Allow extracting fields from a ROW() expression in more cases.

Teach get_expr_result_type() to manufacture a tuple descriptor directly
from a RowExpr node.  If the RowExpr has type RECORD, this is the only
way to get a tupdesc for its result, since even if the rowtype has been
blessed, we don't have its typmod available at this point.  (If the
RowExpr has some named composite type, we continue to let the existing
code handle it, since the RowExpr might well not have the correct column
names embedded in it.)

This fixes assorted corner cases illustrated by the added regression
tests.

This is a back-patch of the v13-era commit 8b7a0f1d1 into previous
branches.  At the time I'd judged it not important enough to back-patch,
but the upcoming fix for bug #18077 includes a test case that depends
on this working correctly; and 8b7a0f1d1 has now aged long enough to
have good confidence that it won't break anything.

Discussion: https://postgr.es/m/10872.1572202006@sss.pgh.pa.us
Discussion: https://postgr.es/m/3607145.1694803130@sss.pgh.pa.us

src/backend/utils/fmgr/funcapi.c

@@ -251,6 +251,38 @@ get_expr_result_type(Node *expr,
 										  NULL,
 										  resultTypeId,
 										  resultTupleDesc);
+	else if (expr && IsA(expr, RowExpr) &&
+			 ((RowExpr *) expr)->row_typeid == RECORDOID)
+	{
+		/* We can resolve the record type by generating the tupdesc directly */
+		RowExpr    *rexpr = (RowExpr *) expr;
+		TupleDesc	tupdesc;
+		AttrNumber	i = 1;
+		ListCell   *lcc,
+				   *lcn;
+
+		tupdesc = CreateTemplateTupleDesc(list_length(rexpr->args), false);
+		Assert(list_length(rexpr->args) == list_length(rexpr->colnames));
+		forboth(lcc, rexpr->args, lcn, rexpr->colnames)
+		{
+			Node	   *col = (Node *) lfirst(lcc);
+			char	   *colname = strVal(lfirst(lcn));
+
+			TupleDescInitEntry(tupdesc, i,
+							   colname,
+							   exprType(col),
+							   exprTypmod(col),
+							   0);
+			TupleDescInitEntryCollation(tupdesc, i,
+										exprCollation(col));
+			i++;
+		}
+		if (resultTypeId)
+			*resultTypeId = rexpr->row_typeid;
+		if (resultTupleDesc)
+			*resultTupleDesc = BlessTupleDesc(tupdesc);
+		return TYPEFUNC_COMPOSITE;
+	}
 	else
 	{
 		/* handle as a generic expression; no chance to resolve RECORD */

src/test/regress/expected/rowtypes.out

@@ -346,6 +346,45 @@ where i8 in (row(123,456)::int8_tbl, '(4567890123456789,123)');
  4567890123456789 | 123
 (2 rows)
 
+-- Check ability to select columns from an anonymous rowtype
+select (row(1, 2.0)).f1;
+ f1 
+----
+  1
+(1 row)
+
+select (row(1, 2.0)).f2;
+ f2  
+-----
+ 2.0
+(1 row)
+
+select (row(1, 2.0)).nosuch;  -- fail
+ERROR:  could not identify column "nosuch" in record data type
+LINE 1: select (row(1, 2.0)).nosuch;
+                ^
+select (row(1, 2.0)).*;
+ f1 | f2  
+----+-----
+  1 | 2.0
+(1 row)
+
+select (r).f1 from (select row(1, 2.0) as r) ss;
+ f1 
+----
+  1
+(1 row)
+
+select (r).f3 from (select row(1, 2.0) as r) ss;  -- fail
+ERROR:  could not identify column "f3" in record data type
+LINE 1: select (r).f3 from (select row(1, 2.0) as r) ss;
+                ^
+select (r).* from (select row(1, 2.0) as r) ss;
+ f1 | f2  
+----+-----
+  1 | 2.0
+(1 row)
+
 -- Check some corner cases involving empty rowtypes
 select ROW();
  row 

src/test/regress/sql/rowtypes.sql

@@ -149,6 +149,15 @@ where i8 in (row(123,456)::int8_tbl, '(4567890123456789,123)');
 select * from int8_tbl i8
 where i8 in (row(123,456)::int8_tbl, '(4567890123456789,123)');
 
+-- Check ability to select columns from an anonymous rowtype
+select (row(1, 2.0)).f1;
+select (row(1, 2.0)).f2;
+select (row(1, 2.0)).nosuch;  -- fail
+select (row(1, 2.0)).*;
+select (r).f1 from (select row(1, 2.0) as r) ss;
+select (r).f3 from (select row(1, 2.0) as r) ss;  -- fail
+select (r).* from (select row(1, 2.0) as r) ss;
+
 -- Check some corner cases involving empty rowtypes
 select ROW();
 select ROW() IS NULL;