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Commit99419d3

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Allow predicate_refuted_by() to deduce that NOT A refutes A.
We had originally made the stronger assumption that NOT A refutes any Bif B implies A, but this fails in three-valued logic, because we need toprove B is false not just that it's not true. However the logic doesgo through if B is equal to A.Recognizing this limited case is enough to handle examples that arise whenwe have simplified "bool_var = true" or "bool_var = false" to just "bool_var"or "NOT bool_var". If we had not done that simplification then thebtree-operator proof logic would have been able to prove that the expressionswere contradictory, but only for identical expressions being compared to theconstants; so handling identical A and B covers all the same cases.The motivation for doing this is to avoid unexpected asymmetrical behaviorwhen a partitioned table uses a boolean partitioning column, as in today'sgripe from Dominik Sander.Back-patch to 8.2, which is as far back as predicate_refuted_by attempts todo anything at all with NOTs.
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‎src/backend/optimizer/util/predtest.c

Lines changed: 38 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -9,7 +9,7 @@
99
*
1010
*
1111
* IDENTIFICATION
12-
* $PostgreSQL: pgsql/src/backend/optimizer/util/predtest.c,v 1.31 2010/02/14 18:42:15 rhaas Exp $
12+
* $PostgreSQL: pgsql/src/backend/optimizer/util/predtest.c,v 1.32 2010/02/25 20:59:53 tgl Exp $
1313
*
1414
*-------------------------------------------------------------------------
1515
*/
@@ -95,6 +95,7 @@ static void arrayexpr_cleanup_fn(PredIterInfo info);
9595
staticboolpredicate_implied_by_simple_clause(Expr*predicate,Node*clause);
9696
staticboolpredicate_refuted_by_simple_clause(Expr*predicate,Node*clause);
9797
staticNode*extract_not_arg(Node*clause);
98+
staticNode*extract_strong_not_arg(Node*clause);
9899
staticboollist_member_strip(List*list,Expr*datum);
99100
staticboolbtree_predicate_proof(Expr*predicate,Node*clause,
100101
boolrefute_it);
@@ -468,6 +469,8 @@ predicate_implied_by_recurse(Node *clause, Node *predicate)
468469
* Unfortunately we *cannot* use
469470
*NOT A R=> B if:B => A
470471
* because this type of reasoning fails to prove that B doesn't yield NULL.
472+
* We can however make the more limited deduction that
473+
*NOT A R=> A
471474
*
472475
* Other comments are as for predicate_implied_by_recurse().
473476
*----------
@@ -651,21 +654,18 @@ predicate_refuted_by_recurse(Node *clause, Node *predicate)
651654

652655
caseCLASS_ATOM:
653656

654-
#ifdefNOT_USED
655-
656657
/*
657-
* If A is a NOT-clause, A R=> B if B=> A's arg
658+
* If A is astrongNOT-clause, A R=> B if Bequals A's arg
658659
*
659-
* Unfortunately not: this would only prove that B is not-TRUE,
660-
* not that it's not NULL either. Keep this code as a comment
661-
* because it would be useful if we ever had a need for the weak
662-
* form of refutation.
660+
* We cannot make the stronger conclusion that B is refuted if
661+
* B implies A's arg; that would only prove that B is not-TRUE,
662+
* not that it's not NULL either. Hence use equal() rather than
663+
* predicate_implied_by_recurse(). We could do the latter if we
664+
* ever had a need for the weak form of refutation.
663665
*/
664-
not_arg=extract_not_arg(clause);
665-
if (not_arg&&
666-
predicate_implied_by_recurse(predicate,not_arg))
666+
not_arg=extract_strong_not_arg(clause);
667+
if (not_arg&&equal(predicate,not_arg))
667668
return true;
668-
#endif
669669

670670
switch (pclass)
671671
{
@@ -1178,6 +1178,32 @@ extract_not_arg(Node *clause)
11781178
returnNULL;
11791179
}
11801180

1181+
/*
1182+
* If clause asserts the falsity of a subclause, return that subclause;
1183+
* otherwise return NULL.
1184+
*/
1185+
staticNode*
1186+
extract_strong_not_arg(Node*clause)
1187+
{
1188+
if (clause==NULL)
1189+
returnNULL;
1190+
if (IsA(clause,BoolExpr))
1191+
{
1192+
BoolExpr*bexpr= (BoolExpr*)clause;
1193+
1194+
if (bexpr->boolop==NOT_EXPR)
1195+
return (Node*)linitial(bexpr->args);
1196+
}
1197+
elseif (IsA(clause,BooleanTest))
1198+
{
1199+
BooleanTest*btest= (BooleanTest*)clause;
1200+
1201+
if (btest->booltesttype==IS_FALSE)
1202+
return (Node*)btest->arg;
1203+
}
1204+
returnNULL;
1205+
}
1206+
11811207

11821208
/*
11831209
* Check whether an Expr is equal() to any member of a list, ignoring

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