diff --git a/src/bin/pg_upgrade/check.c b/src/bin/pg_upgrade/check.c
index e7bf48a739..87b9d328d4 100644
--- a/src/bin/pg_upgrade/check.c
+++ b/src/bin/pg_upgrade/check.c
@@ -108,6 +108,14 @@ check_and_dump_old_cluster(bool live_check)
 	if (GET_MAJOR_VERSION(old_cluster.major_version) <= 1100)
 		check_for_tables_with_oids(&old_cluster);
 
+	/*
+	 * PG 12 changed the 'sql_identifier' type storage to be based on name,
+	 * not varchar, which breaks on-disk format for existing data. So we need
+	 * to prevent upgrade when used in user objects (tables, indexes, ...).
+	 */
+	if (GET_MAJOR_VERSION(old_cluster.major_version) <= 1100)
+		old_11_check_for_sql_identifier_data_type_usage(&old_cluster);
+
 	/*
 	 * Pre-PG 10 allowed tables with 'unknown' type columns and non WAL logged
 	 * hash indexes
diff --git a/src/bin/pg_upgrade/pg_upgrade.h b/src/bin/pg_upgrade/pg_upgrade.h
index f724ecf9ca..729f86aa32 100644
--- a/src/bin/pg_upgrade/pg_upgrade.h
+++ b/src/bin/pg_upgrade/pg_upgrade.h
@@ -451,6 +451,8 @@ void		old_9_6_check_for_unknown_data_type_usage(ClusterInfo *cluster);
 void		old_9_6_invalidate_hash_indexes(ClusterInfo *cluster,
 											bool check_mode);
 
+void		old_11_check_for_sql_identifier_data_type_usage(ClusterInfo *cluster);
+
 /* parallel.c */
 void		parallel_exec_prog(const char *log_file, const char *opt_log_file,
 							   const char *fmt,...) pg_attribute_printf(3, 4);
diff --git a/src/bin/pg_upgrade/version.c b/src/bin/pg_upgrade/version.c
index 42f1ce7757..f3fd8fa273 100644
--- a/src/bin/pg_upgrade/version.c
+++ b/src/bin/pg_upgrade/version.c
@@ -399,3 +399,122 @@ old_9_6_invalidate_hash_indexes(ClusterInfo *cluster, bool check_mode)
 	else
 		check_ok();
 }
+
+/*
+ * old_11_check_for_sql_identifier_data_type_usage()
+ *	11 -> 12
+ *	In 12, the sql_identifier data type was switched from name to varchar,
+ *	which does affect the storage (name is by-ref, but not varlena). This
+ *	means user tables using sql_identifier for columns are broken because
+ *	the on-disk format is different.
+ *
+ *	We need to check all objects that might store sql_identifier on disk,
+ *	i.e. tables, matviews and indexes. Also check composite types in case
+ *	they are used in this context.
+ */
+void
+old_11_check_for_sql_identifier_data_type_usage(ClusterInfo *cluster)
+{
+	int			dbnum;
+	FILE	   *script = NULL;
+	bool		found = false;
+	char		output_path[MAXPGPATH];
+
+	prep_status("Checking for invalid \"sql_identifier\" user columns");
+
+	snprintf(output_path, sizeof(output_path), "tables_using_sql_identifier.txt");
+
+	for (dbnum = 0; dbnum < cluster->dbarr.ndbs; dbnum++)
+	{
+		PGresult   *res;
+		bool		db_used = false;
+		int			ntups;
+		int			rowno;
+		int			i_nspname,
+					i_relname,
+					i_attname;
+		DbInfo	   *active_db = &cluster->dbarr.dbs[dbnum];
+		PGconn	   *conn = connectToServer(cluster, active_db->db_name);
+
+		/*
+		 * We need the recursive CTE because the sql_identifier may be wrapped
+		 * either in a domain or composite type, or both (in arbitrary order).
+		 */
+		res = executeQueryOrDie(conn,
+								"WITH RECURSIVE oids AS ( "
+		/* the sql_identifier type itself */
+								"	SELECT 'information_schema.sql_identifier'::regtype AS oid "
+								"	UNION ALL "
+								"	SELECT * FROM ( "
+		/* domains on the type */
+								"		WITH x AS (SELECT oid FROM oids) "
+								"			SELECT t.oid FROM pg_catalog.pg_type t, x WHERE typbasetype = x.oid AND typtype = 'd' "
+								"			UNION "
+		/* composite types containing the type */
+								"			SELECT t.oid FROM pg_catalog.pg_type t, pg_catalog.pg_class c, pg_catalog.pg_attribute a, x "
+								"			WHERE t.typtype = 'c' AND "
+								"				  t.oid = c.reltype AND "
+								"				  c.oid = a.attrelid AND "
+								"				  NOT a.attisdropped AND "
+								"				  a.atttypid = x.oid "
+								"	) foo "
+								") "
+								"SELECT n.nspname, c.relname, a.attname "
+								"FROM	pg_catalog.pg_class c, "
+								"		pg_catalog.pg_namespace n, "
+								"		pg_catalog.pg_attribute a "
+								"WHERE	c.oid = a.attrelid AND "
+								"		NOT a.attisdropped AND "
+								"		a.atttypid IN (SELECT oid FROM oids) AND "
+								"		c.relkind IN ("
+								CppAsString2(RELKIND_RELATION) ", "
+								CppAsString2(RELKIND_MATVIEW) ", "
+								CppAsString2(RELKIND_INDEX) ") AND "
+								"		c.relnamespace = n.oid AND "
+		/* exclude possible orphaned temp tables */
+								"		n.nspname !~ '^pg_temp_' AND "
+								"		n.nspname !~ '^pg_toast_temp_' AND "
+								"		n.nspname NOT IN ('pg_catalog', 'information_schema')");
+
+		ntups = PQntuples(res);
+		i_nspname = PQfnumber(res, "nspname");
+		i_relname = PQfnumber(res, "relname");
+		i_attname = PQfnumber(res, "attname");
+		for (rowno = 0; rowno < ntups; rowno++)
+		{
+			found = true;
+			if (script == NULL && (script = fopen_priv(output_path, "w")) == NULL)
+				pg_fatal("could not open file \"%s\": %s\n", output_path,
+						 strerror(errno));
+			if (!db_used)
+			{
+				fprintf(script, "Database: %s\n", active_db->db_name);
+				db_used = true;
+			}
+			fprintf(script, "  %s.%s.%s\n",
+					PQgetvalue(res, rowno, i_nspname),
+					PQgetvalue(res, rowno, i_relname),
+					PQgetvalue(res, rowno, i_attname));
+		}
+
+		PQclear(res);
+
+		PQfinish(conn);
+	}
+
+	if (script)
+		fclose(script);
+
+	if (found)
+	{
+		pg_log(PG_REPORT, "fatal\n");
+		pg_fatal("Your installation contains the \"sql_identifier\" data type in user tables\n"
+				 "and/or indexes.  The on-disk format for this data type has changed, so this\n"
+				 "cluster cannot currently be upgraded.  You can remove the problem tables or\n"
+				 "change the data type to \"name\" and restart the upgrade.\n"
+				 "A list of the problem columns is in the file:\n"
+				 "    %s\n\n", output_path);
+	}
+	else
+		check_ok();
+}