Don't force-assign transaction id when exporting a snapshot.

Previously we required every exported transaction to have an xid
assigned. That was used to check that the exporting transaction is
still running, which in turn is needed to guarantee that that
necessary rows haven't been removed in between exporting and importing
the snapshot.

The exported xid caused unnecessary problems with logical decoding,
because slot creation has to wait for all concurrent xid to finish,
which in turn serializes concurrent slot creation.   It also
prohibited snapshots to be exported on hot-standby replicas.

Instead export the virtual transactionid, which avoids the unnecessary
serialization and the inability to export snapshots on standbys. This
changes the file name of the exported snapshot, but since we never
documented what that one means, that seems ok.

Author: Petr Jelinek, slightly editorialized by me
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/f598b4b8-8cd7-0d54-0939-adda763d8c34@2ndquadrant.com

1 files changed, 7 insertions, 7 deletions

diff --git a/src/backend/replication/logical/snapbuild.c b/src/backend/replication/logical/snapbuild.c
index 8848f5b4ec1..e06aa0992a6 100644
--- a/src/backend/replication/logical/snapbuild.c
+++ b/src/backend/replication/logical/snapbuild.c
@@ -262,7 +262,7 @@ static bool ExportInProgress = false;
 static void SnapBuildPurgeCommittedTxn(SnapBuild *builder);
 
 /* snapshot building/manipulation/distribution functions */
-static Snapshot SnapBuildBuildSnapshot(SnapBuild *builder, TransactionId xid);
+static Snapshot SnapBuildBuildSnapshot(SnapBuild *builder);
 
 static void SnapBuildFreeSnapshot(Snapshot snap);
 
@@ -463,7 +463,7 @@ SnapBuildSnapDecRefcount(Snapshot snap)
  * and ->subxip/subxcnt values.
  */
 static Snapshot
-SnapBuildBuildSnapshot(SnapBuild *builder, TransactionId xid)
+SnapBuildBuildSnapshot(SnapBuild *builder)
 {
 	Snapshot	snapshot;
 	Size		ssize;
@@ -562,7 +562,7 @@ SnapBuildInitialSnapshot(SnapBuild *builder)
 	if (TransactionIdIsValid(MyPgXact->xmin))
 		elog(ERROR, "cannot build an initial slot snapshot when MyPgXact->xmin already is valid");
 
-	snap = SnapBuildBuildSnapshot(builder, GetTopTransactionId());
+	snap = SnapBuildBuildSnapshot(builder);
 
 	/*
 	 * We know that snap->xmin is alive, enforced by the logical xmin
@@ -679,7 +679,7 @@ SnapBuildGetOrBuildSnapshot(SnapBuild *builder, TransactionId xid)
 	/* only build a new snapshot if we don't have a prebuilt one */
 	if (builder->snapshot == NULL)
 	{
-		builder->snapshot = SnapBuildBuildSnapshot(builder, xid);
+		builder->snapshot = SnapBuildBuildSnapshot(builder);
 		/* increase refcount for the snapshot builder */
 		SnapBuildSnapIncRefcount(builder->snapshot);
 	}
@@ -743,7 +743,7 @@ SnapBuildProcessChange(SnapBuild *builder, TransactionId xid, XLogRecPtr lsn)
 		/* only build a new snapshot if we don't have a prebuilt one */
 		if (builder->snapshot == NULL)
 		{
-			builder->snapshot = SnapBuildBuildSnapshot(builder, xid);
+			builder->snapshot = SnapBuildBuildSnapshot(builder);
 			/* increase refcount for the snapshot builder */
 			SnapBuildSnapIncRefcount(builder->snapshot);
 		}
@@ -1061,7 +1061,7 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
 		if (builder->snapshot)
 			SnapBuildSnapDecRefcount(builder->snapshot);
 
-		builder->snapshot = SnapBuildBuildSnapshot(builder, xid);
+		builder->snapshot = SnapBuildBuildSnapshot(builder);
 
 		/* we might need to execute invalidations, add snapshot */
 		if (!ReorderBufferXidHasBaseSnapshot(builder->reorder, xid))
@@ -1831,7 +1831,7 @@ SnapBuildRestore(SnapBuild *builder, XLogRecPtr lsn)
 	{
 		SnapBuildSnapDecRefcount(builder->snapshot);
 	}
-	builder->snapshot = SnapBuildBuildSnapshot(builder, InvalidTransactionId);
+	builder->snapshot = SnapBuildBuildSnapshot(builder);
 	SnapBuildSnapIncRefcount(builder->snapshot);
 
 	ReorderBufferSetRestartPoint(builder->reorder, lsn);