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Commitfffb581

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Adjust constant-folding of CASE expressions so that the simple comparison
form of CASE (eg, CASE 0 WHEN 1 THEN ...) can be constant-folded as itwas in 7.4. Also, avoid constant-folding result expressions that arecertainly unreachable --- the former coding was a bit cavalier about thisand could generate unexpected results for all-constant CASE expressions.Add regression test cases. Per report from Vlad Marchenko.
1 parentb3a7e98 commitfffb581

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3 files changed

+107
-40
lines changed

3 files changed

+107
-40
lines changed

‎src/backend/optimizer/util/clauses.c

Lines changed: 82 additions & 40 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/optimizer/util/clauses.c,v 1.187 2005/01/28 19:34:07 tgl Exp $
11+
* $PostgreSQL: pgsql/src/backend/optimizer/util/clauses.c,v 1.188 2005/02/02 21:49:07 tgl Exp $
1212
*
1313
* HISTORY
1414
* AUTHORDATEMAJOR EVENT
@@ -49,6 +49,7 @@
4949
typedefstruct
5050
{
5151
List*active_fns;
52+
Node*case_val;
5253
boolestimate;
5354
}eval_const_expressions_context;
5455

@@ -1195,6 +1196,7 @@ eval_const_expressions(Node *node)
11951196
eval_const_expressions_contextcontext;
11961197

11971198
context.active_fns=NIL;/* nothing being recursively simplified */
1199+
context.case_val=NULL;/* no CASE being examined */
11981200
context.estimate= false;/* safe transformations only */
11991201
returneval_const_expressions_mutator(node,&context);
12001202
}
@@ -1219,6 +1221,7 @@ estimate_expression_value(Node *node)
12191221
eval_const_expressions_contextcontext;
12201222

12211223
context.active_fns=NIL;/* nothing being recursively simplified */
1224+
context.case_val=NULL;/* no CASE being examined */
12221225
context.estimate= true;/* unsafe transformations OK */
12231226
returneval_const_expressions_mutator(node,&context);
12241227
}
@@ -1592,71 +1595,98 @@ eval_const_expressions_mutator(Node *node,
15921595
* If there are no non-FALSE alternatives, we simplify the entire
15931596
* CASE to the default result (ELSE result).
15941597
*
1595-
* If we have a simple-form CASE with constant test expression and
1596-
* one or more constant comparison expressions, we could run the
1597-
* implied comparisons and potentially reduce those arms to constants.
1598-
* This is not yet implemented, however. At present, the
1599-
* CaseTestExpr placeholder will always act as a non-constant node
1600-
* and prevent the comparison boolean expressions from being reduced
1601-
* to Const nodes.
1598+
* If we have a simple-form CASE with constant test expression,
1599+
* we substitute the constant value for contained CaseTestExpr
1600+
* placeholder nodes, so that we have the opportunity to reduce
1601+
* constant test conditions. For example this allows
1602+
*CASE 0 WHEN 0 THEN 1 ELSE 1/0 END
1603+
* to reduce to 1 rather than drawing a divide-by-0 error.
16021604
*----------
16031605
*/
16041606
CaseExpr*caseexpr= (CaseExpr*)node;
16051607
CaseExpr*newcase;
1608+
Node*save_case_val;
16061609
Node*newarg;
16071610
List*newargs;
1608-
Node*defresult;
1609-
Const*const_input;
1611+
boolconst_true_cond;
1612+
Node*defresult=NULL;
16101613
ListCell*arg;
16111614

16121615
/* Simplify the test expression, if any */
16131616
newarg=eval_const_expressions_mutator((Node*)caseexpr->arg,
16141617
context);
16151618

1619+
/* Set up for contained CaseTestExpr nodes */
1620+
save_case_val=context->case_val;
1621+
if (newarg&&IsA(newarg,Const))
1622+
context->case_val=newarg;
1623+
else
1624+
context->case_val=NULL;
1625+
16161626
/* Simplify the WHEN clauses */
16171627
newargs=NIL;
1628+
const_true_cond= false;
16181629
foreach(arg,caseexpr->args)
16191630
{
1620-
/* Simplify this alternative's condition and result */
1621-
CaseWhen*casewhen= (CaseWhen*)
1622-
expression_tree_mutator((Node*)lfirst(arg),
1623-
eval_const_expressions_mutator,
1624-
(void*)context);
1625-
1626-
Assert(IsA(casewhen,CaseWhen));
1627-
if (casewhen->expr==NULL||
1628-
!IsA(casewhen->expr,Const))
1629-
{
1630-
newargs=lappend(newargs,casewhen);
1631-
continue;
1632-
}
1633-
const_input= (Const*)casewhen->expr;
1634-
if (const_input->constisnull||
1635-
!DatumGetBool(const_input->constvalue))
1636-
continue;/* drop alternative with FALSE condition */
1631+
CaseWhen*oldcasewhen= (CaseWhen*)lfirst(arg);
1632+
Node*casecond;
1633+
Node*caseresult;
1634+
1635+
Assert(IsA(oldcasewhen,CaseWhen));
1636+
1637+
/* Simplify this alternative's test condition */
1638+
casecond=
1639+
eval_const_expressions_mutator((Node*)oldcasewhen->expr,
1640+
context);
16371641

16381642
/*
1639-
*Found a TRUE condition.If it's the first (un-dropped)
1640-
*alternative, the CASE reduces to just this alternative.
1643+
*If the test condition is constant FALSE (or NULL), then drop
1644+
*this WHEN clause completely, without processing the result.
16411645
*/
1642-
if (newargs==NIL)
1643-
return (Node*)casewhen->result;
1646+
if (casecond&&IsA(casecond,Const))
1647+
{
1648+
Const*const_input= (Const*)casecond;
1649+
1650+
if (const_input->constisnull||
1651+
!DatumGetBool(const_input->constvalue))
1652+
continue;/* drop alternative with FALSE condition */
1653+
/* Else it's constant TRUE */
1654+
const_true_cond= true;
1655+
}
1656+
1657+
/* Simplify this alternative's result value */
1658+
caseresult=
1659+
eval_const_expressions_mutator((Node*)oldcasewhen->result,
1660+
context);
16441661

1662+
/* If non-constant test condition, emit a new WHEN node */
1663+
if (!const_true_cond)
1664+
{
1665+
CaseWhen*newcasewhen=makeNode(CaseWhen);
1666+
1667+
newcasewhen->expr= (Expr*)casecond;
1668+
newcasewhen->result= (Expr*)caseresult;
1669+
newargs=lappend(newargs,newcasewhen);
1670+
continue;
1671+
}
1672+
16451673
/*
1646-
* Otherwise, add it to the list, and drop all the rest.
1674+
* Found a TRUE condition, so none of the remaining alternatives
1675+
* can be reached. We treat the result as the default result.
16471676
*/
1648-
newargs=lappend(newargs,casewhen);
1677+
defresult=caseresult;
16491678
break;
16501679
}
16511680

1652-
/* Simplify the default result */
1653-
defresult=eval_const_expressions_mutator((Node*)caseexpr->defresult,
1654-
context);
1681+
/* Simplify the default result, unless we replaced it above */
1682+
if (!const_true_cond)
1683+
defresult=
1684+
eval_const_expressions_mutator((Node*)caseexpr->defresult,
1685+
context);
16551686

1656-
/*
1657-
* If no non-FALSE alternatives, CASE reduces to the default
1658-
* result
1659-
*/
1687+
context->case_val=save_case_val;
1688+
1689+
/* If no non-FALSE alternatives, CASE reduces to the default result */
16601690
if (newargs==NIL)
16611691
returndefresult;
16621692
/* Otherwise we need a new CASE node */
@@ -1667,6 +1697,18 @@ eval_const_expressions_mutator(Node *node,
16671697
newcase->defresult= (Expr*)defresult;
16681698
return (Node*)newcase;
16691699
}
1700+
if (IsA(node,CaseTestExpr))
1701+
{
1702+
/*
1703+
* If we know a constant test value for the current CASE
1704+
* construct, substitute it for the placeholder. Else just
1705+
* return the placeholder as-is.
1706+
*/
1707+
if (context->case_val)
1708+
returncopyObject(context->case_val);
1709+
else
1710+
returncopyObject(node);
1711+
}
16701712
if (IsA(node,ArrayExpr))
16711713
{
16721714
ArrayExpr*arrayexpr= (ArrayExpr*)node;

‎src/test/regress/expected/case.out

Lines changed: 17 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -72,6 +72,23 @@ SELECT '6' AS "One",
7272
6 | 6
7373
(1 row)
7474

75+
-- Constant-expression folding shouldn't evaluate unreachable subexpressions
76+
SELECT CASE WHEN 1=0 THEN 1/0 WHEN 1=1 THEN 1 ELSE 2/0 END;
77+
case
78+
------
79+
1
80+
(1 row)
81+
82+
SELECT CASE 1 WHEN 0 THEN 1/0 WHEN 1 THEN 1 ELSE 2/0 END;
83+
case
84+
------
85+
1
86+
(1 row)
87+
88+
-- However we do not currently suppress folding of potentially
89+
-- reachable subexpressions
90+
SELECT CASE WHEN i > 100 THEN 1/0 ELSE 0 END FROM case_tbl;
91+
ERROR: division by zero
7592
-- Test for cases involving untyped literals in test expression
7693
SELECT CASE 'a' WHEN 'a' THEN 1 ELSE 2 END;
7794
case

‎src/test/regress/sql/case.sql

Lines changed: 8 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -58,6 +58,14 @@ SELECT '6' AS "One",
5858
ELSE7
5959
ENDAS"Two WHEN with default";
6060

61+
-- Constant-expression folding shouldn't evaluate unreachable subexpressions
62+
SELECT CASE WHEN1=0 THEN1/0 WHEN1=1 THEN1 ELSE2/0 END;
63+
SELECT CASE1 WHEN0 THEN1/0 WHEN1 THEN1 ELSE2/0 END;
64+
65+
-- However we do not currently suppress folding of potentially
66+
-- reachable subexpressions
67+
SELECT CASE WHEN i>100 THEN1/0 ELSE0 ENDFROM case_tbl;
68+
6169
-- Test for cases involving untyped literals in test expression
6270
SELECT CASE'a' WHEN'a' THEN1 ELSE2 END;
6371

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