Refactor attribute mappings used in logical tuple conversion

Tuple conversion support in tupconvert.c is able to convert rowtypes
between two relations, inner and outer, which are logically equivalent
but have a different ordering or even dropped columns (used mainly for
inheritance tree and partitions).  This makes use of attribute mappings,
which are simple arrays made of AttrNumber elements with a length
matching the number of attributes of the outer relation.  The length of
the attribute mapping has been treated as completely independent of the
mapping itself until now, making it easy to pass down an incorrect
mapping length.

This commit refactors the code related to attribute mappings and moves
it into an independent facility called attmap.c, extracted from
tupconvert.c.  This merges the attribute mapping with its length,
avoiding to try to guess what is the length of a mapping to use as this
is computed once, when the map is built.

This will avoid mistakes like what has been fixed in dc816e58, which has
used an incorrect mapping length by matching it with the number of
attributes of an inner relation (a child partition) instead of an outer
relation (a partitioned table).

Author: Michael Paquier
Reviewed-by: Amit Langote
Discussion: https://postgr.es/m/20191121042556.GD153437@paquier.xyz

1 files changed, 52 insertions, 0 deletions

diff --git a/src/include/access/attmap.h b/src/include/access/attmap.h
new file mode 100644
index 00000000000..57de8b65721
--- /dev/null
+++ b/src/include/access/attmap.h
@@ -0,0 +1,52 @@
+/*-------------------------------------------------------------------------
+ *
+ * attmap.h
+ *	  Definitions for PostgreSQL attribute mappings
+ *
+ *
+ * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/access/attmap.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef ATTMAP_H
+#define ATTMAP_H
+
+#include "access/attnum.h"
+#include "access/tupdesc.h"
+
+/*
+ * Attribute mapping structure
+ *
+ * This maps attribute numbers between a pair of relations, designated
+ * 'input' and 'output' (most typically inheritance parent and child
+ * relations), whose common columns may have different attribute numbers.
+ * Such difference may arise due to the columns being ordered differently
+ * in the two relations or the two relations having dropped columns at
+ * different positions.
+ *
+ * 'maplen' is set to the number of attributes of the 'output' relation,
+ * taking into account any of its dropped attributes, with the corresponding
+ * elements of the 'attnums' array set to 0.
+ */
+typedef struct AttrMap
+{
+	AttrNumber *attnums;
+	int			maplen;
+} AttrMap;
+
+extern AttrMap *make_attrmap(int maplen);
+extern void free_attrmap(AttrMap *map);
+
+/* Convertion routines to build mappings */
+extern AttrMap *build_attrmap_by_name(TupleDesc indesc,
+									  TupleDesc outdesc);
+extern AttrMap *build_attrmap_by_name_if_req(TupleDesc indesc,
+											 TupleDesc outdesc);
+extern AttrMap *build_attrmap_by_position(TupleDesc indesc,
+										  TupleDesc outdesc,
+										  const char *msg);
+
+#endif							/* ATTMAP_H */