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/commands/explain.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994-5, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/commands/explain.h,v 1.35 2008/01/01 19:45:57 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/commands/explain.h,v 1.36 2008/11/19 01:10:23 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -41,4 +41,7 @@ extern void ExplainOneUtility(Node *utilityStmt, ExplainStmt *stmt,
 extern void ExplainOnePlan(PlannedStmt *plannedstmt, ParamListInfo params,
 			   ExplainStmt *stmt, TupOutputState *tstate);
 
+extern void ExplainPrintPlan(StringInfo str, QueryDesc *queryDesc,
+							 bool analyze, bool verbose);
+
 #endif   /* EXPLAIN_H */