Replicate generated columns when specified in the column list.

This commit allows logical replication to publish and replicate generated
columns when explicitly listed in the column list. We also ensured that
the generated columns were copied during the initial tablesync when they
were published.

We will allow to replicate generated columns even when they are not
specified in the column list (via a new publication option) in a separate
commit.

The motivation of this work is to allow replication for cases where the
client doesn't have generated columns. For example, the case where one is
trying to replicate data from Postgres to the non-Postgres database.

Author: Shubham Khanna, Vignesh C, Hou Zhijie
Reviewed-by: Peter Smith, Hayato Kuroda, Shlok Kyal, Amit Kapila
Discussion: https://postgr.es/m/B80D17B2-2C8E-4C7D-87F2-E5B4BE3C069E@gmail.com

doc/src/sgml/protocol.sgml

@@ -6544,7 +6544,7 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
 
      <para>
       Next, the following message part appears for each column included in
-      the publication (except generated columns):
+      the publication:
      </para>
 
      <variablelist>
@@ -7477,7 +7477,7 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
      </variablelist>
 
      <para>
-      Next, one of the following submessages appears for each column (except generated columns):
+      Next, one of the following submessages appears for each column:
 
       <variablelist>
        <varlistentry>

doc/src/sgml/ref/create_publication.sgml

@@ -89,7 +89,8 @@ CREATE PUBLICATION <replaceable class="parameter">name</replaceable>
 
      <para>
       When a column list is specified, only the named columns are replicated.
-      If no column list is specified, all columns of the table are replicated
+      The column list can contain generated columns as well. If no column list
+      is specified, all table columns (except generated columns) are replicated
       through this publication, including any columns added later. It has no
       effect on <literal>TRUNCATE</literal> commands. See
       <xref linkend="logical-replication-col-lists"/> for details about column

src/backend/catalog/pg_publication.c

@@ -500,8 +500,7 @@ publication_add_relation(Oid pubid, PublicationRelInfo *pri,
  * pub_collist_validate
  *		Process and validate the 'columns' list and ensure the columns are all
  *		valid to use for a publication.  Checks for and raises an ERROR for
- * 		any; unknown columns, system columns, duplicate columns or generated
- *		columns.
+ * 		any unknown columns, system columns, or duplicate columns.
  *
  * Looks up each column's attnum and returns a 0-based Bitmapset of the
  * corresponding attnums.
@@ -511,7 +510,6 @@ pub_collist_validate(Relation targetrel, List *columns)
 {
 	Bitmapset  *set = NULL;
 	ListCell   *lc;
-	TupleDesc	tupdesc = RelationGetDescr(targetrel);
 
 	foreach(lc, columns)
 	{
@@ -530,12 +528,6 @@ pub_collist_validate(Relation targetrel, List *columns)
 					errmsg("cannot use system column \"%s\" in publication column list",
 						   colname));
 
-		if (TupleDescAttr(tupdesc, attnum - 1)->attgenerated)
-			ereport(ERROR,
-					errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
-					errmsg("cannot use generated column \"%s\" in publication column list",
-						   colname));
-
 		if (bms_is_member(attnum, set))
 			ereport(ERROR,
 					errcode(ERRCODE_DUPLICATE_OBJECT),

src/backend/replication/logical/proto.c

@@ -40,19 +40,6 @@ static void logicalrep_read_tuple(StringInfo in, LogicalRepTupleData *tuple);
 static void logicalrep_write_namespace(StringInfo out, Oid nspid);
 static const char *logicalrep_read_namespace(StringInfo in);
 
-/*
- * Check if a column is covered by a column list.
- *
- * Need to be careful about NULL, which is treated as a column list covering
- * all columns.
- */
-static bool
-column_in_column_list(int attnum, Bitmapset *columns)
-{
-	return (columns == NULL || bms_is_member(attnum, columns));
-}
-
-
 /*
  * Write BEGIN to the output stream.
  */
@@ -781,10 +768,7 @@ logicalrep_write_tuple(StringInfo out, Relation rel, TupleTableSlot *slot,
 	{
 		Form_pg_attribute att = TupleDescAttr(desc, i);
 
-		if (att->attisdropped || att->attgenerated)
-			continue;
-
-		if (!column_in_column_list(att->attnum, columns))
+		if (!logicalrep_should_publish_column(att, columns))
 			continue;
 
 		nliveatts++;
@@ -802,10 +786,7 @@ logicalrep_write_tuple(StringInfo out, Relation rel, TupleTableSlot *slot,
 		Form_pg_type typclass;
 		Form_pg_attribute att = TupleDescAttr(desc, i);
 
-		if (att->attisdropped || att->attgenerated)
-			continue;
-
-		if (!column_in_column_list(att->attnum, columns))
+		if (!logicalrep_should_publish_column(att, columns))
 			continue;
 
 		if (isnull[i])
@@ -938,10 +919,7 @@ logicalrep_write_attrs(StringInfo out, Relation rel, Bitmapset *columns)
 	{
 		Form_pg_attribute att = TupleDescAttr(desc, i);
 
-		if (att->attisdropped || att->attgenerated)
-			continue;
-
-		if (!column_in_column_list(att->attnum, columns))
+		if (!logicalrep_should_publish_column(att, columns))
 			continue;
 
 		nliveatts++;
@@ -959,10 +937,7 @@ logicalrep_write_attrs(StringInfo out, Relation rel, Bitmapset *columns)
 		Form_pg_attribute att = TupleDescAttr(desc, i);
 		uint8		flags = 0;
 
-		if (att->attisdropped || att->attgenerated)
-			continue;
-
-		if (!column_in_column_list(att->attnum, columns))
+		if (!logicalrep_should_publish_column(att, columns))
 			continue;
 
 		/* REPLICA IDENTITY FULL means all columns are sent as part of key. */
@@ -1269,3 +1244,33 @@ logicalrep_message_type(LogicalRepMsgType action)
 
 	return err_unknown;
 }
+
+/*
+ * Check if the column 'att' of a table should be published.
+ *
+ * 'columns' represents the column list specified for that table in the
+ * publication.
+ *
+ * Note that generated columns can be present only in 'columns' list.
+ */
+bool
+logicalrep_should_publish_column(Form_pg_attribute att, Bitmapset *columns)
+{
+	if (att->attisdropped)
+		return false;
+
+	/*
+	 * Skip publishing generated columns if they are not included in the
+	 * column list.
+	 */
+	if (!columns && att->attgenerated)
+		return false;
+
+	/*
+	 * Check if a column is covered by a column list.
+	 */
+	if (columns && !bms_is_member(att->attnum, columns))
+		return false;
+
+	return true;
+}

src/backend/replication/logical/tablesync.c

@@ -787,23 +787,27 @@ copy_read_data(void *outbuf, int minread, int maxread)
 
 /*
  * Get information about remote relation in similar fashion the RELATION
- * message provides during replication. This function also returns the relation
- * qualifications to be used in the COPY command.
+ * message provides during replication.
+ *
+ * This function also returns (a) the relation qualifications to be used in
+ * the COPY command, and (b) whether the remote relation has published any
+ * generated column.
  */
 static void
-fetch_remote_table_info(char *nspname, char *relname,
-						LogicalRepRelation *lrel, List **qual)
+fetch_remote_table_info(char *nspname, char *relname, LogicalRepRelation *lrel,
+						List **qual, bool *gencol_published)
 {
 	WalRcvExecResult *res;
 	StringInfoData cmd;
 	TupleTableSlot *slot;
 	Oid			tableRow[] = {OIDOID, CHAROID, CHAROID};
-	Oid			attrRow[] = {INT2OID, TEXTOID, OIDOID, BOOLOID};
+	Oid			attrRow[] = {INT2OID, TEXTOID, OIDOID, BOOLOID, BOOLOID};
 	Oid			qualRow[] = {TEXTOID};
 	bool		isnull;
 	int			natt;
 	StringInfo	pub_names = NULL;
 	Bitmapset  *included_cols = NULL;
+	int			server_version = walrcv_server_version(LogRepWorkerWalRcvConn);
 
 	lrel->nspname = nspname;
 	lrel->relname = relname;
@@ -851,7 +855,7 @@ fetch_remote_table_info(char *nspname, char *relname,
 	 * We need to do this before fetching info about column names and types,
 	 * so that we can skip columns that should not be replicated.
 	 */
-	if (walrcv_server_version(LogRepWorkerWalRcvConn) >= 150000)
+	if (server_version >= 150000)
 	{
 		WalRcvExecResult *pubres;
 		TupleTableSlot *tslot;
@@ -941,7 +945,13 @@ fetch_remote_table_info(char *nspname, char *relname,
 					 "SELECT a.attnum,"
 					 "       a.attname,"
 					 "       a.atttypid,"
-					 "       a.attnum = ANY(i.indkey)"
+					 "       a.attnum = ANY(i.indkey)");
+
+	/* Generated columns can be replicated since version 18. */
+	if (server_version >= 180000)
+		appendStringInfo(&cmd, ", a.attgenerated != ''");
+
+	appendStringInfo(&cmd,
 					 "  FROM pg_catalog.pg_attribute a"
 					 "  LEFT JOIN pg_catalog.pg_index i"
 					 "       ON (i.indexrelid = pg_get_replica_identity_index(%u))"
@@ -950,11 +960,11 @@ fetch_remote_table_info(char *nspname, char *relname,
 					 "   AND a.attrelid = %u"
 					 " ORDER BY a.attnum",
 					 lrel->remoteid,
-					 (walrcv_server_version(LogRepWorkerWalRcvConn) >= 120000 ?
+					 (server_version >= 120000 && server_version < 180000 ?
 					  "AND a.attgenerated = ''" : ""),
 					 lrel->remoteid);
 	res = walrcv_exec(LogRepWorkerWalRcvConn, cmd.data,
-					  lengthof(attrRow), attrRow);
+					  server_version >= 180000 ? lengthof(attrRow) : lengthof(attrRow) - 1, attrRow);
 
 	if (res->status != WALRCV_OK_TUPLES)
 		ereport(ERROR,
@@ -998,6 +1008,13 @@ fetch_remote_table_info(char *nspname, char *relname,
 		if (DatumGetBool(slot_getattr(slot, 4, &isnull)))
 			lrel->attkeys = bms_add_member(lrel->attkeys, natt);
 
+		/* Remember if the remote table has published any generated column. */
+		if (server_version >= 180000 && !(*gencol_published))
+		{
+			*gencol_published = DatumGetBool(slot_getattr(slot, 5, &isnull));
+			Assert(!isnull);
+		}
+
 		/* Should never happen. */
 		if (++natt >= MaxTupleAttributeNumber)
 			elog(ERROR, "too many columns in remote table \"%s.%s\"",
@@ -1030,7 +1047,7 @@ fetch_remote_table_info(char *nspname, char *relname,
 	 * 3) one of the subscribed publications is declared as TABLES IN SCHEMA
 	 * that includes this relation
 	 */
-	if (walrcv_server_version(LogRepWorkerWalRcvConn) >= 150000)
+	if (server_version >= 150000)
 	{
 		/* Reuse the already-built pub_names. */
 		Assert(pub_names != NULL);
@@ -1106,10 +1123,12 @@ copy_table(Relation rel)
 	List	   *attnamelist;
 	ParseState *pstate;
 	List	   *options = NIL;
+	bool		gencol_published = false;
 
 	/* Get the publisher relation info. */
 	fetch_remote_table_info(get_namespace_name(RelationGetNamespace(rel)),
-							RelationGetRelationName(rel), &lrel, &qual);
+							RelationGetRelationName(rel), &lrel, &qual,
+							&gencol_published);
 
 	/* Put the relation into relmap. */
 	logicalrep_relmap_update(&lrel);
@@ -1121,8 +1140,8 @@ copy_table(Relation rel)
 	/* Start copy on the publisher. */
 	initStringInfo(&cmd);
 
-	/* Regular table with no row filter */
-	if (lrel.relkind == RELKIND_RELATION && qual == NIL)
+	/* Regular table with no row filter or generated columns */
+	if (lrel.relkind == RELKIND_RELATION && qual == NIL && !gencol_published)
 	{
 		appendStringInfo(&cmd, "COPY %s",
 						 quote_qualified_identifier(lrel.nspname, lrel.relname));
@@ -1153,9 +1172,14 @@ copy_table(Relation rel)
 	{
 		/*
 		 * For non-tables and tables with row filters, we need to do COPY
-		 * (SELECT ...), but we can't just do SELECT * because we need to not
-		 * copy generated columns. For tables with any row filters, build a
-		 * SELECT query with OR'ed row filters for COPY.
+		 * (SELECT ...), but we can't just do SELECT * because we may need to
+		 * copy only subset of columns including generated columns. For tables
+		 * with any row filters, build a SELECT query with OR'ed row filters
+		 * for COPY.
+		 *
+		 * We also need to use this same COPY (SELECT ...) syntax when
+		 * generated columns are published, because copy of generated columns
+		 * is not supported by the normal COPY.
 		 */
 		appendStringInfoString(&cmd, "COPY (SELECT ");
 		for (int i = 0; i < lrel.natts; i++)

src/backend/replication/pgoutput/pgoutput.c

@@ -766,16 +766,12 @@ send_relation_and_attrs(Relation relation, TransactionId xid,
 	{
 		Form_pg_attribute att = TupleDescAttr(desc, i);
 
-		if (att->attisdropped || att->attgenerated)
+		if (!logicalrep_should_publish_column(att, columns))
 			continue;
 
 		if (att->atttypid < FirstGenbkiObjectId)
 			continue;
 
-		/* Skip this attribute if it's not present in the column list */
-		if (columns != NULL && !bms_is_member(att->attnum, columns))
-			continue;
-
 		OutputPluginPrepareWrite(ctx, false);
 		logicalrep_write_typ(ctx->out, xid, att->atttypid);
 		OutputPluginWrite(ctx, false);
@@ -1074,6 +1070,7 @@ pgoutput_column_list_init(PGOutputData *data, List *publications,
 					int			i;
 					int			nliveatts = 0;
 					TupleDesc	desc = RelationGetDescr(relation);
+					bool		att_gen_present = false;
 
 					pgoutput_ensure_entry_cxt(data, entry);
 
@@ -1085,17 +1082,30 @@ pgoutput_column_list_init(PGOutputData *data, List *publications,
 					{
 						Form_pg_attribute att = TupleDescAttr(desc, i);
 
-						if (att->attisdropped || att->attgenerated)
+						if (att->attisdropped)
 							continue;
 
+						if (att->attgenerated)
+						{
+							/*
+							 * Generated cols are skipped unless they are
+							 * present in a column list.
+							 */
+							if (!bms_is_member(att->attnum, cols))
+								continue;
+
+							att_gen_present = true;
+						}
+
 						nliveatts++;
 					}
 
 					/*
-					 * If column list includes all the columns of the table,
-					 * set it to NULL.
+					 * Generated attributes are published only when they are
+					 * present in the column list. Otherwise, a NULL column
+					 * list means publish all columns.
 					 */
-					if (bms_num_members(cols) == nliveatts)
+					if (!att_gen_present && bms_num_members(cols) == nliveatts)
 					{
 						bms_free(cols);
 						cols = NULL;

src/include/replication/logicalproto.h

@@ -270,5 +270,7 @@ extern void logicalrep_read_stream_abort(StringInfo in,
 										 LogicalRepStreamAbortData *abort_data,
 										 bool read_abort_info);
 extern const char *logicalrep_message_type(LogicalRepMsgType action);
+extern bool logicalrep_should_publish_column(Form_pg_attribute att,
+											 Bitmapset *columns);
 
 #endif							/* LOGICAL_PROTO_H */

src/test/regress/expected/publication.out

@@ -687,9 +687,6 @@ UPDATE testpub_tbl5 SET a = 1;
 ERROR:  cannot update table "testpub_tbl5"
 DETAIL:  Column list used by the publication does not cover the replica identity.
 ALTER PUBLICATION testpub_fortable DROP TABLE testpub_tbl5;
--- error: generated column "d" can't be in list
-ALTER PUBLICATION testpub_fortable ADD TABLE testpub_tbl5 (a, d);
-ERROR:  cannot use generated column "d" in publication column list
 -- error: system attributes "ctid" not allowed in column list
 ALTER PUBLICATION testpub_fortable ADD TABLE testpub_tbl5 (a, ctid);
 ERROR:  cannot use system column "ctid" in publication column list
@@ -717,6 +714,9 @@ UPDATE testpub_tbl5 SET a = 1;
 ERROR:  cannot update table "testpub_tbl5"
 DETAIL:  Column list used by the publication does not cover the replica identity.
 ALTER PUBLICATION testpub_fortable DROP TABLE testpub_tbl5;
+-- ok: generated column "d" can be in the list too
+ALTER PUBLICATION testpub_fortable ADD TABLE testpub_tbl5 (a, d);
+ALTER PUBLICATION testpub_fortable DROP TABLE testpub_tbl5;
 -- error: change the replica identity to "b", and column list to (a, c)
 -- then update fails, because (a, c) does not cover replica identity
 ALTER TABLE testpub_tbl5 REPLICA IDENTITY USING INDEX testpub_tbl5_b_key;

src/test/regress/sql/publication.sql

@@ -413,8 +413,6 @@ ALTER PUBLICATION testpub_fortable ADD TABLE testpub_tbl5 (a, x);
 ALTER PUBLICATION testpub_fortable ADD TABLE testpub_tbl5 (b, c);
 UPDATE testpub_tbl5 SET a = 1;
 ALTER PUBLICATION testpub_fortable DROP TABLE testpub_tbl5;
--- error: generated column "d" can't be in list
-ALTER PUBLICATION testpub_fortable ADD TABLE testpub_tbl5 (a, d);
 -- error: system attributes "ctid" not allowed in column list
 ALTER PUBLICATION testpub_fortable ADD TABLE testpub_tbl5 (a, ctid);
 ALTER PUBLICATION testpub_fortable SET TABLE testpub_tbl1 (id, ctid);
@@ -435,6 +433,10 @@ ALTER TABLE testpub_tbl5 REPLICA IDENTITY USING INDEX testpub_tbl5_b_key;
 UPDATE testpub_tbl5 SET a = 1;
 ALTER PUBLICATION testpub_fortable DROP TABLE testpub_tbl5;
 
+-- ok: generated column "d" can be in the list too
+ALTER PUBLICATION testpub_fortable ADD TABLE testpub_tbl5 (a, d);
+ALTER PUBLICATION testpub_fortable DROP TABLE testpub_tbl5;
+
 -- error: change the replica identity to "b", and column list to (a, c)
 -- then update fails, because (a, c) does not cover replica identity
 ALTER TABLE testpub_tbl5 REPLICA IDENTITY USING INDEX testpub_tbl5_b_key;

src/test/subscription/t/031_column_list.pl

@@ -1202,9 +1202,10 @@ $result = $node_publisher->safe_psql(
 is( $result, qq(t
 t), 'check the number of columns in the old tuple');
 
-# TEST: Generated and dropped columns are not considered for the column list.
-# So, the publication having a column list except for those columns and a
-# publication without any column (aka all columns as part of the columns
+# TEST: Dropped columns are not considered for the column list, and generated
+# columns are not replicated if they are not explicitly included in the column
+# list. So, the publication having a column list except for those columns and a
+# publication without any column list (aka all columns as part of the columns
 # list) are considered to have the same column list.
 $node_publisher->safe_psql(
 	'postgres', qq(
@@ -1275,6 +1276,40 @@ ok( $stderr =~
 	  qr/cannot use different column lists for table "public.test_mix_1" in different publications/,
 	'different column lists detected');
 
+# TEST: Generated columns are considered for the column list.
+$node_publisher->safe_psql(
+	'postgres', qq(
+	CREATE TABLE test_gen (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a + 1) STORED);
+	INSERT INTO test_gen VALUES (0);
+	CREATE PUBLICATION pub_gen FOR TABLE test_gen (a, b);
+));
+
+$node_subscriber->safe_psql(
+	'postgres', qq(
+	CREATE TABLE test_gen (a int PRIMARY KEY, b int);
+	CREATE SUBSCRIPTION sub_gen CONNECTION '$publisher_connstr' PUBLICATION pub_gen;
+));
+
+$node_subscriber->wait_for_subscription_sync;
+
+is( $node_subscriber->safe_psql(
+		'postgres', "SELECT * FROM test_gen ORDER BY a"),
+	qq(0|1),
+	'initial replication with generated columns in column list');
+
+$node_publisher->safe_psql(
+	'postgres', qq(
+	INSERT INTO test_gen VALUES (1);
+));
+
+$node_publisher->wait_for_catchup('sub_gen');
+
+is( $node_subscriber->safe_psql(
+		'postgres', "SELECT * FROM test_gen ORDER BY a"),
+	qq(0|1
+1|2),
+	'replication with generated columns in column list');
+
 # TEST: If the column list is changed after creating the subscription, we
 # should catch the error reported by walsender.