Support index-only scans using the visibility map to avoid heap fetches.

When a btree index contains all columns required by the query, and the
visibility map shows that all tuples on a target heap page are
visible-to-all, we don't need to fetch that heap page.  This patch depends
on the previous patches that made the visibility map reliable.

There's a fair amount left to do here, notably trying to figure out a less
chintzy way of estimating the cost of an index-only scan, but the core
functionality seems ready to commit.

Robert Haas and Ibrar Ahmed, with some previous work by Heikki Linnakangas.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/include/nodes/plannodes.h b/src/include/nodes/plannodes.h
index 535eca77a7e..60467f52769 100644
--- a/src/include/nodes/plannodes.h
+++ b/src/include/nodes/plannodes.h
@@ -300,6 +300,10 @@ typedef Scan SeqScan;
  * that the sort ordering is fully determinable from the top-level operators.
  * indexorderbyorig is unused at run time, but is needed for EXPLAIN.
  * (Note these fields are used for amcanorderbyop cases, not amcanorder cases.)
+ *
+ * indexorderdir specifies the scan ordering, for indexscans on amcanorder
+ * indexes (for other indexes it should be "don't care").  indexonly specifies
+ * an index-only scan, for indexscans on amcanreturn indexes.
  * ----------------
  */
 typedef struct IndexScan
@@ -311,6 +315,7 @@ typedef struct IndexScan
 	List	   *indexorderby;	/* list of index ORDER BY exprs */
 	List	   *indexorderbyorig;		/* the same in original form */
 	ScanDirection indexorderdir;	/* forward or backward or don't care */
+	bool		indexonly;		/* attempt to skip heap fetches? */
 } IndexScan;
 
 /* ----------------