Fix misprocessing of equivalence classes involving record_eq().

canonicalize_ec_expression() is supposed to agree with coerce_type() as to
whether a RelabelType should be inserted to make a subexpression be valid
input for the operators of a given opclass.  However, it did the wrong
thing with named-composite-type inputs to record_eq(): it put in a
RelabelType to RECORDOID, which the parser doesn't.  In some cases this was
harmless because all code paths involving a particular equivalence class
did the same thing, but in other cases this would result in failing to
recognize a composite-type expression as being a member of an equivalence
class that it actually is a member of.  The most obvious bad effect was to
fail to recognize that an index on a composite column could provide the
sort order needed for a mergejoin on that column, as reported by Teodor
Sigaev.  I think there might be other, subtler, cases that result in
misoptimization.  It also seems possible that an unwanted RelabelType
would sometimes get into an emitted plan --- but because record_eq and
friends don't examine the declared type of their input expressions, that
would not create any visible problems.

To fix, just treat RECORDOID as if it were a polymorphic type, which in
some sense it is.  We might want to consider formalizing that a bit more
someday, but for the moment this seems to be the only place where an
IsPolymorphicType() test ought to include RECORDOID as well.

This has been broken for a long time, so back-patch to all supported
branches.

Discussion: https://postgr.es/m/a6b22369-e3bf-4d49-f59d-0c41d3551e81@sigaev.ru

src/backend/optimizer/path/equivclass.c

@@ -497,8 +497,9 @@ canonicalize_ec_expression(Expr *expr, Oid req_type, Oid req_collation)
 
 	/*
 	 * For a polymorphic-input-type opclass, just keep the same exposed type.
+	 * RECORD opclasses work like polymorphic-type ones for this purpose.
 	 */
-	if (IsPolymorphicType(req_type))
+	if (IsPolymorphicType(req_type) || req_type == RECORDOID)
 		req_type = expr_type;
 
 	/*

src/test/regress/expected/join.out

@@ -2672,6 +2672,36 @@ select * from a left join b on i = x and i = y and x = i;
 ---+---+---
 (0 rows)
 
+rollback;
+--
+-- test handling of merge clauses using record_ops
+--
+begin;
+create type mycomptype as (id int, v bigint);
+create temp table tidv (idv mycomptype);
+create index on tidv (idv);
+explain (costs off)
+select a.idv, b.idv from tidv a, tidv b where a.idv = b.idv;
+                        QUERY PLAN                        
+----------------------------------------------------------
+ Merge Join
+   Merge Cond: (a.idv = b.idv)
+   ->  Index Only Scan using tidv_idv_idx on tidv a
+   ->  Materialize
+         ->  Index Only Scan using tidv_idv_idx on tidv b
+(5 rows)
+
+set enable_mergejoin = 0;
+explain (costs off)
+select a.idv, b.idv from tidv a, tidv b where a.idv = b.idv;
+                     QUERY PLAN                     
+----------------------------------------------------
+ Nested Loop
+   ->  Seq Scan on tidv a
+   ->  Index Only Scan using tidv_idv_idx on tidv b
+         Index Cond: (idv = a.idv)
+(4 rows)
+
 rollback;
 --
 -- test NULL behavior of whole-row Vars, per bug #5025

src/test/regress/sql/join.sql

@@ -673,6 +673,26 @@ select * from a left join b on i = x and i = y and x = i;
 
 rollback;
 
+--
+-- test handling of merge clauses using record_ops
+--
+begin;
+
+create type mycomptype as (id int, v bigint);
+
+create temp table tidv (idv mycomptype);
+create index on tidv (idv);
+
+explain (costs off)
+select a.idv, b.idv from tidv a, tidv b where a.idv = b.idv;
+
+set enable_mergejoin = 0;
+
+explain (costs off)
+select a.idv, b.idv from tidv a, tidv b where a.idv = b.idv;
+
+rollback;
+
 --
 -- test NULL behavior of whole-row Vars, per bug #5025
 --