Some infrastructure changes for the upcoming auto-explain contrib module:

* Refactor explain.c slightly to export a convenient-to-use subroutine
for printing EXPLAIN results.

* Provide hooks for plugins to get control at ExecutorStart and ExecutorEnd
as well as ExecutorRun.

* Add some minimal support for tracking the total runtime of ExecutorRun.
This code won't actually do anything unless a plugin prods it to.

* Change the API of the DefineCustomXXXVariable functions to allow nonzero
"flags" to be specified for a custom GUC variable.  While at it, also make
the "bootstrap" default value for custom GUCs be explicitly specified as a
parameter to these functions.  This is to eliminate confusion over where the
default comes from, as has been expressed in the past by some users of the
custom-variable facility.

* Refactor GUC code a bit to ensure that a custom variable gets initialized to
something valid (like its default value) even if the placeholder value was
invalid.

src/include/executor/execdesc.h

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/executor/execdesc.h,v 1.37 2008/01/01 19:45:57 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/executor/execdesc.h,v 1.38 2008/11/19 01:10:23 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -47,6 +47,9 @@ typedef struct QueryDesc
 	TupleDesc	tupDesc;		/* descriptor for result tuples */
 	EState	   *estate;			/* executor's query-wide state */
 	PlanState  *planstate;		/* tree of per-plan-node state */
+
+	/* This is always set NULL by the core system, but plugins can change it */
+	struct Instrumentation *totaltime;	/* total time spent in ExecutorRun */
 } QueryDesc;
 
 /* in pquery.c */