Improve plpgsql's ability to cope with rowtypes containing dropped columns,

by supporting conversions in places that used to demand exact rowtype match.

Since this issue is certain to come up elsewhere (in fact, already has,
in ExecEvalConvertRowtype), factor out the support code into new core
functions for tuple conversion.  I chose to put these in a new source
file since heaptuple.c is already overly long.

Heavily revised version of a patch by Pavel Stehule.

src/include/access/tupconvert.h

@@ -0,0 +1,44 @@
+/*-------------------------------------------------------------------------
+ *
+ * tupconvert.h
+ *	  Tuple conversion support.
+ *
+ *
+ * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * $PostgreSQL: pgsql/src/include/access/tupconvert.h,v 1.1 2009/08/06 20:44:31 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef TUPCONVERT_H
+#define TUPCONVERT_H
+
+#include "access/htup.h"
+
+
+typedef struct TupleConversionMap
+{
+	TupleDesc	indesc;			/* tupdesc for source rowtype */
+	TupleDesc	outdesc;		/* tupdesc for result rowtype */
+	AttrNumber *attrMap;		/* indexes of input fields, or 0 for null */
+	Datum	   *invalues;		/* workspace for deconstructing source */
+	bool	   *inisnull;
+	Datum	   *outvalues;		/* workspace for constructing result */
+	bool	   *outisnull;
+} TupleConversionMap;
+
+
+extern TupleConversionMap *convert_tuples_by_position(TupleDesc indesc,
+													  TupleDesc outdesc,
+													  const char *msg);
+
+extern TupleConversionMap *convert_tuples_by_name(TupleDesc indesc,
+												  TupleDesc outdesc,
+												  const char *msg);
+
+extern HeapTuple do_convert_tuple(HeapTuple tuple, TupleConversionMap *map);
+
+extern void free_conversion_map(TupleConversionMap *map);
+
+#endif   /* TUPCONVERT_H */