Merge the various forms of transaction commit & abort records.

Since 465883b0a two versions of commit records have existed. A compact
version that was used when no cache invalidations, smgr unlinks and
similar were needed, and a full version that could deal with all
that. Additionally the full version was embedded into twophase commit
records.

That resulted in a measurable reduction in the size of the logged WAL in
some workloads. But more recently additions like logical decoding, which
e.g. needs information about the database something was executed on,
made it applicable in fewer situations. The static split generally made
it hard to expand the commit record, because concerns over the size made
it hard to add anything to the compact version.

Additionally it's not particularly pretty to have twophase.c insert
RM_XACT records.

Rejigger things so that the commit and abort records only have one form
each, including the twophase equivalents. The presence of the various
optional (in the sense of not being in every record) pieces is indicated
by a bits in the 'xinfo' flag.  That flag previously was not included in
compact commit records. To prevent an increase in size due to its
presence, it's only included if necessary; signalled by a bit in the
xl_info bits available for xact.c, similar to heapam.c's
XLOG_HEAP_OPMASK/XLOG_HEAP_INIT_PAGE.

Twophase commit/aborts are now the same as their normal
counterparts. The original transaction's xid is included in an optional
data field.

This means that commit records generally are smaller, except in the case
of a transaction with subtransactions, but no other special cases; the
increase there is four bytes, which seems acceptable given that the more
common case of not having subtransactions shrank.  The savings are
especially measurable for twophase commits, which previously always used
the full version; but will in practice only infrequently have required
that.

The motivation for this work are not the space savings and and
deduplication though; it's that it makes it easier to extend commit
records with additional information. That's just a few lines of code
now; without impacting the common case where that information is not
needed.

Discussion: 20150220152150.GD4149@awork2.anarazel.de,
    235610.92468.qm%40web29004.mail.ird.yahoo.com

Reviewed-By: Heikki Linnakangas, Simon Riggs

1 files changed, 40 insertions, 93 deletions

diff --git a/src/backend/replication/logical/decode.c b/src/backend/replication/logical/decode.c
index e7614bd515a..eb7293f2f33 100644
--- a/src/backend/replication/logical/decode.c
+++ b/src/backend/replication/logical/decode.c
@@ -64,12 +64,9 @@ static void DecodeUpdate(LogicalDecodingContext *ctx, XLogRecordBuffer *buf);
 static void DecodeDelete(LogicalDecodingContext *ctx, XLogRecordBuffer *buf);
 static void DecodeMultiInsert(LogicalDecodingContext *ctx, XLogRecordBuffer *buf);
 static void DecodeCommit(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
-			 TransactionId xid, Oid dboid,
-			 TimestampTz commit_time,
-			 int nsubxacts, TransactionId *sub_xids,
-			 int ninval_msgs, SharedInvalidationMessage *msg);
-static void DecodeAbort(LogicalDecodingContext *ctx, XLogRecPtr lsn,
-			TransactionId xid, TransactionId *sub_xids, int nsubxacts);
+						 xl_xact_parsed_commit *parsed, TransactionId xid);
+static void DecodeAbort(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
+						xl_xact_parsed_abort *parsed, TransactionId xid);
 
 /* common function to decode tuples */
 static void DecodeXLogTuple(char *data, Size len, ReorderBufferTupleBuf *tup);
@@ -188,7 +185,7 @@ DecodeXactOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
 	SnapBuild  *builder = ctx->snapshot_builder;
 	ReorderBuffer *reorder = ctx->reorder;
 	XLogReaderState *r = buf->record;
-	uint8		info = XLogRecGetInfo(r) & ~XLR_INFO_MASK;
+	uint8		info = XLogRecGetInfo(r) & XLOG_XACT_OPMASK;
 
 	/* no point in doing anything yet, data could not be decoded anyway */
 	if (SnapBuildCurrentState(builder) < SNAPBUILD_FULL_SNAPSHOT)
@@ -197,87 +194,41 @@ DecodeXactOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
 	switch (info)
 	{
 		case XLOG_XACT_COMMIT:
-			{
-				xl_xact_commit *xlrec;
-				TransactionId *subxacts = NULL;
-				SharedInvalidationMessage *invals = NULL;
-
-				xlrec = (xl_xact_commit *) XLogRecGetData(r);
-
-				subxacts = (TransactionId *) &(xlrec->xnodes[xlrec->nrels]);
-				invals = (SharedInvalidationMessage *) &(subxacts[xlrec->nsubxacts]);
-
-				DecodeCommit(ctx, buf, XLogRecGetXid(r), xlrec->dbId,
-							 xlrec->xact_time,
-							 xlrec->nsubxacts, subxacts,
-							 xlrec->nmsgs, invals);
-
-				break;
-			}
 		case XLOG_XACT_COMMIT_PREPARED:
 			{
-				xl_xact_commit_prepared *prec;
 				xl_xact_commit *xlrec;
-				TransactionId *subxacts;
-				SharedInvalidationMessage *invals = NULL;
-
-				/* Prepared commits contain a normal commit record... */
-				prec = (xl_xact_commit_prepared *) XLogRecGetData(r);
-				xlrec = &prec->crec;
+				xl_xact_parsed_commit parsed;
+				TransactionId xid;
 
-				subxacts = (TransactionId *) &(xlrec->xnodes[xlrec->nrels]);
-				invals = (SharedInvalidationMessage *) &(subxacts[xlrec->nsubxacts]);
-
-				DecodeCommit(ctx, buf, prec->xid, xlrec->dbId,
-							 xlrec->xact_time,
-							 xlrec->nsubxacts, subxacts,
-							 xlrec->nmsgs, invals);
-
-				break;
-			}
-		case XLOG_XACT_COMMIT_COMPACT:
-			{
-				xl_xact_commit_compact *xlrec;
+				xlrec = (xl_xact_commit *) XLogRecGetData(r);
+				ParseCommitRecord(XLogRecGetInfo(buf->record), xlrec, &parsed);
 
-				xlrec = (xl_xact_commit_compact *) XLogRecGetData(r);
+				if (!TransactionIdIsValid(parsed.twophase_xid))
+					xid = XLogRecGetXid(r);
+				else
+					xid = parsed.twophase_xid;
 
-				DecodeCommit(ctx, buf, XLogRecGetXid(r), InvalidOid,
-							 xlrec->xact_time,
-							 xlrec->nsubxacts, xlrec->subxacts,
-							 0, NULL);
+				DecodeCommit(ctx, buf, &parsed, xid);
 				break;
 			}
 		case XLOG_XACT_ABORT:
-			{
-				xl_xact_abort *xlrec;
-				TransactionId *sub_xids;
-
-				xlrec = (xl_xact_abort *) XLogRecGetData(r);
-
-				sub_xids = (TransactionId *) &(xlrec->xnodes[xlrec->nrels]);
-
-				DecodeAbort(ctx, buf->origptr, XLogRecGetXid(r),
-							sub_xids, xlrec->nsubxacts);
-				break;
-			}
 		case XLOG_XACT_ABORT_PREPARED:
 			{
-				xl_xact_abort_prepared *prec;
 				xl_xact_abort *xlrec;
-				TransactionId *sub_xids;
+				xl_xact_parsed_abort parsed;
+				TransactionId xid;
 
-				/* prepared abort contain a normal commit abort... */
-				prec = (xl_xact_abort_prepared *) XLogRecGetData(r);
-				xlrec = &prec->arec;
+				xlrec = (xl_xact_abort *) XLogRecGetData(r);
+				ParseAbortRecord(XLogRecGetInfo(buf->record), xlrec, &parsed);
 
-				sub_xids = (TransactionId *) &(xlrec->xnodes[xlrec->nrels]);
+				if (!TransactionIdIsValid(parsed.twophase_xid))
+					xid = XLogRecGetXid(r);
+				else
+					xid = parsed.twophase_xid;
 
-				/* r->xl_xid is committed in a separate record */
-				DecodeAbort(ctx, buf->origptr, prec->xid,
-							sub_xids, xlrec->nsubxacts);
+				DecodeAbort(ctx, buf, &parsed, xid);
 				break;
 			}
-
 		case XLOG_XACT_ASSIGNMENT:
 			{
 				xl_xact_assignment *xlrec;
@@ -477,10 +428,7 @@ DecodeHeapOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
  */
 static void
 DecodeCommit(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
-			 TransactionId xid, Oid dboid,
-			 TimestampTz commit_time,
-			 int nsubxacts, TransactionId *sub_xids,
-			 int ninval_msgs, SharedInvalidationMessage *msgs)
+			 xl_xact_parsed_commit *parsed, TransactionId xid)
 {
 	int			i;
 
@@ -489,15 +437,15 @@ DecodeCommit(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
 	 * transaction's contents, since the various caches need to always be
 	 * consistent.
 	 */
-	if (ninval_msgs > 0)
+	if (parsed->nmsgs > 0)
 	{
 		ReorderBufferAddInvalidations(ctx->reorder, xid, buf->origptr,
-									  ninval_msgs, msgs);
+									  parsed->nmsgs, parsed->msgs);
 		ReorderBufferXidSetCatalogChanges(ctx->reorder, xid, buf->origptr);
 	}
 
 	SnapBuildCommitTxn(ctx->snapshot_builder, buf->origptr, xid,
-					   nsubxacts, sub_xids);
+					   parsed->nsubxacts, parsed->subxacts);
 
 	/* ----
 	 * Check whether we are interested in this specific transaction, and tell
@@ -524,12 +472,11 @@ DecodeCommit(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
 	 * ---
 	 */
 	if (SnapBuildXactNeedsSkip(ctx->snapshot_builder, buf->origptr) ||
-		(dboid != InvalidOid && dboid != ctx->slot->data.database))
+		(parsed->dbId != InvalidOid && parsed->dbId != ctx->slot->data.database))
 	{
-		for (i = 0; i < nsubxacts; i++)
+		for (i = 0; i < parsed->nsubxacts; i++)
 		{
-			ReorderBufferForget(ctx->reorder, *sub_xids, buf->origptr);
-			sub_xids++;
+			ReorderBufferForget(ctx->reorder, parsed->subxacts[i], buf->origptr);
 		}
 		ReorderBufferForget(ctx->reorder, xid, buf->origptr);
 
@@ -537,16 +484,15 @@ DecodeCommit(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
 	}
 
 	/* tell the reorderbuffer about the surviving subtransactions */
-	for (i = 0; i < nsubxacts; i++)
+	for (i = 0; i < parsed->nsubxacts; i++)
 	{
-		ReorderBufferCommitChild(ctx->reorder, xid, *sub_xids,
+		ReorderBufferCommitChild(ctx->reorder, xid, parsed->subxacts[i],
 								 buf->origptr, buf->endptr);
-		sub_xids++;
 	}
 
 	/* replay actions of all transaction + subtransactions in order */
 	ReorderBufferCommit(ctx->reorder, xid, buf->origptr, buf->endptr,
-						commit_time);
+						parsed->xact_time);
 }
 
 /*
@@ -554,20 +500,21 @@ DecodeCommit(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
  * snapbuild.c and reorderbuffer.c
  */
 static void
-DecodeAbort(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid,
-			TransactionId *sub_xids, int nsubxacts)
+DecodeAbort(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
+			xl_xact_parsed_abort *parsed, TransactionId xid)
 {
 	int			i;
 
-	SnapBuildAbortTxn(ctx->snapshot_builder, lsn, xid, nsubxacts, sub_xids);
+	SnapBuildAbortTxn(ctx->snapshot_builder, buf->record->EndRecPtr, xid,
+					  parsed->nsubxacts, parsed->subxacts);
 
-	for (i = 0; i < nsubxacts; i++)
+	for (i = 0; i < parsed->nsubxacts; i++)
 	{
-		ReorderBufferAbort(ctx->reorder, *sub_xids, lsn);
-		sub_xids++;
+		ReorderBufferAbort(ctx->reorder, parsed->subxacts[i],
+						   buf->record->EndRecPtr);
 	}
 
-	ReorderBufferAbort(ctx->reorder, xid, lsn);
+	ReorderBufferAbort(ctx->reorder, xid, buf->record->EndRecPtr);
 }
 
 /*