Rearrange the implementation of index-only scans.

This commit changes index-only scans so that data is read directly from the
index tuple without first generating a faux heap tuple.  The only immediate
benefit is that indexes on system columns (such as OID) can be used in
index-only scans, but this is necessary infrastructure if we are ever to
support index-only scans on expression indexes.  The executor is now ready
for that, though the planner still needs substantial work to recognize
the possibility.

To do this, Vars in index-only plan nodes have to refer to index columns
not heap columns.  I introduced a new special varno, INDEX_VAR, to mark
such Vars to avoid confusion.  (In passing, this commit renames the two
existing special varnos to OUTER_VAR and INNER_VAR.)  This allows
ruleutils.c to handle them with logic similar to what we use for subplan
reference Vars.

Since index-only scans are now fundamentally different from regular
indexscans so far as their expression subtrees are concerned, I also chose
to change them to have their own plan node type (and hence, their own
executor source file).

1 files changed, 34 insertions, 2 deletions

diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 3885fa0099d..0a89f189d7c 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1226,7 +1226,6 @@ 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
@@ -1243,10 +1242,43 @@ typedef struct IndexScanState
 	ExprContext *iss_RuntimeContext;
 	Relation	iss_RelationDesc;
 	IndexScanDesc iss_ScanDesc;
-	Buffer		iss_VMBuffer;
 } IndexScanState;
 
 /* ----------------
+ *	 IndexOnlyScanState information
+ *
+ *		indexqual		   execution state for indexqual expressions
+ *		ScanKeys		   Skey structures for index quals
+ *		NumScanKeys		   number of ScanKeys
+ *		OrderByKeys		   Skey structures for index ordering operators
+ *		NumOrderByKeys	   number of OrderByKeys
+ *		RuntimeKeys		   info about Skeys that must be evaluated at runtime
+ *		NumRuntimeKeys	   number of RuntimeKeys
+ *		RuntimeKeysReady   true if runtime Skeys have been computed
+ *		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 IndexOnlyScanState
+{
+	ScanState	ss;				/* its first field is NodeTag */
+	List	   *indexqual;
+	ScanKey		ioss_ScanKeys;
+	int			ioss_NumScanKeys;
+	ScanKey		ioss_OrderByKeys;
+	int			ioss_NumOrderByKeys;
+	IndexRuntimeKeyInfo *ioss_RuntimeKeys;
+	int			ioss_NumRuntimeKeys;
+	bool		ioss_RuntimeKeysReady;
+	ExprContext *ioss_RuntimeContext;
+	Relation	ioss_RelationDesc;
+	IndexScanDesc ioss_ScanDesc;
+	Buffer		ioss_VMBuffer;
+} IndexOnlyScanState;
+
+/* ----------------
  *	 BitmapIndexScanState information
  *
  *		result			   bitmap to return output into, or NULL