Logical decoding of sequences

This extends the logical decoding to also decode sequence increments.
We differentiate between sequences created in the current (in-progress)
transaction, and sequences created earlier. This mixed behavior is
necessary because while sequences are not transactional (increments are
not subject to ROLLBACK), relfilenode changes are. So we do this:

* Changes for sequences created in the same top-level transaction are
  treated as transactional, i.e. just like any other change from that
  transaction, and discarded in case of a rollback.

* Changes for sequences created earlier are applied immediately, as if
  performed outside any transaction. This applies also after ALTER
  SEQUENCE, which may create a new relfilenode.

Moreover, if we ever get support for DDL replication, the sequence
won't exist until the transaction gets applied.

Sequences created in the current transaction are tracked in a simple
hash table, identified by a relfilenode. That means a sequence may
already exist, but if a transaction does ALTER SEQUENCE then the
increments for the new relfilenode will be treated as transactional.

For each relfilenode we track the XID of (sub)transaction that created
it, which is needed for cleanup at transaction end. We don't need to
check the XID to decide if an increment is transactional - if we find a
match in the hash table, it has to be the same transaction.

This requires two minor changes to WAL-logging. Firstly, we need to
ensure the sequence record has a valid XID - until now the the increment
might have XID 0 if it was the first change in a subxact. But the
sequence might have been created in the same top-level transaction. So
we ensure the XID is assigned when WAL-logging increments.

The other change is addition of "created" flag, marking increments for
newly created relfilenodes. This makes it easier to maintain the hash
table of sequences that need transactional handling.
Note: This is needed because of subxacts. A XID 0 might still have the
sequence created in a different subxact of the same top-level xact.

This does not include any changes to test_decoding and/or the built-in
replication - those will be committed in separate patches.

A patch adding decoding of sequences was originally submitted by Cary
Huang. This commit reworks various important aspects (e.g. the WAL
logging and transactional/non-transactional handling). However, the
original patch and reviews were very useful.

Author: Tomas Vondra, Cary Huang
Reviewed-by: Peter Eisentraut, Hannu Krosing, Andres Freund
Discussion: https://postgr.es/m/d045f3c2-6cfb-06d3-5540-e63c320df8bc@enterprisedb.com
Discussion: https://postgr.es/m/1710ed7e13b.cd7177461430746.3372264562543607781@highgo.ca

4 files changed, 654 insertions, 0 deletions

diff --git a/src/backend/commands/sequence.c b/src/backend/commands/sequence.c
index 27cb6307581..ab592ce2f15 100644
--- a/src/backend/commands/sequence.c
+++ b/src/backend/commands/sequence.c
@@ -378,8 +378,13 @@ fill_seq_with_data(Relation rel, HeapTuple tuple)
 
 	/* check the comment above nextval_internal()'s equivalent call. */
 	if (RelationNeedsWAL(rel))
+	{
 		GetTopTransactionId();
 
+		if (XLogLogicalInfoActive())
+			GetCurrentTransactionId();
+	}
+
 	START_CRIT_SECTION();
 
 	MarkBufferDirty(buf);
@@ -399,6 +404,7 @@ fill_seq_with_data(Relation rel, HeapTuple tuple)
 		XLogRegisterBuffer(0, buf, REGBUF_WILL_INIT);
 
 		xlrec.node = rel->rd_node;
+		xlrec.created = true;
 
 		XLogRegisterData((char *) &xlrec, sizeof(xl_seq_rec));
 		XLogRegisterData((char *) tuple->t_data, tuple->t_len);
@@ -764,10 +770,28 @@ nextval_internal(Oid relid, bool check_permissions)
 	 * It's sufficient to ensure the toplevel transaction has an xid, no need
 	 * to assign xids subxacts, that'll already trigger an appropriate wait.
 	 * (Have to do that here, so we're outside the critical section)
+	 *
+	 * We have to ensure we have a proper XID, which will be included in
+	 * the XLOG record by XLogRecordAssemble. Otherwise the first nextval()
+	 * in a subxact (without any preceding changes) would get XID 0, and it
+	 * would then be impossible to decide which top xact it belongs to.
+	 * It'd also trigger assert in DecodeSequence. We only do that with
+	 * wal_level=logical, though.
+	 *
+	 * XXX This might seem unnecessary, because if there's no XID the xact
+	 * couldn't have done anything important yet, e.g. it could not have
+	 * created a sequence. But that's incorrect, because of subxacts. The
+	 * current subtransaction might not have done anything yet (thus no XID),
+	 * but an earlier one might have created the sequence.
 	 */
 	if (logit && RelationNeedsWAL(seqrel))
+	{
 		GetTopTransactionId();
 
+		if (XLogLogicalInfoActive())
+			GetCurrentTransactionId();
+	}
+
 	/* ready to change the on-disk (or really, in-buffer) tuple */
 	START_CRIT_SECTION();
 
@@ -803,6 +827,7 @@ nextval_internal(Oid relid, bool check_permissions)
 		seq->log_cnt = 0;
 
 		xlrec.node = seqrel->rd_node;
+		xlrec.created = false;
 
 		XLogRegisterData((char *) &xlrec, sizeof(xl_seq_rec));
 		XLogRegisterData((char *) seqdatatuple.t_data, seqdatatuple.t_len);
@@ -977,8 +1002,13 @@ do_setval(Oid relid, int64 next, bool iscalled)
 
 	/* check the comment above nextval_internal()'s equivalent call. */
 	if (RelationNeedsWAL(seqrel))
+	{
 		GetTopTransactionId();
 
+		if (XLogLogicalInfoActive())
+			GetCurrentTransactionId();
+	}
+
 	/* ready to change the on-disk (or really, in-buffer) tuple */
 	START_CRIT_SECTION();
 
@@ -999,6 +1029,8 @@ do_setval(Oid relid, int64 next, bool iscalled)
 		XLogRegisterBuffer(0, buf, REGBUF_WILL_INIT);
 
 		xlrec.node = seqrel->rd_node;
+		xlrec.created = false;
+
 		XLogRegisterData((char *) &xlrec, sizeof(xl_seq_rec));
 		XLogRegisterData((char *) seqdatatuple.t_data, seqdatatuple.t_len);
 
diff --git a/src/backend/replication/logical/decode.c b/src/backend/replication/logical/decode.c
index 3fb5a92a1a1..18cf9318221 100644
--- a/src/backend/replication/logical/decode.c
+++ b/src/backend/replication/logical/decode.c
@@ -42,6 +42,7 @@
 #include "replication/reorderbuffer.h"
 #include "replication/snapbuild.h"
 #include "storage/standby.h"
+#include "commands/sequence.h"
 
 /* individual record(group)'s handlers */
 static void DecodeInsert(LogicalDecodingContext *ctx, XLogRecordBuffer *buf);
@@ -63,6 +64,7 @@ static void DecodePrepare(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
 
 /* common function to decode tuples */
 static void DecodeXLogTuple(char *data, Size len, ReorderBufferTupleBuf *tup);
+static void DecodeSeqTuple(char *data, Size len, ReorderBufferTupleBuf *tuple);
 
 /* helper functions for decoding transactions */
 static inline bool FilterPrepare(LogicalDecodingContext *ctx,
@@ -1250,3 +1252,130 @@ DecodeTXNNeedSkip(LogicalDecodingContext *ctx, XLogRecordBuffer *buf,
 			(txn_dbid != InvalidOid && txn_dbid != ctx->slot->data.database) ||
 			ctx->fast_forward || FilterByOrigin(ctx, origin_id));
 }
+
+/*
+ * DecodeSeqTuple
+ *		decode tuple describing the sequence increment
+ *
+ * Sequences are represented as a table with a single row, which gets updated
+ * by nextval(). The tuple is stored in WAL right after the xl_seq_rec, so we
+ * simply copy it into the tuplebuf (similar to seq_redo).
+ */
+static void
+DecodeSeqTuple(char *data, Size len, ReorderBufferTupleBuf *tuple)
+{
+	int			datalen = len - sizeof(xl_seq_rec) - SizeofHeapTupleHeader;
+
+	Assert(datalen >= 0);
+
+	tuple->tuple.t_len = datalen + SizeofHeapTupleHeader;
+
+	ItemPointerSetInvalid(&tuple->tuple.t_self);
+
+	tuple->tuple.t_tableOid = InvalidOid;
+
+	memcpy(((char *) tuple->tuple.t_data),
+		   data + sizeof(xl_seq_rec),
+		   SizeofHeapTupleHeader);
+
+	memcpy(((char *) tuple->tuple.t_data) + SizeofHeapTupleHeader,
+		   data + sizeof(xl_seq_rec) + SizeofHeapTupleHeader,
+		   datalen);
+}
+
+/*
+ * Handle sequence decode
+ *
+ * Decoding sequences is a bit tricky, because while most sequence actions
+ * are non-transactional (not subject to rollback), some need to be handled
+ * as transactional.
+ *
+ * By default, a sequence increment is non-transactional - we must not queue
+ * it in a transaction as other changes, because the transaction might get
+ * rolled back and we'd discard the increment. The downstream would not be
+ * notified about the increment, which is wrong.
+ *
+ * On the other hand, the sequence may be created in a transaction. In this
+ * case we *should* queue the change as other changes in the transaction,
+ * because we don't want to send the increments for unknown sequence to the
+ * plugin - it might get confused about which sequence it's related to etc.
+ */
+void
+sequence_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
+{
+	SnapBuild  *builder = ctx->snapshot_builder;
+	ReorderBufferTupleBuf *tuplebuf;
+	RelFileNode target_node;
+	XLogReaderState *r = buf->record;
+	char	   *tupledata = NULL;
+	Size		tuplelen;
+	Size		datalen = 0;
+	TransactionId xid = XLogRecGetXid(r);
+	uint8		info = XLogRecGetInfo(buf->record) & ~XLR_INFO_MASK;
+	xl_seq_rec *xlrec;
+	Snapshot	snapshot;
+	RepOriginId origin_id = XLogRecGetOrigin(r);
+	bool		transactional;
+
+	/* only decode changes flagged with XLOG_SEQ_LOG */
+	if (info != XLOG_SEQ_LOG)
+		elog(ERROR, "unexpected RM_SEQ_ID record type: %u", info);
+
+	/*
+	 * If we don't have snapshot or we are just fast-forwarding, there is no
+	 * point in decoding messages.
+	 */
+	if (SnapBuildCurrentState(builder) < SNAPBUILD_FULL_SNAPSHOT ||
+		ctx->fast_forward)
+		return;
+
+	/* only interested in our database */
+	XLogRecGetBlockTag(r, 0, &target_node, NULL, NULL);
+	if (target_node.dbNode != ctx->slot->data.database)
+		return;
+
+	/* output plugin doesn't look for this origin, no need to queue */
+	if (FilterByOrigin(ctx, XLogRecGetOrigin(r)))
+		return;
+
+	tupledata = XLogRecGetData(r);
+	datalen = XLogRecGetDataLen(r);
+	tuplelen = datalen - SizeOfHeapHeader - sizeof(xl_seq_rec);
+
+	/* extract the WAL record, with "created" flag */
+	xlrec = (xl_seq_rec *) XLogRecGetData(r);
+
+	/* XXX how could we have sequence change without data? */
+	if(!datalen || !tupledata)
+		return;
+
+	tuplebuf = ReorderBufferGetTupleBuf(ctx->reorder, tuplelen);
+	DecodeSeqTuple(tupledata, datalen, tuplebuf);
+
+	/*
+	 * Should we handle the sequence increment as transactional or not?
+	 *
+	 * If the sequence was created in a still-running transaction, treat
+	 * it as transactional and queue the increments. Otherwise it needs
+	 * to be treated as non-transactional, in which case we send it to
+	 * the plugin right away.
+	 */
+	transactional = ReorderBufferSequenceIsTransactional(ctx->reorder,
+														 target_node,
+														 xlrec->created);
+
+	/* Skip the change if already processed (per the snapshot). */
+	if (transactional &&
+		!SnapBuildProcessChange(builder, xid, buf->origptr))
+		return;
+	else if (!transactional &&
+			 (SnapBuildCurrentState(builder) != SNAPBUILD_CONSISTENT ||
+			  SnapBuildXactNeedsSkip(builder, buf->origptr)))
+		return;
+
+	/* Queue the increment (or send immediately if not transactional). */
+	snapshot = SnapBuildGetOrBuildSnapshot(builder, xid);
+	ReorderBufferQueueSequence(ctx->reorder, xid, snapshot, buf->endptr,
+							   origin_id, target_node, transactional,
+							   xlrec->created, tuplebuf);
+}
diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index 9bc3a2d8deb..934aa13f2d3 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -73,6 +73,10 @@ static void truncate_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 static void message_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 							   XLogRecPtr message_lsn, bool transactional,
 							   const char *prefix, Size message_size, const char *message);
+static void sequence_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
+								XLogRecPtr sequence_lsn, Relation rel,
+								bool transactional,
+								int64 last_value, int64 log_cnt, bool is_called);
 
 /* streaming callbacks */
 static void stream_start_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
@@ -90,6 +94,10 @@ static void stream_change_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn
 static void stream_message_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 									  XLogRecPtr message_lsn, bool transactional,
 									  const char *prefix, Size message_size, const char *message);
+static void stream_sequence_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
+									   XLogRecPtr sequence_lsn, Relation rel,
+									   bool transactional,
+									   int64 last_value, int64 log_cnt, bool is_called);
 static void stream_truncate_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 									   int nrelations, Relation relations[], ReorderBufferChange *change);
 
@@ -218,6 +226,7 @@ StartupDecodingContext(List *output_plugin_options,
 	ctx->reorder->apply_truncate = truncate_cb_wrapper;
 	ctx->reorder->commit = commit_cb_wrapper;
 	ctx->reorder->message = message_cb_wrapper;
+	ctx->reorder->sequence = sequence_cb_wrapper;
 
 	/*
 	 * To support streaming, we require start/stop/abort/commit/change
@@ -234,6 +243,7 @@ StartupDecodingContext(List *output_plugin_options,
 		(ctx->callbacks.stream_commit_cb != NULL) ||
 		(ctx->callbacks.stream_change_cb != NULL) ||
 		(ctx->callbacks.stream_message_cb != NULL) ||
+		(ctx->callbacks.stream_sequence_cb != NULL) ||
 		(ctx->callbacks.stream_truncate_cb != NULL);
 
 	/*
@@ -251,6 +261,7 @@ StartupDecodingContext(List *output_plugin_options,
 	ctx->reorder->stream_commit = stream_commit_cb_wrapper;
 	ctx->reorder->stream_change = stream_change_cb_wrapper;
 	ctx->reorder->stream_message = stream_message_cb_wrapper;
+	ctx->reorder->stream_sequence = stream_sequence_cb_wrapper;
 	ctx->reorder->stream_truncate = stream_truncate_cb_wrapper;
 
 
@@ -1204,6 +1215,42 @@ message_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 }
 
 static void
+sequence_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
+					XLogRecPtr sequence_lsn, Relation rel, bool transactional,
+					int64 last_value, int64 log_cnt, bool is_called)
+{
+	LogicalDecodingContext *ctx = cache->private_data;
+	LogicalErrorCallbackState state;
+	ErrorContextCallback errcallback;
+
+	Assert(!ctx->fast_forward);
+
+	if (ctx->callbacks.sequence_cb == NULL)
+		return;
+
+	/* Push callback + info on the error context stack */
+	state.ctx = ctx;
+	state.callback_name = "sequence";
+	state.report_location = sequence_lsn;
+	errcallback.callback = output_plugin_error_callback;
+	errcallback.arg = (void *) &state;
+	errcallback.previous = error_context_stack;
+	error_context_stack = &errcallback;
+
+	/* set output state */
+	ctx->accept_writes = true;
+	ctx->write_xid = txn != NULL ? txn->xid : InvalidTransactionId;
+	ctx->write_location = sequence_lsn;
+
+	/* do the actual work: call callback */
+	ctx->callbacks.sequence_cb(ctx, txn, sequence_lsn, rel, transactional,
+							   last_value, log_cnt, is_called);
+
+	/* Pop the error context stack */
+	error_context_stack = errcallback.previous;
+}
+
+static void
 stream_start_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 						XLogRecPtr first_lsn)
 {
@@ -1509,6 +1556,47 @@ stream_message_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 }
 
 static void
+stream_sequence_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
+						   XLogRecPtr sequence_lsn, Relation rel,
+						   bool transactional,
+						   int64 last_value, int64 log_cnt, bool is_called)
+{
+	LogicalDecodingContext *ctx = cache->private_data;
+	LogicalErrorCallbackState state;
+	ErrorContextCallback errcallback;
+
+	Assert(!ctx->fast_forward);
+
+	/* We're only supposed to call this when streaming is supported. */
+	Assert(ctx->streaming);
+
+	/* this callback is optional */
+	if (ctx->callbacks.stream_sequence_cb == NULL)
+		return;
+
+	/* Push callback + info on the error context stack */
+	state.ctx = ctx;
+	state.callback_name = "stream_sequence";
+	state.report_location = sequence_lsn;
+	errcallback.callback = output_plugin_error_callback;
+	errcallback.arg = (void *) &state;
+	errcallback.previous = error_context_stack;
+	error_context_stack = &errcallback;
+
+	/* set output state */
+	ctx->accept_writes = true;
+	ctx->write_xid = txn != NULL ? txn->xid : InvalidTransactionId;
+	ctx->write_location = sequence_lsn;
+
+	/* do the actual work: call callback */
+	ctx->callbacks.sequence_cb(ctx, txn, sequence_lsn, rel, transactional,
+							   last_value, log_cnt, is_called);
+
+	/* Pop the error context stack */
+	error_context_stack = errcallback.previous;
+}
+
+static void
 stream_truncate_cb_wrapper(ReorderBuffer *cache, ReorderBufferTXN *txn,
 						   int nrelations, Relation relations[],
 						   ReorderBufferChange *change)
diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
index 19b2ba2000c..c2d9be81fae 100644
--- a/src/backend/replication/logical/reorderbuffer.c
+++ b/src/backend/replication/logical/reorderbuffer.c
@@ -77,6 +77,40 @@
  *	  a bit more memory to the oldest subtransactions, because it's likely
  *	  they are the source for the next sequence of changes.
  *
+ *	  When decoding sequences, we differentiate between a sequences created
+ *	  in a (running) transaction, and sequences created in other (already
+ *	  committed) transactions. Changes for sequences created in the same
+ *	  top-level transaction are treated as "transactional" i.e. just like
+ *	  any other change from that transaction (and discarded in case of a
+ *	  rollback). Changes for sequences created earlier are treated as not
+ *	  transactional - are processed immediately, as if performed outside
+ *	  any transaction (and thus not rolled back).
+ *
+ *	  This mixed behavior is necessary - sequences are non-transactional
+ *	  (e.g. ROLLBACK does not undo the sequence increments). But for new
+ *	  sequences, we need to handle them in a transactional way, because if
+ *	  we ever get some DDL support, the sequence won't exist until the
+ *	  transaction gets applied. So we need to ensure the increments don't
+ *	  happen until the sequence gets created.
+ *
+ *	  To differentiate which sequences are "old" and which were created
+ *	  in a still-running transaction, we track sequences created in running
+ *	  transactions in a hash table. Sequences are identified by relfilenode,
+ *	  and we track XID of the (sub)transaction that created it. This means
+ *	  that if a transaction does something that changes the relfilenode
+ *	  (like an alter / reset of a sequence), the new relfilenode will be
+ *	  treated as if created in the transaction. The list of sequences gets
+ *	  discarded when the transaction completes (commit/rollback).
+ *
+ *	  We don't use the XID to check if it's the same top-level transaction.
+ *	  It's enough to know it was created in an in-progress transaction,
+ *	  and we know it must be the current one because otherwise it wouldn't
+ *	  see the sequence object.
+ *
+ *	  The XID may be valid even for non-transactional sequences - we simply
+ *	  keep the XID logged to WAL, it's up to the reorderbuffer to decide if
+ *	  the increment is transactional.
+ *
  * -------------------------------------------------------------------------
  */
 #include "postgres.h"
@@ -91,6 +125,7 @@
 #include "access/xact.h"
 #include "access/xlog_internal.h"
 #include "catalog/catalog.h"
+#include "commands/sequence.h"
 #include "lib/binaryheap.h"
 #include "miscadmin.h"
 #include "pgstat.h"
@@ -116,6 +151,13 @@ typedef struct ReorderBufferTXNByIdEnt
 	ReorderBufferTXN *txn;
 } ReorderBufferTXNByIdEnt;
 
+/* entry for hash table we use to track sequences created in running xacts */
+typedef struct ReorderBufferSequenceEnt
+{
+	RelFileNode		rnode;
+	TransactionId	xid;
+} ReorderBufferSequenceEnt;
+
 /* data structures for (relfilenode, ctid) => (cmin, cmax) mapping */
 typedef struct ReorderBufferTupleCidKey
 {
@@ -339,6 +381,14 @@ ReorderBufferAllocate(void)
 	buffer->by_txn = hash_create("ReorderBufferByXid", 1000, &hash_ctl,
 								 HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
 
+	/* hash table of sequences, mapping relfilenode to XID of transaction */
+	hash_ctl.keysize = sizeof(RelFileNode);
+	hash_ctl.entrysize = sizeof(ReorderBufferSequenceEnt);
+	hash_ctl.hcxt = buffer->context;
+
+	buffer->sequences = hash_create("ReorderBufferSequenceHash", 1000, &hash_ctl,
+								 HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
+
 	buffer->by_txn_last_xid = InvalidTransactionId;
 	buffer->by_txn_last_txn = NULL;
 
@@ -525,6 +575,13 @@ ReorderBufferReturnChange(ReorderBuffer *rb, ReorderBufferChange *change,
 				change->data.truncate.relids = NULL;
 			}
 			break;
+		case REORDER_BUFFER_CHANGE_SEQUENCE:
+			if (change->data.sequence.tuple)
+			{
+				ReorderBufferReturnTupleBuf(rb, change->data.sequence.tuple);
+				change->data.sequence.tuple = NULL;
+			}
+			break;
 		case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_CONFIRM:
 		case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_ABORT:
 		case REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID:
@@ -860,6 +917,230 @@ ReorderBufferQueueMessage(ReorderBuffer *rb, TransactionId xid,
 }
 
 /*
+ * Treat the sequence increment as transactional?
+ *
+ * The hash table tracks all sequences created in in-progress transactions,
+ * so we simply do a lookup (the sequence is identified by relfilende). If
+ * we find a match, the increment should be handled as transactional.
+ */
+bool
+ReorderBufferSequenceIsTransactional(ReorderBuffer *rb,
+									 RelFileNode rnode, bool created)
+{
+	bool	found = false;
+
+	if (created)
+		return true;
+
+	hash_search(rb->sequences,
+				(void *) &rnode,
+				HASH_FIND,
+				&found);
+
+	return found;
+}
+
+/*
+ * Cleanup sequences created in in-progress transactions.
+ *
+ * There's no way to search by XID, so we simply do a seqscan of all
+ * the entries in the hash table. Hopefully there are only a couple
+ * entries in most cases - people generally don't create many new
+ * sequences over and over.
+ */
+static void
+ReorderBufferSequenceCleanup(ReorderBuffer *rb, TransactionId xid)
+{
+	HASH_SEQ_STATUS scan_status;
+	ReorderBufferSequenceEnt *ent;
+
+	hash_seq_init(&scan_status, rb->sequences);
+	while ((ent = (ReorderBufferSequenceEnt *) hash_seq_search(&scan_status)) != NULL)
+	{
+		/* skip sequences not from this transaction */
+		if (ent->xid != xid)
+			continue;
+
+		(void) hash_search(rb->sequences,
+					   (void *) &(ent->rnode),
+					   HASH_REMOVE, NULL);
+	}
+}
+
+/*
+ * A transactional sequence increment is queued to be processed upon commit
+ * and a non-transactional increment gets processed immediately.
+ *
+ * A sequence update may be both transactional and non-transactional. When
+ * created in a running transaction, treat it as transactional and queue
+ * the change in it. Otherwise treat it as non-transactional, so that we
+ * don't forget the increment in case of a rollback.
+ */
+void
+ReorderBufferQueueSequence(ReorderBuffer *rb, TransactionId xid,
+						   Snapshot snapshot, XLogRecPtr lsn, RepOriginId origin_id,
+						   RelFileNode rnode, bool transactional, bool created,
+						   ReorderBufferTupleBuf *tuplebuf)
+{
+	/*
+	 * Change needs to be handled as transactional, because the sequence was
+	 * created in a transaction that is still running. In that case all the
+	 * changes need to be queued in that transaction, we must not send them
+	 * to the downstream until the transaction commits.
+	 *
+	 * There's a bit of a trouble with subtransactions - we can't queue it
+	 * into the subxact, because it might be rolled back and we'd lose the
+	 * increment. We need to queue it into the same (sub)xact that created
+	 * the sequence, which is why we track the XID in the hash table.
+	 */
+	if (transactional)
+	{
+		MemoryContext oldcontext;
+		ReorderBufferChange *change;
+
+		/* lookup sequence by relfilenode */
+		ReorderBufferSequenceEnt   *ent;
+		bool						found;
+
+		/* transactional changes require a transaction */
+		Assert(xid != InvalidTransactionId);
+
+		/* search the lookup table (we ignore the return value, found is enough) */
+		ent = hash_search(rb->sequences,
+						  (void *) &rnode,
+						  created ? HASH_ENTER : HASH_FIND,
+						  &found);
+
+		/*
+		 * If this is the "create" increment, we must not have found any
+		 * pre-existing entry in the hash table (i.e. there must not be
+		 * any conflicting sequence).
+		 */
+		Assert(!(created && found));
+
+		/* But we must have either created or found an existing entry. */
+		Assert(created || found);
+
+		/*
+		 * When creating the sequence, remember the XID of the transaction
+		 * that created id.
+		 */
+		if (created)
+			ent->xid = xid;
+
+		/* XXX Maybe check that we're still in the same top-level xact? */
+
+		/* OK, allocate and queue the change */
+		oldcontext = MemoryContextSwitchTo(rb->context);
+
+		change = ReorderBufferGetChange(rb);
+
+		change->action = REORDER_BUFFER_CHANGE_SEQUENCE;
+		change->origin_id = origin_id;
+
+		memcpy(&change->data.sequence.relnode, &rnode, sizeof(RelFileNode));
+
+		change->data.sequence.tuple = tuplebuf;
+
+		/* add it to the same subxact that created the sequence */
+		ReorderBufferQueueChange(rb, ent->xid, lsn, change, false);
+
+		MemoryContextSwitchTo(oldcontext);
+	}
+	else
+	{
+		/*
+		 * This increment is for a sequence that was not created in any
+		 * running transaction, so we treat it as non-transactional and
+		 * just send it to the output plugin directly.
+		 */
+		ReorderBufferTXN *txn = NULL;
+		volatile Snapshot snapshot_now = snapshot;
+		bool	using_subtxn;
+
+#ifdef USE_ASSERT_CHECKING
+		/* All "creates" have to be handled as transactional. */
+		Assert(!created);
+
+		/* Make sure the sequence is not in the hash table. */
+		{
+			bool	found;
+			hash_search(rb->sequences,
+						(void *) &rnode,
+						HASH_FIND, &found);
+			Assert(!found);
+		}
+#endif
+
+		if (xid != InvalidTransactionId)
+			txn = ReorderBufferTXNByXid(rb, xid, true, NULL, lsn, true);
+
+		/* setup snapshot to allow catalog access */
+		SetupHistoricSnapshot(snapshot_now, NULL);
+
+		/*
+		 * Decoding needs access to syscaches et al., which in turn use
+		 * heavyweight locks and such. Thus we need to have enough state around to
+		 * keep track of those.  The easiest way is to simply use a transaction
+		 * internally.  That also allows us to easily enforce that nothing writes
+		 * to the database by checking for xid assignments.
+		 *
+		 * When we're called via the SQL SRF there's already a transaction
+		 * started, so start an explicit subtransaction there.
+		 */
+		using_subtxn = IsTransactionOrTransactionBlock();
+
+		PG_TRY();
+		{
+			Relation	relation;
+			HeapTuple	tuple;
+			Form_pg_sequence_data seq;
+			Oid			reloid;
+
+			if (using_subtxn)
+				BeginInternalSubTransaction("sequence");
+			else
+				StartTransactionCommand();
+
+			reloid = RelidByRelfilenode(rnode.spcNode, rnode.relNode);
+
+			if (reloid == InvalidOid)
+				elog(ERROR, "could not map filenode \"%s\" to relation OID",
+					 relpathperm(rnode,
+								 MAIN_FORKNUM));
+
+			relation = RelationIdGetRelation(reloid);
+			tuple = &tuplebuf->tuple;
+			seq = (Form_pg_sequence_data) GETSTRUCT(tuple);
+
+			rb->sequence(rb, txn, lsn, relation, transactional,
+						 seq->last_value, seq->log_cnt, seq->is_called);
+
+			RelationClose(relation);
+
+			TeardownHistoricSnapshot(false);
+
+			AbortCurrentTransaction();
+
+			if (using_subtxn)
+				RollbackAndReleaseCurrentSubTransaction();
+		}
+		PG_CATCH();
+		{
+			TeardownHistoricSnapshot(true);
+
+			AbortCurrentTransaction();
+
+			if (using_subtxn)
+				RollbackAndReleaseCurrentSubTransaction();
+
+			PG_RE_THROW();
+		}
+		PG_END_TRY();
+	}
+}
+
+/*
  * AssertTXNLsnOrder
  *		Verify LSN ordering of transaction lists in the reorderbuffer
  *
@@ -1535,6 +1816,9 @@ ReorderBufferCleanupTXN(ReorderBuffer *rb, ReorderBufferTXN *txn)
 				&found);
 	Assert(found);
 
+	/* Remove sequences created in this transaction (if any). */
+	ReorderBufferSequenceCleanup(rb, txn->xid);
+
 	/* remove entries spilled to disk */
 	if (rbtxn_is_serialized(txn))
 		ReorderBufferRestoreCleanup(rb, txn);
@@ -1951,6 +2235,29 @@ ReorderBufferApplyMessage(ReorderBuffer *rb, ReorderBufferTXN *txn,
 }
 
 /*
+ * Helper function for ReorderBufferProcessTXN for applying sequences.
+ */
+static inline void
+ReorderBufferApplySequence(ReorderBuffer *rb, ReorderBufferTXN *txn,
+						   Relation relation, ReorderBufferChange *change,
+						   bool streaming)
+{
+	HeapTuple	tuple;
+	Form_pg_sequence_data seq;
+
+	tuple = &change->data.sequence.tuple->tuple;
+	seq = (Form_pg_sequence_data) GETSTRUCT(tuple);
+
+	/* Only ever called from ReorderBufferApplySequence, so transational. */
+	if (streaming)
+		rb->stream_sequence(rb, txn, change->lsn, relation, true,
+							seq->last_value, seq->log_cnt, seq->is_called);
+	else
+		rb->sequence(rb, txn, change->lsn, relation, true,
+					 seq->last_value, seq->log_cnt, seq->is_called);
+}
+
+/*
  * Function to store the command id and snapshot at the end of the current
  * stream so that we can reuse the same while sending the next stream.
  */
@@ -2392,6 +2699,31 @@ ReorderBufferProcessTXN(ReorderBuffer *rb, ReorderBufferTXN *txn,
 				case REORDER_BUFFER_CHANGE_INTERNAL_TUPLECID:
 					elog(ERROR, "tuplecid value in changequeue");
 					break;
+
+				case REORDER_BUFFER_CHANGE_SEQUENCE:
+					Assert(snapshot_now);
+
+					reloid = RelidByRelfilenode(change->data.sequence.relnode.spcNode,
+												change->data.sequence.relnode.relNode);
+
+					if (reloid == InvalidOid)
+						elog(ERROR, "could not map filenode \"%s\" to relation OID",
+							 relpathperm(change->data.sequence.relnode,
+										 MAIN_FORKNUM));
+
+					relation = RelationIdGetRelation(reloid);
+
+					if (!RelationIsValid(relation))
+						elog(ERROR, "could not open relation with OID %u (for filenode \"%s\")",
+							 reloid,
+							 relpathperm(change->data.sequence.relnode,
+										 MAIN_FORKNUM));
+
+					if (RelationIsLogicallyLogged(relation))
+						ReorderBufferApplySequence(rb, txn, relation, change, streaming);
+
+					RelationClose(relation);
+					break;
 			}
 		}
 
@@ -3778,6 +4110,39 @@ ReorderBufferSerializeChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 
 				break;
 			}
+		case REORDER_BUFFER_CHANGE_SEQUENCE:
+			{
+				char	   *data;
+				ReorderBufferTupleBuf *tup;
+				Size		len = 0;
+
+				tup = change->data.sequence.tuple;
+
+				if (tup)
+				{
+					sz += sizeof(HeapTupleData);
+					len = tup->tuple.t_len;
+					sz += len;
+				}
+
+				/* make sure we have enough space */
+				ReorderBufferSerializeReserve(rb, sz);
+
+				data = ((char *) rb->outbuf) + sizeof(ReorderBufferDiskChange);
+				/* might have been reallocated above */
+				ondisk = (ReorderBufferDiskChange *) rb->outbuf;
+
+				if (len)
+				{
+					memcpy(data, &tup->tuple, sizeof(HeapTupleData));
+					data += sizeof(HeapTupleData);
+
+					memcpy(data, tup->tuple.t_data, len);
+					data += len;
+				}
+
+				break;
+			}
 		case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_CONFIRM:
 		case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_ABORT:
 		case REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID:
@@ -4042,6 +4407,22 @@ ReorderBufferChangeSize(ReorderBufferChange *change)
 
 				break;
 			}
+		case REORDER_BUFFER_CHANGE_SEQUENCE:
+			{
+				ReorderBufferTupleBuf *tup;
+				Size		len = 0;
+
+				tup = change->data.sequence.tuple;
+
+				if (tup)
+				{
+					sz += sizeof(HeapTupleData);
+					len = tup->tuple.t_len;
+					sz += len;
+				}
+
+				break;
+			}
 		case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_CONFIRM:
 		case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_ABORT:
 		case REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID:
@@ -4341,6 +4722,30 @@ ReorderBufferRestoreChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 
 				break;
 			}
+
+		case REORDER_BUFFER_CHANGE_SEQUENCE:
+			if (change->data.sequence.tuple)
+			{
+				uint32		tuplelen = ((HeapTuple) data)->t_len;
+
+				change->data.sequence.tuple =
+					ReorderBufferGetTupleBuf(rb, tuplelen - SizeofHeapTupleHeader);
+
+				/* restore ->tuple */
+				memcpy(&change->data.sequence.tuple->tuple, data,
+					   sizeof(HeapTupleData));
+				data += sizeof(HeapTupleData);
+
+				/* reset t_data pointer into the new tuplebuf */
+				change->data.sequence.tuple->tuple.t_data =
+					ReorderBufferTupleBufData(change->data.sequence.tuple);
+
+				/* restore tuple data itself */
+				memcpy(change->data.sequence.tuple->tuple.t_data, data, tuplelen);
+				data += tuplelen;
+			}
+			break;
+
 		case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_CONFIRM:
 		case REORDER_BUFFER_CHANGE_INTERNAL_SPEC_ABORT:
 		case REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID: