From d7304244e26b92d533f7e44f0afbcf1ff2f46286 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 12 Nov 2010 15:14:51 -0500
Subject: [PATCH] Fix old oversight in const-simplification of COALESCE()
expressions.
Once we have found a non-null constant argument, there is no need to
examine additional arguments of the COALESCE. The previous coding got it
right only if the constant was in the first argument position; otherwise
it tried to simplify following arguments too, leading to unexpected
behavior like this:
regression=# select coalesce(f1, 42, 1/0) from int4_tbl;
ERROR: division by zero
It's a minor corner case, but a bug is a bug, so back-patch all the way.
---
src/backend/optimizer/util/clauses.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index e948bccc1a2..de2e66b0f16 100644
--- a/src/backend/optimizer/util/clauses.c
+++ b/src/backend/optimizer/util/clauses.c
@@ -2746,7 +2746,9 @@ eval_const_expressions_mutator(Node *node,
/*
* We can remove null constants from the list. For a non-null
* constant, if it has not been preceded by any other
- * non-null-constant expressions then that is the result.
+ * non-null-constant expressions then it is the result. Otherwise,
+ * it's the next argument, but we can drop following arguments
+ * since they will never be reached.
*/
if (IsA(e, Const))
{
@@ -2754,6 +2756,8 @@ eval_const_expressions_mutator(Node *node,
continue; /* drop null constant */
if (newargs == NIL)
return e; /* first expr */
+ newargs = lappend(newargs, e);
+ break;
}
newargs = lappend(newargs, e);
}
--
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