Change postgres_fdw to show casts as casts, not underlying function calls.

On reflection this method seems to be exposing an unreasonable amount of
implementation detail.  It wouldn't matter when talking to a remote server
of the identical Postgres version, but it seems likely to make things worse
not better if the remote is a different version with different casting
infrastructure.  Instead adopt ruleutils.c's policy of regurgitating the
cast as it was originally specified; including not showing it at all, if
it was implicit to start with.  (We must do that because for some datatypes
explicit and implicit casts have different semantics.)

contrib/postgres_fdw/deparse.c

@@ -855,10 +855,6 @@ deparseArrayRef(StringInfo buf, ArrayRef *node, PlannerInfo *root)
 
 /*
  * Deparse given node which represents a function call into buf.
- *
- * Here not only explicit function calls and explicit casts but also implicit
- * casts are deparsed to avoid problems caused by different cast settings
- * between local and remote.
  */
 static void
 deparseFuncExpr(StringInfo buf, FuncExpr *node, PlannerInfo *root)
@@ -870,6 +866,37 @@ deparseFuncExpr(StringInfo buf, FuncExpr *node, PlannerInfo *root)
 	bool		first;
 	ListCell   *arg;
 
+	/*
+	 * If the function call came from an implicit coercion, then just show the
+	 * first argument.
+	 */
+	if (node->funcformat == COERCE_IMPLICIT_CAST)
+	{
+		deparseExpr(buf, (Expr *) linitial(node->args), root);
+		return;
+	}
+
+	/*
+	 * If the function call came from a cast, then show the first argument
+	 * plus an explicit cast operation.
+	 */
+	if (node->funcformat == COERCE_EXPLICIT_CAST)
+	{
+		Oid			rettype = node->funcresulttype;
+		int32		coercedTypmod;
+
+		/* Get the typmod if this is a length-coercion function */
+		(void) exprIsLengthCoercion((Node *) node, &coercedTypmod);
+
+		deparseExpr(buf, (Expr *) linitial(node->args), root);
+		appendStringInfo(buf, "::%s",
+						 format_type_with_typemod(rettype, coercedTypmod));
+		return;
+	}
+
+	/*
+	 * Normal function: display as proname(args).
+	 */
 	proctup = SearchSysCache1(PROCOID, ObjectIdGetDatum(node->funcid));
 	if (!HeapTupleIsValid(proctup))
 		elog(ERROR, "cache lookup failed for function %u", node->funcid);
@@ -1062,6 +1089,7 @@ static void
 deparseRelabelType(StringInfo buf, RelabelType *node, PlannerInfo *root)
 {
 	deparseExpr(buf, node->arg, root);
+	if (node->relabelformat != COERCE_IMPLICIT_CAST)
 		appendStringInfo(buf, "::%s",
 						 format_type_with_typemod(node->resulttype,
 												  node->resulttypmod));

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -189,10 +189,10 @@ SELECT * FROM ft1 WHERE false;
 -- with WHERE clause
 EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE t1.c1 = 101 AND t1.c6 = '1' AND t1.c7 >= '1';
                                                                    QUERY PLAN                                                                   
-------------------------------------------------------------------------------------------------------------------------------------------------------
+------------------------------------------------------------------------------------------------------------------------------------------------
  Foreign Scan on public.ft1 t1
    Output: c1, c2, c3, c4, c5, c6, c7, c8
-   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE ((c7 >= '1'::bpchar)) AND (("C 1" = 101)) AND ((c6::text = '1'::text))
+   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE ((c7 >= '1'::bpchar)) AND (("C 1" = 101)) AND ((c6 = '1'::text))
 (3 rows)
 
 SELECT * FROM ft1 t1 WHERE t1.c1 = 101 AND t1.c6 = '1' AND t1.c7 >= '1';
@@ -354,10 +354,10 @@ EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE c1 IS NOT NULL;    --
 
 EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE round(abs(c1), 0) = 1; -- FuncExpr
                                                      QUERY PLAN                                                      
---------------------------------------------------------------------------------------------------------------------------------
+---------------------------------------------------------------------------------------------------------------------
  Foreign Scan on public.ft1 t1
    Output: c1, c2, c3, c4, c5, c6, c7, c8
-   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE ((round("numeric"(abs("C 1")), 0) = 1::numeric))
+   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE ((round(abs("C 1"), 0) = 1::numeric))
 (3 rows)
 
 EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE c1 = -c1;          -- OpExpr(l)
@@ -370,10 +370,10 @@ EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE c1 = -c1;          --
 
 EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE 1 = c1!;           -- OpExpr(r)
                                                 QUERY PLAN                                                
-----------------------------------------------------------------------------------------------------------------
+----------------------------------------------------------------------------------------------------------
  Foreign Scan on public.ft1 t1
    Output: c1, c2, c3, c4, c5, c6, c7, c8
-   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE ((1::numeric = (int8("C 1") !)))
+   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE ((1::numeric = ("C 1" !)))
 (3 rows)
 
 EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE (c1 IS NOT NULL) IS DISTINCT FROM (c1 IS NOT NULL); -- DistinctExpr
@@ -402,10 +402,10 @@ EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE c1 = (ARRAY[c1,c2,3])[
 
 EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE c6 = E'foo''s\\bar';  -- check special chars
                                                  QUERY PLAN                                                  
--------------------------------------------------------------------------------------------------------------------
+-------------------------------------------------------------------------------------------------------------
  Foreign Scan on public.ft1 t1
    Output: c1, c2, c3, c4, c5, c6, c7, c8
-   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE ((c6::text = E'foo''s\\bar'::text))
+   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE ((c6 = E'foo''s\\bar'::text))
 (3 rows)
 
 EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE c8 = 'foo';  -- can't be sent to remote