Prevent invoking I/O conversion casts via functional/attribute notation.

PG 8.4 added a built-in feature for casting pretty much any data type to
string types (text, varchar, etc).  We allowed this to work in any of the
historically-allowed syntaxes: CAST(x AS text), x::text, text(x), or
x.text.  However, multiple complaints have shown that it's too easy to
invoke such casts unintentionally in the latter two styles, particularly
field selection.  To cure the problem with the narrowest possible change
of behavior, disallow use of I/O conversion casts from composite types to
string types via functional/attribute syntax.  The new functionality is
still available via cast syntax.

In passing, document the equivalence of functional and attribute syntax
in a more visible place.

1 files changed, 24 insertions, 4 deletions

diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index b50bce44872..9bb100e0c1e 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -985,8 +985,13 @@ func_get_detail(List *funcname,
 		 * can't write "foo[] (something)" as a function call.  In theory
 		 * someone might want to invoke it as "_foo (something)" but we have
 		 * never supported that historically, so we can insist that people
-		 * write it as a normal cast instead.  Lack of historical support is
-		 * also the reason for not considering composite-type casts here.
+		 * write it as a normal cast instead.
+		 *
+		 * We also reject the specific case of COERCEVIAIO for a composite
+		 * source type and a string-category target type.  This is a case that
+		 * find_coercion_pathway() allows by default, but experience has shown
+		 * that it's too commonly invoked by mistake.  So, again, insist that
+		 * people use cast syntax if they want to do that.
 		 *
 		 * NB: it's important that this code does not exceed what coerce_type
 		 * can do, because the caller will try to apply coerce_type if we
@@ -1017,8 +1022,23 @@ func_get_detail(List *funcname,
 					cpathtype = find_coercion_pathway(targetType, sourceType,
 													  COERCION_EXPLICIT,
 													  &cfuncid);
-					iscoercion = (cpathtype == COERCION_PATH_RELABELTYPE ||
-								  cpathtype == COERCION_PATH_COERCEVIAIO);
+					switch (cpathtype)
+					{
+						case COERCION_PATH_RELABELTYPE:
+							iscoercion = true;
+							break;
+						case COERCION_PATH_COERCEVIAIO:
+							if ((sourceType == RECORDOID ||
+								 ISCOMPLEX(sourceType)) &&
+								TypeCategory(targetType) == TYPCATEGORY_STRING)
+								iscoercion = false;
+							else
+								iscoercion = true;
+							break;
+						default:
+							iscoercion = false;
+							break;
+					}
 				}
 
 				if (iscoercion)