Avoid superfluous work for commits during logical slot creation.

Before 955a684e04 logical decoding snapshot maintenance needed to
cope with transactions it might not have seen in their entirety. For
such transactions we'd to assume they modified the catalog (could have
happened before we were watching), and thus a new snapshot had to be
built, and distributed to concurrently running transactions.

That's problematic because building a new snapshot isn't that cheap ,
especially as the the array of committed transactions needs to be
sorted.  When creating a slot on a server with a lot of transactions,
this could make logical slot creation infeasibly expensive.

After 955a684e04 there's no need to deal with transaction that
aren't guaranteed to be fully observable.  That allows to avoid
building snapshots for transactions that haven't modified catalog,
even before reaching consistency.

While this isn't necessarily a bugfix, slot creation being impossible
in some production workloads, is severe enough to warrant
backpatching.

Author: Andres Freund, based on a quite different patch from Petr Jelinek
Analyzed-By: Petr Jelinek
Reviewed-By: Petr Jelinek
Discussion: https://postgr.es/m/f37e975c-908f-858e-707f-058d3b1eb214@2ndquadrant.com
Backpatch: 9.4-, where logical decoding has been introduced

src/backend/replication/logical/snapbuild.c

@@ -929,21 +929,26 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
 {
 	int			nxact;
 
-	bool		forced_timetravel = false;
+	bool		needs_snapshot = false;
+	bool		needs_timetravel = false;
 	bool		sub_needs_timetravel = false;
-	bool		top_needs_timetravel = false;
 
 	TransactionId xmax = xid;
 
 	/*
-	 * If we couldn't observe every change of a transaction because it was
-	 * already running at the point we started to observe we have to assume it
-	 * made catalog changes.
-	 *
-	 * This has the positive benefit that we afterwards have enough
-	 * information to build an exportable snapshot that's usable by pg_dump et
-	 * al.
+	 * Transactions preceding BUILDING_SNAPSHOT will neither be decoded, nor
+	 * will they be part of a snapshot.  So we don't need to record anything.
 	 */
+	if (builder->state == SNAPBUILD_START ||
+		(builder->state == SNAPBUILD_BUILDING_SNAPSHOT &&
+		 TransactionIdPrecedes(xid, SnapBuildNextPhaseAt(builder))))
+	{
+		/* ensure that only commits after this are getting replayed */
+		if (builder->start_decoding_at <= lsn)
+			builder->start_decoding_at = lsn + 1;
+		return;
+	}
+
 	if (builder->state < SNAPBUILD_CONSISTENT)
 	{
 		/* ensure that only commits after this are getting replayed */
@@ -951,12 +956,13 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
 			builder->start_decoding_at = lsn + 1;
 
 		/*
-		 * We could avoid treating !SnapBuildTxnIsRunning transactions as
-		 * timetravel ones, but we want to be able to export a snapshot when
-		 * we reached consistency.
+		 * If building an exportable snapshot, force xid to be tracked, even
+		 * if the transaction didn't modify the catalog.
 		 */
-		forced_timetravel = true;
-		elog(DEBUG1, "forced to assume catalog changes for xid %u because it was running too early", xid);
+		if (builder->building_full_snapshot)
+		{
+			needs_timetravel = true;
+		}
 	}
 
 	for (nxact = 0; nxact < nsubxacts; nxact++)
@@ -964,70 +970,81 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
 		TransactionId subxid = subxacts[nxact];
 
 		/*
-		 * If we're forcing timetravel we also need visibility information
-		 * about subtransaction, so keep track of subtransaction's state.
+		 * Add subtransaction to base snapshot if catalog modifying, we don't
+		 * distinguish to toplevel transactions there.
 		 */
-		if (forced_timetravel)
+		if (ReorderBufferXidHasCatalogChanges(builder->reorder, subxid))
 		{
+			sub_needs_timetravel = true;
+			needs_snapshot = true;
+
+			elog(DEBUG1, "found subtransaction %u:%u with catalog changes",
+				 xid, subxid);
+
 			SnapBuildAddCommittedTxn(builder, subxid);
+
 			if (NormalTransactionIdFollows(subxid, xmax))
 				xmax = subxid;
 		}
-
 		/*
-		 * Add subtransaction to base snapshot if it DDL, we don't distinguish
-		 * to toplevel transactions there.
+		 * If we're forcing timetravel we also need visibility information
+		 * about subtransaction, so keep track of subtransaction's state, even
+		 * if not catalog modifying.  Don't need to distribute a snapshot in
+		 * that case.
 		 */
-		else if (ReorderBufferXidHasCatalogChanges(builder->reorder, subxid))
+		else if (needs_timetravel)
 		{
-			sub_needs_timetravel = true;
-
-			elog(DEBUG1, "found subtransaction %u:%u with catalog changes.",
-				 xid, subxid);
-
 			SnapBuildAddCommittedTxn(builder, subxid);
-
 			if (NormalTransactionIdFollows(subxid, xmax))
 				xmax = subxid;
 		}
 	}
 
-	if (forced_timetravel)
+	/* if top-level modified catalog, it'll need a snapshot */
+	if (ReorderBufferXidHasCatalogChanges(builder->reorder, xid))
 	{
-		elog(DEBUG2, "forced transaction %u to do timetravel.", xid);
-
+		elog(DEBUG2, "found top level transaction %u, with catalog changes",
+			 xid);
+		needs_snapshot = true;
+		needs_timetravel = true;
 		SnapBuildAddCommittedTxn(builder, xid);
 	}
-	/* add toplevel transaction to base snapshot */
-	else if (ReorderBufferXidHasCatalogChanges(builder->reorder, xid))
+	else if (sub_needs_timetravel)
 	{
-		elog(DEBUG2, "found top level transaction %u, with catalog changes!",
-			 xid);
-
-		top_needs_timetravel = true;
+		/* track toplevel txn as well, subxact alone isn't meaningful */
 		SnapBuildAddCommittedTxn(builder, xid);
 	}
-	else if (sub_needs_timetravel)
+	else if (needs_timetravel)
 	{
-		/* mark toplevel txn as timetravel as well */
+		elog(DEBUG2, "forced transaction %u to do timetravel", xid);
+
 		SnapBuildAddCommittedTxn(builder, xid);
 	}
 
-	/* if there's any reason to build a historic snapshot, do so now */
-	if (forced_timetravel || top_needs_timetravel || sub_needs_timetravel)
+	if (!needs_timetravel)
 	{
+		/* record that we cannot export a general snapshot anymore */
+		builder->committed.includes_all_transactions = false;
+	}
+
+	Assert(!needs_snapshot || needs_timetravel);
+
 	/*
 	 * Adjust xmax of the snapshot builder, we only do that for committed,
 	 * catalog modifying, transactions, everything else isn't interesting
 	 * for us since we'll never look at the respective rows.
 	 */
-		if (!TransactionIdIsValid(builder->xmax) ||
-			TransactionIdFollowsOrEquals(xmax, builder->xmax))
+	if (needs_timetravel &&
+		(!TransactionIdIsValid(builder->xmax) ||
+		 TransactionIdFollowsOrEquals(xmax, builder->xmax)))
 	{
 		builder->xmax = xmax;
 		TransactionIdAdvance(builder->xmax);
 	}
 
+	/* if there's any reason to build a historic snapshot, do so now */
+	if (needs_snapshot)
+	{
 		/*
 		 * If we haven't built a complete snapshot yet there's no need to hand
 		 * it out, it wouldn't (and couldn't) be used anyway.
@@ -1059,11 +1076,6 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
 		/* add a new Snapshot to all currently running transactions */
 		SnapBuildDistributeNewCatalogSnapshot(builder, lsn);
 	}
-	else
-	{
-		/* record that we cannot export a general snapshot anymore */
-		builder->committed.includes_all_transactions = false;
-	}
 }