0002-Skip-unnecessary-snapshot-builds.patch

From 31ea8f255e531c9327f7a5d55c5c0c756b37739c Mon Sep 17 00:00:00 2001
From: Petr Jelinek <pjmodos@pjmodos.net>
Date: Sat, 10 Dec 2016 22:22:13 +0100
Subject: [PATCH 2/2] Skip unnecessary snapshot builds

---
 src/backend/replication/logical/snapbuild.c | 38 +++++++++++++++++++++--------
 1 file changed, 28 insertions(+), 10 deletions(-)

diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c
index 5836d52..3cf4829 100644
--- a/src/backend/replication/logical/snapbuild.c
+++ b/src/backend/replication/logical/snapbuild.c
@@ -955,6 +955,7 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
 	bool		forced_timetravel = false;
 	bool		sub_needs_timetravel = false;
 	bool		top_needs_timetravel = false;
+	bool		build_snapshot = true;
 
 	TransactionId xmax = xid;
 
@@ -976,10 +977,19 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
 		/*
 		 * We could avoid treating !SnapBuildTxnIsRunning transactions as
 		 * timetravel ones, but we want to be able to export a snapshot when
-		 * we reached consistency.
+		 * we reached consistency so we need to keep track of them.
 		 */
 		forced_timetravel = true;
 		elog(DEBUG1, "forced to assume catalog changes for xid %u because it was running too early", xid);
+
+		/*
+		 * It is however desirable to skip building new snapshot for
+		 * !SnapBuildTxnIsRunning transactions as otherwise we might end up
+		 * building thousands of unused snapshots on busy servers which can
+		 * be very expensive.
+		 */
+		if (!SnapBuildTxnIsRunning(builder, xid))
+			build_snapshot = false;
 	}
 
 	for (nxact = 0; nxact < nsubxacts; nxact++)
@@ -1069,15 +1079,25 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
 		if (builder->state < SNAPBUILD_FULL_SNAPSHOT)
 			return;
 
+		/* Make sure we always build snapshot if there is no existing one. */
+		build_snapshot = build_snapshot || !builder->snapshot;
+
 		/*
-		 * Decrease the snapshot builder's refcount of the old snapshot, note
-		 * that it still will be used if it has been handed out to the
-		 * reorderbuffer earlier.
+		 * Decrease the snapshot builder's refcount of the old snapshot if we
+		 * plan to build new one, note that it still will be used if it has
+		 * been handed out to the reorderbuffer earlier.
 		 */
-		if (builder->snapshot)
+		if (builder->snapshot && build_snapshot)
 			SnapBuildSnapDecRefcount(builder->snapshot);
 
-		builder->snapshot = SnapBuildBuildSnapshot(builder, xid);
+		/* Build new snapshot unless asked not to. */
+		if (build_snapshot)
+		{
+			builder->snapshot = SnapBuildBuildSnapshot(builder, xid);
+
+			/* refcount of the snapshot builder for the new snapshot */
+			SnapBuildSnapIncRefcount(builder->snapshot);
+		}
 
 		/* we might need to execute invalidations, add snapshot */
 		if (!ReorderBufferXidHasBaseSnapshot(builder->reorder, xid))
@@ -1087,11 +1107,9 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
 										 builder->snapshot);
 		}
 
-		/* refcount of the snapshot builder for the new snapshot */
-		SnapBuildSnapIncRefcount(builder->snapshot);
-
 		/* add a new Snapshot to all currently running transactions */
-		SnapBuildDistributeNewCatalogSnapshot(builder, lsn);
+		if (build_snapshot)
+			SnapBuildDistributeNewCatalogSnapshot(builder, lsn);
 	}
 	else
 	{
-- 
2.7.4