I've had some time to look at this issue again. There are several
cases in plancat.c where specific expressions need to be run through
eval_const_expressions because the planner will be comparing them to
similarly-processed qual clauses. This includes constraint
expressions, statistics expressions, index expressions and index
predicates -- as highlighted by this bug report.
For a constraint expression, we run it through eval_const_expressions
before fixing its Vars to have the correct varno. This is problematic
because eval_const_expressions reduces NullTest quals based on varno,
so we must ensure the Vars have the correct varnos before calling it.
I think we can fix it by reordering the calls to eval_const_expressions
and ChangeVarNodes in get_relation_constraints().
For statistics expressions, we have the same issue as with constraint
expressions, plus we don't pass a valid root to eval_const_expressions.
I think we can fix this by adding "root" as a new parameter to
get_relation_statistics() and passing it to eval_const_expressions.
For index expressions and index predicates, however, the situation is
more complex, because we cache the constant-simplified expressions in
the relcache entry. This means we can't fix the Vars before applying
eval_const_expressions to them. One possible solution is to run
eval_const_expressions twice: first with "root" set to NULL, before
fixing the Vars, and then again with a valid "root" after the Vars
have been fixed. Please see the attached draft patch.
(This is basically what David's proposed patch does. However, I don't
think we should limit ourselves to the case where info->indpred
contains only a single reducible NullTest qual, since there may be
more than one.)
However, running eval_const_expressions twice for index expressions and
index predicates looks ugly, and one could argue that it might have a
performance impact.
BTW, all of these solutions are only possible after e2debb643, which
is only in master. In v17 and v18, we don't have a way to reduce
NullTest quals for these expressions, since that reduction doesn't
occur during const-simplification. I don't know what we should do for
v17 and v18.
Thoughts?
Thanks
Richard
