Implement "join removal" for cases where the inner side of a left join

is unique and is not referenced above the join.  In this case the inner
side doesn't affect the query result and can be thrown away entirely.
Although perhaps nobody would ever write such a thing by hand, it's
a reasonably common case in machine-generated SQL.

The current implementation only recognizes the case where the inner side
is a simple relation with a unique index matching the query conditions.
This is enough for the use-cases that have been shown so far, but we
might want to try to handle other cases later.

Robert Haas, somewhat rewritten by Tom

1 files changed, 17 insertions, 1 deletions

diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index 4432252a72f..9b59d63b2b3 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/nodes/relation.h,v 1.174 2009/07/16 20:55:44 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/relation.h,v 1.175 2009/09/17 20:49:29 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -784,6 +784,22 @@ typedef struct UniquePath
 } UniquePath;
 
 /*
+ * NoOpPath represents exactly the same plan as its subpath.  This is used
+ * when we have determined that a join can be eliminated.  The difference
+ * between the NoOpPath and its subpath is just that the NoOpPath's parent
+ * is the whole join relation while the subpath is for one of the joined
+ * relations (and the other one isn't needed).
+ *
+ * Note: path.pathtype is always T_Join, but this won't actually give rise
+ * to a Join plan node.
+ */
+typedef struct NoOpPath
+{
+	Path		path;
+	Path	   *subpath;
+} NoOpPath;
+
+/*
  * All join-type paths share these fields.
  */