Create hooks to let a loadable plugin monitor (or even replace) the planner

and/or create plans for hypothetical situations; in particular, investigate
plans that would be generated using hypothetical indexes.  This is a
heavily-rewritten version of the hooks proposed by Gurjeet Singh for his
Index Advisor project.  In this formulation, the index advisor can be
entirely a loadable module instead of requiring a significant part to be
in the core backend, and plans can be generated for hypothetical indexes
without requiring the creation and rolling-back of system catalog entries.

The index advisor patch as-submitted is not compatible with these hooks,
but it needs significant work anyway due to other 8.2-to-8.3 planner
changes.  With these hooks in the core backend, development of the advisor
can proceed as a pgfoundry project.

1 files changed, 2 insertions, 19 deletions

diff --git a/src/backend/commands/prepare.c b/src/backend/commands/prepare.c
index 997f66c8188..de999a36376 100644
--- a/src/backend/commands/prepare.c
+++ b/src/backend/commands/prepare.c
@@ -10,7 +10,7 @@
  * Copyright (c) 2002-2007, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/prepare.c,v 1.75 2007/04/27 22:05:47 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/prepare.c,v 1.76 2007/05/25 17:54:25 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -678,8 +678,6 @@ ExplainExecuteQuery(ExecuteStmt *execstmt, ExplainStmt *stmt,
 
 		if (IsA(pstmt, PlannedStmt))
 		{
-			QueryDesc  *qdesc;
-
 			if (execstmt->into)
 			{
 				if (pstmt->commandType != CMD_SELECT ||
@@ -694,22 +692,7 @@ ExplainExecuteQuery(ExecuteStmt *execstmt, ExplainStmt *stmt,
 				pstmt->intoClause = execstmt->into;
 			}
 
-			/*
-			 * Update snapshot command ID to ensure this query sees results of
-			 * any previously executed queries.  (It's a bit cheesy to modify
-			 * ActiveSnapshot without making a copy, but for the limited ways
-			 * in which EXPLAIN can be invoked, I think it's OK, because the
-			 * active snapshot shouldn't be shared with anything else anyway.)
-			 */
-			ActiveSnapshot->curcid = GetCurrentCommandId();
-
-			/* Create a QueryDesc requesting no output */
-			qdesc = CreateQueryDesc(pstmt,
-									ActiveSnapshot, InvalidSnapshot,
-									None_Receiver,
-									paramLI, stmt->analyze);
-
-			ExplainOnePlan(qdesc, stmt, tstate);
+			ExplainOnePlan(pstmt, paramLI, stmt, tstate);
 		}
 		else
 		{