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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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15
src/backend/executor/execQual.c
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@ -3815,6 +3815,21 @@ ExecEvalNullTest(NullTestState *nstate,
if (ntest->argisrow && !(*isNull))
{
/*
* The SQL standard defines IS [NOT] NULL for a non-null rowtype
* argument as:
*
* "R IS NULL" is true if every field is the null value.
*
* "R IS NOT NULL" is true if no field is the null value.
*
* This definition is (apparently intentionally) not recursive; so our
* tests on the fields are primitive attisnull tests, not recursive
* checks to see if they are all-nulls or no-nulls rowtypes.
*
* The standard does not consider the possibility of zero-field rows,
* but here we consider them to vacuously satisfy both predicates.
*/
HeapTupleHeader tuple;
Oid tupType;
int32 tupTypmod;