Improve error message for replication of generated columns.

Currently, logical replication produces a generic error message when
targeting a subscriber-side table column that is either missing or
generated. The error message can be misleading for generated columns.

This patch introduces a specific error message to clarify the issue when
generated columns are involved.

Author: Shubham Khanna
Reviewed-by: Peter Smith, Vignesh C, Amit Kapila
Discussion: https://postgr.es/m/CAHv8RjJBvYtqU7OAofBizOmQOK2Q8h+w9v2_cQWxT_gO7er3Aw@mail.gmail.com

src/backend/replication/logical/relation.c

@@ -220,41 +220,63 @@ logicalrep_rel_att_by_name(LogicalRepRelation *remoterel, const char *attname)
 }
 
 /*
- * Report error with names of the missing local relation column(s), if any.
+ * Returns a comma-separated string of attribute names based on the provided
+ * relation and bitmap indicating which attributes to include.
+ */
+static char *
+logicalrep_get_attrs_str(LogicalRepRelation *remoterel, Bitmapset *atts)
+{
+	StringInfoData attsbuf;
+	int			attcnt = 0;
+	int			i = -1;
+
+	Assert(!bms_is_empty(atts));
+
+	initStringInfo(&attsbuf);
+
+	while ((i = bms_next_member(atts, i)) >= 0)
+	{
+		attcnt++;
+		if (attcnt > 1)
+			appendStringInfo(&attsbuf, _(", "));
+
+		appendStringInfo(&attsbuf, _("\"%s\""), remoterel->attnames[i]);
+	}
+
+	return attsbuf.data;
+}
+
+/*
+ * If attempting to replicate missing or generated columns, report an error.
+ * Prioritize 'missing' errors if both occur though the prioritization is
+ * arbitrary.
  */
 static void
-logicalrep_report_missing_attrs(LogicalRepRelation *remoterel,
+logicalrep_report_missing_or_gen_attrs(LogicalRepRelation *remoterel,
-								Bitmapset *missingatts)
+									   Bitmapset *missingatts,
+									   Bitmapset *generatedatts)
 {
 	if (!bms_is_empty(missingatts))
-	{
-		StringInfoData missingattsbuf;
-		int			missingattcnt = 0;
-		int			i;
-
-		initStringInfo(&missingattsbuf);
-
-		i = -1;
-		while ((i = bms_next_member(missingatts, i)) >= 0)
-		{
-			missingattcnt++;
-			if (missingattcnt == 1)
-				appendStringInfo(&missingattsbuf, _("\"%s\""),
-								 remoterel->attnames[i]);
-			else
-				appendStringInfo(&missingattsbuf, _(", \"%s\""),
-								 remoterel->attnames[i]);
-		}
-
 		ereport(ERROR,
-				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-				 errmsg_plural("logical replication target relation \"%s.%s\" is missing replicated column: %s",
+				errmsg_plural("logical replication target relation \"%s.%s\" is missing replicated column: %s",
-							   "logical replication target relation \"%s.%s\" is missing replicated columns: %s",
+							  "logical replication target relation \"%s.%s\" is missing replicated columns: %s",
-							   missingattcnt,
+							  bms_num_members(missingatts),
-							   remoterel->nspname,
+							  remoterel->nspname,
-							   remoterel->relname,
+							  remoterel->relname,
-							   missingattsbuf.data)));
+							  logicalrep_get_attrs_str(remoterel,
-	}
+													   missingatts)));
+
+	if (!bms_is_empty(generatedatts))
+		ereport(ERROR,
+				errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				errmsg_plural("logical replication target relation \"%s.%s\" has incompatible generated column: %s",
+							  "logical replication target relation \"%s.%s\" has incompatible generated columns: %s",
+							  bms_num_members(generatedatts),
+							  remoterel->nspname,
+							  remoterel->relname,
+							  logicalrep_get_attrs_str(remoterel,
+													   generatedatts)));
 }
 
 /*
@@ -380,6 +402,7 @@ logicalrep_rel_open(LogicalRepRelId remoteid, LOCKMODE lockmode)
 		MemoryContext oldctx;
 		int			i;
 		Bitmapset  *missingatts;
+		Bitmapset  *generatedattrs = NULL;
 
 		/* Release the no-longer-useful attrmap, if any. */
 		if (entry->attrmap)
@@ -421,7 +444,7 @@ logicalrep_rel_open(LogicalRepRelId remoteid, LOCKMODE lockmode)
 			int			attnum;
 			Form_pg_attribute attr = TupleDescAttr(desc, i);
 
-			if (attr->attisdropped || attr->attgenerated)
+			if (attr->attisdropped)
 			{
 				entry->attrmap->attnums[i] = -1;
 				continue;
@@ -432,12 +455,20 @@ logicalrep_rel_open(LogicalRepRelId remoteid, LOCKMODE lockmode)
 
 			entry->attrmap->attnums[i] = attnum;
 			if (attnum >= 0)
+			{
+				/* Remember which subscriber columns are generated. */
+				if (attr->attgenerated)
+					generatedattrs = bms_add_member(generatedattrs, attnum);
+
 				missingatts = bms_del_member(missingatts, attnum);
+			}
 		}
 
-		logicalrep_report_missing_attrs(remoterel, missingatts);
+		logicalrep_report_missing_or_gen_attrs(remoterel, missingatts,
+											   generatedattrs);
 
 		/* be tidy */
+		bms_free(generatedattrs);
 		bms_free(missingatts);
 
 		/*

src/test/subscription/t/011_generated.pl

@@ -326,4 +326,46 @@ is( $result, qq(|2|
 $node_subscriber->safe_psql('postgres', "DROP SUBSCRIPTION sub1");
 $node_publisher->safe_psql('postgres', "DROP PUBLICATION pub1");
 
+# =============================================================================
+# The following test verifies the expected error when replicating to a
+# generated subscriber column. Test the following combinations:
+# - regular -> generated
+# - generated -> generated
+# =============================================================================
+
+# --------------------------------------------------
+# A "regular -> generated" or "generated -> generated" replication fails,
+# reporting an error that the generated column on the subscriber side cannot
+# be replicated.
+#
+# Test Case: regular -> generated and generated -> generated
+# Publisher table has regular column 'c2' and generated column 'c3'.
+# Subscriber table has generated columns 'c2' and 'c3'.
+# --------------------------------------------------
+
+# Create table and publication. Insert data into the table.
+$node_publisher->safe_psql(
+	'postgres', qq(
+	CREATE TABLE t1(c1 int, c2 int, c3 int GENERATED ALWAYS AS (c1 * 2) STORED);
+	CREATE PUBLICATION pub1 for table t1(c1, c2, c3);
+	INSERT INTO t1 VALUES (1);
+));
+
+# Create table and subscription.
+$node_subscriber->safe_psql(
+	'postgres', qq(
+	CREATE TABLE t1(c1 int, c2 int GENERATED ALWAYS AS (c1 + 2) STORED, c3 int GENERATED ALWAYS AS (c1 + 2) STORED);
+	CREATE SUBSCRIPTION sub1 CONNECTION '$publisher_connstr' PUBLICATION pub1;
+));
+
+# Verify that an error occurs.
+my $offset = -s $node_subscriber->logfile;
+$node_subscriber->wait_for_log(
+	qr/ERROR: ( [A-Z0-9]:)? logical replication target relation "public.t1" has incompatible generated columns: "c2", "c3"/,
+	$offset);
+
+# cleanup
+$node_subscriber->safe_psql('postgres', "DROP SUBSCRIPTION sub1");
+$node_publisher->safe_psql('postgres', "DROP PUBLICATION pub1");
+
 done_testing();