diff options
Diffstat (limited to 'src/backend/optimizer/util/clauses.c')
| -rw-r--r-- | src/backend/optimizer/util/clauses.c | 16 |
1 files changed, 15 insertions, 1 deletions
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c index fa84164b068..ccf913b35cf 100644 --- a/src/backend/optimizer/util/clauses.c +++ b/src/backend/optimizer/util/clauses.c @@ -2583,7 +2583,18 @@ eval_const_expressions_mutator(Node *node, * placeholder nodes, so that we have the opportunity to reduce * constant test conditions. For example this allows * CASE 0 WHEN 0 THEN 1 ELSE 1/0 END - * to reduce to 1 rather than drawing a divide-by-0 error. + * to reduce to 1 rather than drawing a divide-by-0 error. Note + * that when the test expression is constant, we don't have to + * include it in the resulting CASE; for example + * CASE 0 WHEN x THEN y ELSE z END + * is transformed by the parser to + * CASE 0 WHEN CaseTestExpr = x THEN y ELSE z END + * which we can simplify to + * CASE WHEN 0 = x THEN y ELSE z END + * It is not necessary for the executor to evaluate the "arg" + * expression when executing the CASE, since any contained + * CaseTestExprs that might have referred to it will have been + * replaced by the constant. *---------- */ CaseExpr *caseexpr = (CaseExpr *) node; @@ -2602,7 +2613,10 @@ eval_const_expressions_mutator(Node *node, /* Set up for contained CaseTestExpr nodes */ save_case_val = context->case_val; if (newarg && IsA(newarg, Const)) + { context->case_val = newarg; + newarg = NULL; /* not needed anymore, see comment above */ + } else context->case_val = NULL; |