Improve snapshot manager by keeping explicit track of snapshots.

There are two ways to track a snapshot: there's the "registered" list, which
is used for arbitrary long-lived snapshots; and there's the "active stack",
which is used for the snapshot that is considered "active" at any time.
This also allows users of snapshots to stop worrying about snapshot memory
allocation and freeing, and about using PG_TRY blocks around ActiveSnapshot
assignment.  This is all done automatically now.

As a consequence, this allows us to reset MyProc->xmin when there are no
more snapshots registered in the current backend, reducing the impact that
long-running transactions have on VACUUM.

1 files changed, 7 insertions, 8 deletions

diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index cbcc2a67e1c..6fbdabd1d2b 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994-5, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/explain.c,v 1.173 2008/04/18 01:42:17 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/explain.c,v 1.174 2008/05/12 20:01:59 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -229,17 +229,14 @@ ExplainOnePlan(PlannedStmt *plannedstmt, ParamListInfo params,
 	int			eflags;
 
 	/*
-	 * 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.)
+	 * Use a snapshot with an updated command ID to ensure this query sees
+	 * results of any previously executed queries.
 	 */
-	ActiveSnapshot->curcid = GetCurrentCommandId(false);
+	PushUpdatedSnapshot(GetActiveSnapshot());
 
 	/* Create a QueryDesc requesting no output */
 	queryDesc = CreateQueryDesc(plannedstmt,
-								ActiveSnapshot, InvalidSnapshot,
+								GetActiveSnapshot(), InvalidSnapshot,
 								None_Receiver, params,
 								stmt->analyze);
 
@@ -324,6 +321,8 @@ ExplainOnePlan(PlannedStmt *plannedstmt, ParamListInfo params,
 
 	FreeQueryDesc(queryDesc);
 
+	PopActiveSnapshot();
+
 	/* We need a CCI just in case query expanded to multiple plans */
 	if (stmt->analyze)
 		CommandCounterIncrement();