Skip WAL for new relfilenodes, under wal_level=minimal.

Until now, only selected bulk operations (e.g. COPY) did this.  If a
given relfilenode received both a WAL-skipping COPY and a WAL-logged
operation (e.g. INSERT), recovery could lose tuples from the COPY.  See
src/backend/access/transam/README section "Skipping WAL for New
RelFileNode" for the new coding rules.  Maintainers of table access
methods should examine that section.

To maintain data durability, just before commit, we choose between an
fsync of the relfilenode and copying its contents to WAL.  A new GUC,
wal_skip_threshold, guides that choice.  If this change slows a workload
that creates small, permanent relfilenodes under wal_level=minimal, try
adjusting wal_skip_threshold.  Users setting a timeout on COMMIT may
need to adjust that timeout, and log_min_duration_statement analysis
will reflect time consumption moving to COMMIT from commands like COPY.

Internally, this requires a reliable determination of whether
RollbackAndReleaseCurrentSubTransaction() would unlink a relation's
current relfilenode.  Introduce rd_firstRelfilenodeSubid.  Amend the
specification of rd_createSubid such that the field is zero when a new
rel has an old rd_node.  Make relcache.c retain entries for certain
dropped relations until end of transaction.

Bump XLOG_PAGE_MAGIC, since this introduces XLOG_GIST_ASSIGN_LSN.
Future servers accept older WAL, so this bump is discretionary.

Kyotaro Horiguchi, reviewed (in earlier, similar versions) by Robert
Haas.  Heikki Linnakangas and Michael Paquier implemented earlier
designs that materially clarified the problem.  Reviewed, in earlier
designs, by Andrew Dunstan, Andres Freund, Alvaro Herrera, Tom Lane,
Fujii Masao, and Simon Riggs.  Reported by Martijn van Oosterhout.

Discussion: https://postgr.es/m/20150702220524.GA9392@svana.org

1 files changed, 26 insertions, 5 deletions

diff --git a/src/backend/access/gist/gistutil.c b/src/backend/access/gist/gistutil.c
index dd975b164c..765329bbcd 100644
--- a/src/backend/access/gist/gistutil.c
+++ b/src/backend/access/gist/gistutil.c
@@ -1004,23 +1004,44 @@ gistproperty(Oid index_oid, int attno,
 }
 
 /*
- * Temporary and unlogged GiST indexes are not WAL-logged, but we need LSNs
- * to detect concurrent page splits anyway. This function provides a fake
- * sequence of LSNs for that purpose.
+ * Some indexes are not WAL-logged, but we need LSNs to detect concurrent page
+ * splits anyway. This function provides a fake sequence of LSNs for that
+ * purpose.
  */
 XLogRecPtr
 gistGetFakeLSN(Relation rel)
 {
-	static XLogRecPtr counter = FirstNormalUnloggedLSN;
-
 	if (rel->rd_rel->relpersistence == RELPERSISTENCE_TEMP)
 	{
 		/*
 		 * Temporary relations are only accessible in our session, so a simple
 		 * backend-local counter will do.
 		 */
+		static XLogRecPtr counter = FirstNormalUnloggedLSN;
+
 		return counter++;
 	}
+	else if (rel->rd_rel->relpersistence == RELPERSISTENCE_PERMANENT)
+	{
+		/*
+		 * WAL-logging on this relation will start after commit, so its LSNs
+		 * must be distinct numbers smaller than the LSN at the next commit.
+		 * Emit a dummy WAL record if insert-LSN hasn't advanced after the
+		 * last call.
+		 */
+		static XLogRecPtr lastlsn = InvalidXLogRecPtr;
+		XLogRecPtr	currlsn = GetXLogInsertRecPtr();
+
+		/* Shouldn't be called for WAL-logging relations */
+		Assert(!RelationNeedsWAL(rel));
+
+		/* No need for an actual record if we already have a distinct LSN */
+		if (!XLogRecPtrIsInvalid(lastlsn) && lastlsn == currlsn)
+			currlsn = gistXLogAssignLSN();
+
+		lastlsn = currlsn;
+		return currlsn;
+	}
 	else
 	{
 		/*