Create the infrastructure for planner support functions.

Rename/repurpose pg_proc.protransform as "prosupport".  The idea is
still that it names an internal function that provides knowledge to
the planner about the behavior of the function it's attached to;
but redesign the API specification so that it's not limited to doing
just one thing, but can support an extensible set of requests.

The original purpose of simplifying a function call is handled by
the first request type to be invented, SupportRequestSimplify.
Adjust all the existing transform functions to handle this API,
and rename them fron "xxx_transform" to "xxx_support" to reflect
the potential generalization of what they do.  (Since we never
previously provided any way for extensions to add transform functions,
this change doesn't create an API break for them.)

Also add DDL and pg_dump support for attaching a support function to a
user-defined function.  Unfortunately, DDL access has to be restricted
to superusers, at least for now; but seeing that support functions
will pretty much have to be written in C, that limitation is just
theoretical.  (This support is untested in this patch, but a follow-on
patch will add cases that exercise it.)

Discussion: https://postgr.es/m/15193.1548028093@sss.pgh.pa.us

1 files changed, 29 insertions, 19 deletions

diff --git a/src/backend/utils/adt/varchar.c b/src/backend/utils/adt/varchar.c
index 5cf927e27f2..c866af022f2 100644
--- a/src/backend/utils/adt/varchar.c
+++ b/src/backend/utils/adt/varchar.c
@@ -21,6 +21,7 @@
 #include "catalog/pg_type.h"
 #include "libpq/pqformat.h"
 #include "nodes/nodeFuncs.h"
+#include "nodes/supportnodes.h"
 #include "utils/array.h"
 #include "utils/builtins.h"
 #include "utils/varlena.h"
@@ -547,32 +548,41 @@ varcharsend(PG_FUNCTION_ARGS)
 
 
 /*
- * varchar_transform()
- * Flatten calls to varchar's length coercion function that set the new maximum
- * length >= the previous maximum length.  We can ignore the isExplicit
- * argument, since that only affects truncation cases.
+ * varchar_support()
+ *
+ * Planner support function for the varchar() length coercion function.
+ *
+ * Currently, the only interesting thing we can do is flatten calls that set
+ * the new maximum length >= the previous maximum length.  We can ignore the
+ * isExplicit argument, since that only affects truncation cases.
  */
 Datum
-varchar_transform(PG_FUNCTION_ARGS)
+varchar_support(PG_FUNCTION_ARGS)
 {
-	FuncExpr   *expr = castNode(FuncExpr, PG_GETARG_POINTER(0));
+	Node	   *rawreq = (Node *) PG_GETARG_POINTER(0);
 	Node	   *ret = NULL;
-	Node	   *typmod;
 
-	Assert(list_length(expr->args) >= 2);
+	if (IsA(rawreq, SupportRequestSimplify))
+	{
+		SupportRequestSimplify *req = (SupportRequestSimplify *) rawreq;
+		FuncExpr   *expr = req->fcall;
+		Node	   *typmod;
 
-	typmod = (Node *) lsecond(expr->args);
+		Assert(list_length(expr->args) >= 2);
 
-	if (IsA(typmod, Const) &&!((Const *) typmod)->constisnull)
-	{
-		Node	   *source = (Node *) linitial(expr->args);
-		int32		old_typmod = exprTypmod(source);
-		int32		new_typmod = DatumGetInt32(((Const *) typmod)->constvalue);
-		int32		old_max = old_typmod - VARHDRSZ;
-		int32		new_max = new_typmod - VARHDRSZ;
-
-		if (new_typmod < 0 || (old_typmod >= 0 && old_max <= new_max))
-			ret = relabel_to_typmod(source, new_typmod);
+		typmod = (Node *) lsecond(expr->args);
+
+		if (IsA(typmod, Const) &&!((Const *) typmod)->constisnull)
+		{
+			Node	   *source = (Node *) linitial(expr->args);
+			int32		old_typmod = exprTypmod(source);
+			int32		new_typmod = DatumGetInt32(((Const *) typmod)->constvalue);
+			int32		old_max = old_typmod - VARHDRSZ;
+			int32		new_max = new_typmod - VARHDRSZ;
+
+			if (new_typmod < 0 || (old_typmod >= 0 && old_max <= new_max))
+				ret = relabel_to_typmod(source, new_typmod);
+		}
 	}
 
 	PG_RETURN_POINTER(ret);