Add an EXPLAIN (BUFFERS) option to show buffer-usage statistics.

This patch also removes buffer-usage statistics from the track_counts
output, since this (or the global server statistics) is deemed to be a better
interface to this information.

Itagaki Takahiro, reviewed by Euler Taveira de Oliveira.

src/include/executor/execdesc.h

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/executor/execdesc.h,v 1.40 2009/01/02 20:42:00 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/executor/execdesc.h,v 1.41 2009/12/15 04:57:48 rhaas Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -42,7 +42,7 @@ typedef struct QueryDesc
 	Snapshot	crosscheck_snapshot;	/* crosscheck for RI update/delete */
 	DestReceiver *dest;			/* the destination for tuple output */
 	ParamListInfo params;		/* param values being passed in */
-	bool		doInstrument;	/* TRUE requests runtime instrumentation */
+	int			instrument_options;		/* OR of InstrumentOption flags */
 
 	/* These fields are set by ExecutorStart */
 	TupleDesc	tupDesc;		/* descriptor for result tuples */
@@ -60,7 +60,7 @@ extern QueryDesc *CreateQueryDesc(PlannedStmt *plannedstmt,
 				Snapshot crosscheck_snapshot,
 				DestReceiver *dest,
 				ParamListInfo params,
-				bool doInstrument);
+				int instrument_options);
 
 extern QueryDesc *CreateUtilityQueryDesc(Node *utilitystmt,
 					   const char *sourceText,