could become non-simple? If so, between that and the new
hasTargetSRFs test, it might now be impossible for
exec_simple_check_node() to fail.
I think you might be right. The other way that I'm aware of that
could cause interesting things to happen is if someone redefines
a SQL function that had been inlined in the originally-compiled
version of the expression. However, it should be the case that
inline_function() will refuse to inline if the new definition
contains anything "scary", so that the expression as seen by
plpgsql is still simple; any non-simplicity will just be hidden
under a function call.
In fact, I suspect we could get rid of exec_simple_recheck_plan
altogether. It could use a bit more study, but the empty-rtable
check plus the other checks in exec_simple_check_plan (particularly,
hasAggs, hasWindowFuncs, hasTargetSRFs, hasSubLinks) seem like
they are enough to guarantee that what comes out of the planner
will be "simple".
If I recall things correctly, originally there were only the
post-planning simplicity checks that are now embodied in
exec_simple_recheck_plan/exec_simple_check_node. I think I added
on the pre-planning checks in exec_simple_check_plan in order to
try to save some planning cycles. Since the SRF checks were
clearly still necessary at the time, I didn't think hard about
whether any of the other post-planning checks could be got rid of.
regards, tom lane
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I wrote:
Robert Haas <robertmhaas@gmail.com> writes:
I'm a little mystified by exec_simple_check_node().
...
Did that, possibly, remove the last way in which a simple expression
could be could become non-simple? If so, between that and the new
hasTargetSRFs test, it might now be impossible for
exec_simple_check_node() to fail.
In fact, I suspect we could get rid of exec_simple_recheck_plan
altogether. It could use a bit more study, but the empty-rtable
check plus the other checks in exec_simple_check_plan (particularly,
hasAggs, hasWindowFuncs, hasTargetSRFs, hasSubLinks) seem like
they are enough to guarantee that what comes out of the planner
will be "simple".
I did some more studying and it definitely seems like
exec_simple_recheck_plan can never fail anymore. As an experimental
check, converting everything it was testing into Asserts still gets
through check-world. Accordingly, here's a proposed patch that gets
rid of exec_simple_check_node() and simplifies some of the rest of
the logic.
regards, tom lane