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.

3 files changed, 13 insertions, 0 deletions

diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index c8a0b598645..3885fa0099d 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1226,6 +1226,7 @@ typedef struct
  *		RuntimeContext	   expr context for evaling runtime Skeys
  *		RelationDesc	   index relation descriptor
  *		ScanDesc		   index scan descriptor
+ *		VMBuffer		   buffer in use for visibility map testing, if any
  * ----------------
  */
 typedef struct IndexScanState
@@ -1242,6 +1243,7 @@ typedef struct IndexScanState
 	ExprContext *iss_RuntimeContext;
 	Relation	iss_RelationDesc;
 	IndexScanDesc iss_ScanDesc;
+	Buffer		iss_VMBuffer;
 } IndexScanState;
 
 /* ----------------
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;
 
 /* ----------------
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index ecbbc1cd39a..cf48ba433c8 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -482,6 +482,7 @@ typedef struct IndexOptInfo
 	bool		unique;			/* true if a unique index */
 	bool		hypothetical;	/* true if index doesn't really exist */
 	bool		amcanorderbyop; /* does AM support order by operator result? */
+	bool		amcanreturn;	/* does AM know how to return tuples? */
 	bool		amoptionalkey;	/* can query omit key for the first column? */
 	bool		amsearchnulls;	/* can AM search for NULL/NOT NULL entries? */
 	bool		amhasgettuple;	/* does AM have amgettuple interface? */
@@ -672,6 +673,10 @@ typedef struct Path
  * NoMovementScanDirection for an indexscan, but the planner wants to
  * distinguish ordered from unordered indexes for building pathkeys.)
  *
+ * 'indexonly' is TRUE for an index-only scan, that is, the index's access
+ * method has amcanreturn = TRUE and we only need columns available from the
+ * index.
+ *
  * 'indextotalcost' and 'indexselectivity' are saved in the IndexPath so that
  * we need not recompute them when considering using the same index in a
  * bitmap index/heap scan (see BitmapHeapPath).  The costs of the IndexPath
@@ -693,6 +698,7 @@ typedef struct IndexPath
 	List	   *indexorderbys;
 	bool		isjoininner;
 	ScanDirection indexscandir;
+	bool		indexonly;
 	Cost		indextotalcost;
 	Selectivity indexselectivity;
 	double		rows;			/* estimated number of result tuples */