Restructure subtransaction handling to reduce resource consumption,

as per recent discussions.  Invent SubTransactionIds that are managed like
CommandIds (ie, counter is reset at start of each top transaction), and
use these instead of TransactionIds to keep track of subtransaction status
in those modules that need it.  This means that a subtransaction does not
need an XID unless it actually inserts/modifies rows in the database.
Accordingly, don't assign it an XID nor take a lock on the XID until it
tries to do that.  This saves a lot of overhead for subtransactions that
are only used for error recovery (eg plpgsql exceptions).  Also, arrange
to release a subtransaction's XID lock as soon as the subtransaction
exits, in both the commit and abort cases.  This avoids holding many
unique locks after a long series of subtransactions.  The price is some
additional overhead in XactLockTableWait, but that seems acceptable.
Finally, restructure the state machine in xact.c to have a more orthogonal
set of states for subtransactions.

1 files changed, 16 insertions, 16 deletions

diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index ae5ddb54bc7..6fa60185241 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/cache/relcache.c,v 1.210 2004/08/29 05:06:50 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/cache/relcache.c,v 1.211 2004/09/16 16:58:35 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -836,7 +836,7 @@ RelationBuildDesc(RelationBuildDescInfo buildinfo,
 	 */
 	relation->rd_refcnt = 0;
 	relation->rd_isnailed = false;
-	relation->rd_createxact = InvalidTransactionId;
+	relation->rd_createSubid = InvalidSubTransactionId;
 	relation->rd_istemp = isTempNamespace(relation->rd_rel->relnamespace);
 
 	/*
@@ -1287,7 +1287,7 @@ formrdesc(const char *relationName,
 	 * for new or temp relations.
 	 */
 	relation->rd_isnailed = true;
-	relation->rd_createxact = InvalidTransactionId;
+	relation->rd_createSubid = InvalidSubTransactionId;
 	relation->rd_istemp = false;
 
 	/*
@@ -1578,7 +1578,7 @@ RelationClose(Relation relation)
 
 #ifdef RELCACHE_FORCE_RELEASE
 	if (RelationHasReferenceCountZero(relation) &&
-		!TransactionIdIsValid(relation->rd_createxact))
+		relation->rd_createSubid == InvalidSubTransactionId)
 		RelationClearRelation(relation, false);
 #endif
 }
@@ -1736,7 +1736,7 @@ RelationClearRelation(Relation relation, bool rebuild)
 	{
 		/*
 		 * When rebuilding an open relcache entry, must preserve ref count
-		 * and rd_createxact state.  Also attempt to preserve the
+		 * and rd_createSubid state.  Also attempt to preserve the
 		 * tupledesc and rewrite-rule substructures in place.
 		 *
 		 * Note that this process does not touch CurrentResourceOwner; which
@@ -1744,7 +1744,7 @@ RelationClearRelation(Relation relation, bool rebuild)
 		 * necessarily belong to that resource owner.
 		 */
 		int			old_refcnt = relation->rd_refcnt;
-		TransactionId old_createxact = relation->rd_createxact;
+		SubTransactionId old_createSubid = relation->rd_createSubid;
 		TupleDesc	old_att = relation->rd_att;
 		RuleLock   *old_rules = relation->rd_rules;
 		MemoryContext old_rulescxt = relation->rd_rulescxt;
@@ -1765,7 +1765,7 @@ RelationClearRelation(Relation relation, bool rebuild)
 				 buildinfo.i.info_id);
 		}
 		relation->rd_refcnt = old_refcnt;
-		relation->rd_createxact = old_createxact;
+		relation->rd_createSubid = old_createSubid;
 		if (equalTupleDescs(old_att, relation->rd_att))
 		{
 			/* needn't flush typcache here */
@@ -1802,7 +1802,7 @@ RelationFlushRelation(Relation relation)
 {
 	bool		rebuild;
 
-	if (TransactionIdIsValid(relation->rd_createxact))
+	if (relation->rd_createSubid != InvalidSubTransactionId)
 	{
 		/*
 		 * New relcache entries are always rebuilt, not flushed; else we'd
@@ -1948,7 +1948,7 @@ RelationCacheInvalidate(void)
 		}
 
 		/* Ignore new relations, since they are never SI targets */
-		if (TransactionIdIsValid(relation->rd_createxact))
+		if (relation->rd_createSubid != InvalidSubTransactionId)
 			continue;
 
 		relcacheInvalsReceived++;
@@ -2032,10 +2032,10 @@ AtEOXact_RelationCache(bool isCommit)
 		 * flush, the entry will get deleted anyway by shared-cache-inval
 		 * processing of the aborted pg_class insertion.)
 		 */
-		if (TransactionIdIsValid(relation->rd_createxact))
+		if (relation->rd_createSubid != InvalidSubTransactionId)
 		{
 			if (isCommit)
-				relation->rd_createxact = InvalidTransactionId;
+				relation->rd_createSubid = InvalidSubTransactionId;
 			else
 			{
 				RelationClearRelation(relation, false);
@@ -2097,8 +2097,8 @@ AtEOXact_RelationCache(bool isCommit)
  * Note: this must be called *before* processing invalidation messages.
  */
 void
-AtEOSubXact_RelationCache(bool isCommit, TransactionId myXid,
-						  TransactionId parentXid)
+AtEOSubXact_RelationCache(bool isCommit, SubTransactionId mySubid,
+						  SubTransactionId parentSubid)
 {
 	HASH_SEQ_STATUS status;
 	RelIdCacheEnt *idhentry;
@@ -2115,10 +2115,10 @@ AtEOSubXact_RelationCache(bool isCommit, TransactionId myXid,
 		 * During subcommit, mark it as belonging to the parent, instead.
 		 * During subabort, simply delete the relcache entry.
 		 */
-		if (TransactionIdEquals(relation->rd_createxact, myXid))
+		if (relation->rd_createSubid == mySubid)
 		{
 			if (isCommit)
-				relation->rd_createxact = parentXid;
+				relation->rd_createSubid = parentSubid;
 			else
 			{
 				Assert(RelationHasReferenceCountZero(relation));
@@ -2182,7 +2182,7 @@ RelationBuildLocalRelation(const char *relname,
 	rel->rd_refcnt = nailit ? 1 : 0;
 
 	/* it's being created in this transaction */
-	rel->rd_createxact = GetCurrentTransactionId();
+	rel->rd_createSubid = GetCurrentSubTransactionId();
 
 	/* is it a temporary relation? */
 	rel->rd_istemp = isTempNamespace(relnamespace);