Add a reverse-translation column number array to struct AppendRelInfo.

This provides for cheaper mapping of child columns back to parent
columns.  The one existing use-case in examine_simple_variable()
would hardly justify this by itself; but an upcoming bug fix will
make use of this array in a mainstream code path, and it seems
likely that we'll find other uses for it as we continue to build
out the partitioning infrastructure.

Discussion: https://postgr.es/m/12424.1575168015@sss.pgh.pa.us

1 files changed, 16 insertions, 11 deletions

diff --git a/src/include/nodes/pathnodes.h b/src/include/nodes/pathnodes.h
index 23a06d718e3..6dab810d68f 100644
--- a/src/include/nodes/pathnodes.h
+++ b/src/include/nodes/pathnodes.h
@@ -2151,17 +2151,13 @@ struct SpecialJoinInfo
  * "append relation" (essentially, a list of child RTEs), we build an
  * AppendRelInfo for each child RTE.  The list of AppendRelInfos indicates
  * which child RTEs must be included when expanding the parent, and each node
- * carries information needed to translate Vars referencing the parent into
- * Vars referencing that child.
- *
- * These structs are kept in the PlannerInfo node's append_rel_list.
- * Note that we just throw all the structs into one list, and scan the
- * whole list when desiring to expand any one parent.  We could have used
- * a more complex data structure (eg, one list per parent), but this would
- * be harder to update during operations such as pulling up subqueries,
- * and not really any easier to scan.  Considering that typical queries
- * will not have many different append parents, it doesn't seem worthwhile
- * to complicate things.
+ * carries information needed to translate between columns of the parent and
+ * columns of the child.
+ *
+ * These structs are kept in the PlannerInfo node's append_rel_list, with
+ * append_rel_array[] providing a convenient lookup method for the struct
+ * associated with a particular child relid (there can be only one, though
+ * parent rels may have many entries in append_rel_list).
  *
  * Note: after completion of the planner prep phase, any given RTE is an
  * append parent having entries in append_rel_list if and only if its
@@ -2219,6 +2215,15 @@ typedef struct AppendRelInfo
 	List	   *translated_vars;	/* Expressions in the child's Vars */
 
 	/*
+	 * This array simplifies translations in the reverse direction, from
+	 * child's column numbers to parent's.  The entry at [ccolno - 1] is the
+	 * 1-based parent column number for child column ccolno, or zero if that
+	 * child column is dropped or doesn't exist in the parent.
+	 */
+	int			num_child_cols; /* length of array */
+	AttrNumber *parent_colnos;	/* array of parent attnos, or zeroes */
+
+	/*
 	 * We store the parent table's OID here for inheritance, or InvalidOid for
 	 * UNION ALL.  This is only needed to help in generating error messages if
 	 * an attempt is made to reference a dropped parent column.