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, 19 deletions

diff --git a/src/backend/commands/dbcommands.c b/src/backend/commands/dbcommands.c
index 518fed81968..6d744a5bad3 100644
--- a/src/backend/commands/dbcommands.c
+++ b/src/backend/commands/dbcommands.c
@@ -13,7 +13,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/dbcommands.c,v 1.181 2006/05/04 16:07:29 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/dbcommands.c,v 1.182 2006/07/10 16:20:50 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -55,7 +55,7 @@ static bool get_db_info(const char *name, LOCKMODE lockmode,
 			Oid *dbIdP, Oid *ownerIdP,
 			int *encodingP, bool *dbIsTemplateP, bool *dbAllowConnP,
 			Oid *dbLastSysOidP,
-			TransactionId *dbVacuumXidP, TransactionId *dbFrozenXidP,
+			TransactionId *dbVacuumXidP, TransactionId *dbMinXidP,
 			Oid *dbTablespace);
 static bool have_createdb_privilege(void);
 static void remove_dbtablespaces(Oid db_id);
@@ -76,7 +76,7 @@ createdb(const CreatedbStmt *stmt)
 	bool		src_allowconn;
 	Oid			src_lastsysoid;
 	TransactionId src_vacuumxid;
-	TransactionId src_frozenxid;
+	TransactionId src_minxid;
 	Oid			src_deftablespace;
 	volatile Oid dst_deftablespace;
 	Relation	pg_database_rel;
@@ -228,7 +228,7 @@ createdb(const CreatedbStmt *stmt)
 	if (!get_db_info(dbtemplate, ShareLock,
 					 &src_dboid, &src_owner, &src_encoding,
 					 &src_istemplate, &src_allowconn, &src_lastsysoid,
-					 &src_vacuumxid, &src_frozenxid, &src_deftablespace))
+					 &src_vacuumxid, &src_minxid, &src_deftablespace))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_DATABASE),
 				 errmsg("template database \"%s\" does not exist",
@@ -327,16 +327,6 @@ createdb(const CreatedbStmt *stmt)
 	}
 
 	/*
-	 * Normally we mark the new database with the same datvacuumxid and
-	 * datfrozenxid as the source.	However, if the source is not allowing
-	 * connections then we assume it is fully frozen, and we can set the
-	 * current transaction ID as the xid limits.  This avoids immediately
-	 * starting to generate warnings after cloning template0.
-	 */
-	if (!src_allowconn)
-		src_vacuumxid = src_frozenxid = GetCurrentTransactionId();
-
-	/*
 	 * Check for db name conflict.	This is just to give a more friendly
 	 * error message than "unique index violation".  There's a race condition
 	 * but we're willing to accept the less friendly message in that case.
@@ -367,7 +357,7 @@ createdb(const CreatedbStmt *stmt)
 	new_record[Anum_pg_database_datconnlimit - 1] = Int32GetDatum(dbconnlimit);
 	new_record[Anum_pg_database_datlastsysoid - 1] = ObjectIdGetDatum(src_lastsysoid);
 	new_record[Anum_pg_database_datvacuumxid - 1] = TransactionIdGetDatum(src_vacuumxid);
-	new_record[Anum_pg_database_datfrozenxid - 1] = TransactionIdGetDatum(src_frozenxid);
+	new_record[Anum_pg_database_datminxid - 1] = TransactionIdGetDatum(src_minxid);
 	new_record[Anum_pg_database_dattablespace - 1] = ObjectIdGetDatum(dst_deftablespace);
 
 	/*
@@ -1066,7 +1056,7 @@ get_db_info(const char *name, LOCKMODE lockmode,
 			Oid *dbIdP, Oid *ownerIdP,
 			int *encodingP, bool *dbIsTemplateP, bool *dbAllowConnP,
 			Oid *dbLastSysOidP,
-			TransactionId *dbVacuumXidP, TransactionId *dbFrozenXidP,
+			TransactionId *dbVacuumXidP, TransactionId *dbMinXidP,
 			Oid *dbTablespace)
 {
 	bool		result = false;
@@ -1155,9 +1145,9 @@ get_db_info(const char *name, LOCKMODE lockmode,
 				/* limit of vacuumed XIDs */
 				if (dbVacuumXidP)
 					*dbVacuumXidP = dbform->datvacuumxid;
-				/* limit of frozen XIDs */
-				if (dbFrozenXidP)
-					*dbFrozenXidP = dbform->datfrozenxid;
+				/* limit of min XIDs */
+				if (dbMinXidP)
+					*dbMinXidP = dbform->datminxid;
 				/* default tablespace for this database */
 				if (dbTablespace)
 					*dbTablespace = dbform->dattablespace;