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Fix constant-folding of ROW(...) IS [NOT] NULL with composite fields.

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The SQL standard appears to specify that IS [NOT] NULL's tests of field
nullness are non-recursive, ie, we shouldn't consider that a composite
field with value ROW(NULL,NULL) is null for this purpose.
ExecEvalNullTest got this right, but eval_const_expressions did not,
leading to weird inconsistencies depending on whether the expression
was such that the planner could apply constant folding.

Also, adjust the docs to mention that IS [NOT] DISTINCT FROM NULL can be
used as a substitute test if a simple null check is wanted for a rowtype
argument.  That motivated reordering things so that IS [NOT] DISTINCT FROM
is described before IS [NOT] NULL.  In HEAD, I went a bit further and added
a table showing all the comparison-related predicates.

Per bug #14235.  Back-patch to all supported branches, since it's certainly
undesirable that constant-folding should change the semantics.

Report and patch by Andrew Gierth; assorted wordsmithing and revised
regression test cases by me.

Report: <20160708024746.1410.57282@wrigleys.postgresql.org>
This commit is contained in:
Tom Lane
2016-07-26 15:25:02 -04:00
parent c1a9542578
commit 4452000f31
5 changed files with 250 additions and 55 deletions
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13
src/backend/optimizer/util/clauses.c
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@ -3273,7 +3273,7 @@ eval_const_expressions_mutator(Node *node,
arg = eval_const_expressions_mutator((Node *) ntest->arg,
context);
if (arg && IsA(arg, RowExpr))
if (ntest->argisrow && arg && IsA(arg, RowExpr))
{
/*
* We break ROW(...) IS [NOT] NULL into separate tests on
@ -3285,8 +3285,6 @@ eval_const_expressions_mutator(Node *node,
List *newargs = NIL;
ListCell *l;
Assert(ntest->argisrow);
foreach(l, rarg->args)
{
Node *relem = (Node *) lfirst(l);
@ -3305,10 +3303,17 @@ eval_const_expressions_mutator(Node *node,
return makeBoolConst(false, false);
continue;
}
/*
* Else, make a scalar (argisrow == false) NullTest
* for this field. Scalar semantics are required
* because IS [NOT] NULL doesn't recurse; see comments
* in ExecEvalNullTest().
*/
newntest = makeNode(NullTest);
newntest->arg = (Expr *) relem;
newntest->nulltesttype = ntest->nulltesttype;
newntest->argisrow = type_is_rowtype(exprType(relem));
newntest->argisrow = false;
newntest->location = ntest->location;
newargs = lappend(newargs, newntest);
}