Improve vacuum code to track minimum Xids per table instead of per database.

To this end, add a couple of columns to pg_class, relminxid and relvacuumxid,
based on which we calculate the pg_database columns after each vacuum.

We now force all databases to be vacuumed, even template ones.  A backend
noticing too old a database (meaning pg_database.datminxid is in danger of
falling behind Xid wraparound) will signal the postmaster, which in turn will
start an autovacuum iteration to process the offending database.  In principle
this is only there to cope with frozen (non-connectable) databases without
forcing users to set them to connectable, but it could force regular user
database to go through a database-wide vacuum at any time.  Maybe we should
warn users about this somehow.  Of course the real solution will be to use
autovacuum all the time ;-)

There are some additional improvements we could have in this area: for example
the vacuum code could be smarter about not updating pg_database for each table
when called by autovacuum, and do it only once the whole autovacuum iteration
is done.

I updated the system catalogs documentation, but I didn't modify the
maintenance section.  Also having some regression tests for this would be nice
but it's not really a very straightforward thing to do.

Catalog version bumped due to system catalog changes.

1 files changed, 9 insertions, 9 deletions

diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c
index 517d4e1be3a..f1a3924b8cb 100644
--- a/src/backend/access/transam/varsup.c
+++ b/src/backend/access/transam/varsup.c
@@ -6,7 +6,7 @@
  * Copyright (c) 2000-2006, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.70 2006/03/05 15:58:22 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.71 2006/07/10 16:20:49 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -168,11 +168,11 @@ ReadNewTransactionId(void)
 
 /*
  * Determine the last safe XID to allocate given the currently oldest
- * datfrozenxid (ie, the oldest XID that might exist in any database
+ * datminxid (ie, the oldest XID that might exist in any database
  * of our cluster).
  */
 void
-SetTransactionIdLimit(TransactionId oldest_datfrozenxid,
+SetTransactionIdLimit(TransactionId oldest_datminxid,
 					  Name oldest_datname)
 {
 	TransactionId xidWarnLimit;
@@ -180,16 +180,16 @@ SetTransactionIdLimit(TransactionId oldest_datfrozenxid,
 	TransactionId xidWrapLimit;
 	TransactionId curXid;
 
-	Assert(TransactionIdIsValid(oldest_datfrozenxid));
+	Assert(TransactionIdIsValid(oldest_datminxid));
 
 	/*
 	 * The place where we actually get into deep trouble is halfway around
-	 * from the oldest potentially-existing XID.  (This calculation is
-	 * probably off by one or two counts, because the special XIDs reduce the
-	 * size of the loop a little bit.  But we throw in plenty of slop below,
-	 * so it doesn't matter.)
+	 * from the oldest existing XID.  (This calculation is probably off by one
+	 * or two counts, because the special XIDs reduce the size of the loop a
+	 * little bit.  But we throw in plenty of slop below, so it doesn't
+	 * matter.)
 	 */
-	xidWrapLimit = oldest_datfrozenxid + (MaxTransactionId >> 1);
+	xidWrapLimit = oldest_datminxid + (MaxTransactionId >> 1);
 	if (xidWrapLimit < FirstNormalTransactionId)
 		xidWrapLimit += FirstNormalTransactionId;