From: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Date: Thu, 2 Dec 2010 07:42:54 +0000 (-0600)
Subject: Add an assertion to help ensure that clean-up isn't falling down.
X-Git-Url: http://git.postgresql.org/gitweb/irc:/static/gitweb.js?a=commitdiff_plain;h=bf5709a32fdda29bdd07e6d68d8000a938bc224d;p=users%2Fkgrittn%2Fpostgres.git

Add an assertion to help ensure that clean-up isn't falling down.
Add a couple TODO comments for eager cleanup of read-only
transactions.
---

diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index 0f30116391..dcd44169b6 100644
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -819,6 +819,12 @@ RegisterSerializableTransaction(const Snapshot snapshot)
 
 	LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
 
+	/*
+	 * TODO SSI: If there are no serializable transactions which are not
+	 * read-only, we can "opt out" of predicate locking and conflict
+	 * checking.
+	 */
+
 	if (!TransactionIdIsValid(PredTran->SxactGlobalXmin))
 	{
 		Assert(PredTran->SxactGlobalXminCount == 0);
@@ -1824,6 +1830,11 @@ SetNewSxactGlobalXmin(void)
  * If this transaction is committing and is holding any predicate locks,
  * it must be added to a list of completed serializable transaction still
  * holding locks.
+ *
+ * TODO SSI: Maintain a count of non-read-only active serializable
+ * transactions.  Any time this count hits zero, clean up any conflicts and
+ * predicate locks for read-only transactions -- active or committed.  They
+ * need not acquire predicate locks or check for out-conflicts after that.
  */
 void
 ReleasePredicateLocks(const bool isCommit)
@@ -1946,6 +1957,13 @@ ReleasePredicateLocks(const bool isCommit)
 		LWLockRelease(SerializableXactHashLock);
 	}
 
+	/*
+	 * Reality check: There can't be an active serializable transaction
+	 * older than the oldest active transaction.
+	 */
+	Assert(!TransactionIdIsValid(PredTran->SxactGlobalXmin)
+		  || TransactionIdFollowsOrEquals(PredTran->SxactGlobalXmin, RecentGlobalXmin));
+
 	LWLockRelease(SerializableFinishedListLock);
 
 	if (!isCommit)