Create a trigger

If a DDL operation, such as adding a field to a table, is performed in the source of a data migration task, you can use a trigger to capture information about the DDL event and ensure normal synchronization of DML operations.

Background information

When you use OceanBase Migration Service (OMS) for incremental synchronization from a PostgreSQL database, only DML operations, such as INSERT, DELETE, and UPDATE operations, can be synchronized. However, if a DDL operation, such as adding a field to a table, is performed in the source of a data migration task, and OMS does not obtain the new table schema, DML operations related to this table object will fail to be synchronized, thereby interrupting the task. Therefore, you must first create DDL record tables and triggers in the source database, so that OMS can capture DDL operation information to ensure normal synchronization of DML operations.

Limitations

The version of the source PostgreSQL database must be 10.x or later.

Procedure

1. Log in to the source PostgreSQL Database instance and switch to the database from which data is to be migrated.

   In this example, psql is used for login. Run the \c <database name> command in psql to switch to the source database.

2. Create the following two tables to record DDL events.

   • Table that records the information about non-drop events

     ----postgreSQL DDL EVENT TABLE---- 
     
         CREATE TABLE public.oms_non_dropped_ddl_command (
         id bigserial primary key,
         ddl_text text COLLATE pg_catalog."default",
         tag text COLLATE pg_catalog."default",
         database character varying COLLATE pg_catalog."default",
         schema character varying COLLATE pg_catalog."default",
         object_type character varying COLLATE pg_catalog."default",
         objid integer,
         top_objid integer,
         object_identity text COLLATE pg_catalog."default",
         top_object_identity text COLLATE pg_catalog."default",
         top_schema text COLLATE pg_catalog."default",
         all_children_table text COLLATE pg_catalog."default"
         );
         ALTER TABLE public.oms_non_dropped_ddl_command REPLICA IDENTITY FULL;
     

   • Table that records the information about drop events

     ----postgreSQL DROP DDL EVENT TABLE----
     
         CREATE TABLE public.oms_dropped_ddl_command
         (
         id bigserial primary key,
         ddl_text text COLLATE pg_catalog."default",
         tag text COLLATE pg_catalog."default",
         database character varying COLLATE pg_catalog."default",
         schema character varying COLLATE pg_catalog."default",
         objid integer,
         object_type text COLLATE pg_catalog."default",
         object_name text COLLATE pg_catalog."default"
         );
         ALTER TABLE public.oms_dropped_ddl_command REPLICA IDENTITY FULL;
     

3. Create the following two DDL event trigger functions.

   • Non-drop event trigger function

      CREATE OR REPLACE FUNCTION public.oms_capture_ddl_for_non_dropped()
          RETURNS event_trigger
          LANGUAGE 'plpgsql'
          COST 100
          VOLATILE NOT LEAKPROOF SECURITY DEFINER
          AS $BODY$
          DECLARE ddl_text text;
          DECLARE record_object record;
          DECLARE obj record;
          DECLARE top_objid integer;
          DECLARE parent_oid integer;
          DECLARE top_object_identity text; 
          DECLARE top_schema text;
          DECLARE all_children_table text;
     
          BEGIN
          SELECT current_query() INTO ddl_text;
          FOR obj IN (SELECT * FROM pg_event_trigger_ddl_commands() WHERE TG_TAG IN ('CREATE TABLE', 'ALTER TABLE','CREATE INDEX','CREATE SCHEMA', 'COMMENT')) LOOP
          record_object:=obj;
          SELECT inhparent FROM pg_inherits WHERE inhrelid = obj.objid
          AND inhparent::regclass::text NOT LIKE 'pg_%' ORDER BY inhseqno DESC LIMIT 1 INTO parent_oid;
            WHILE parent_oid IS NOT NULL LOOP
              SELECT inhparent
              FROM pg_inherits
              WHERE inhrelid = parent_oid
              AND inhparent::regclass::text NOT LIKE 'pg_%'
              ORDER BY inhseqno DESC
              LIMIT 1
              INTO top_objid;
     
              IF top_objid IS NULL THEN
                  top_objid := parent_oid;
                exit;
              END IF;
              parent_oid := top_objid;
          END LOOP;
          IF top_objid IS NULL THEN
          top_objid :=record_object.objid;
          ELSE
          END IF;
          IF regexp_matches(ddl_text, 'ALTER\s+TABLE\s+.+\s+RENAME\s+TO\s+.+', 'i') IS NOT NULL THEN
          WITH RECURSIVE child_tables AS (
          SELECT oid AS table_objid
          FROM pg_class
          WHERE oid = top_objid
          UNION
              SELECT c.oid AS table_objid
          FROM pg_class c
          JOIN pg_inherits i ON c.oid = i.inhrelid
          JOIN child_tables ct ON i.inhparent = ct.table_objid
          )
          SELECT string_agg(table_objid::text, ', ') INTO all_children_table FROM child_tables WHERE table_objid <> top_objid;
          ELSE
          END IF;
          SELECT pn.nspname, ss.relname INTO obj from pg_catalog.pg_class ss JOIN pg_catalog.pg_namespace pn ON ss.relnamespace = pn.oid WHERE ss.oid = top_objid;
          top_object_identity:=obj.relname;
          top_schema:=obj.nspname;
          IF TG_TAG ='CREATE TABLE' AND record_object.object_type='table' THEN
          EXECUTE 'ALTER TABLE ' || record_object.object_identity || ' REPLICA IDENTITY FULL';
          ELSE
          END IF;
          INSERT INTO public.oms_non_dropped_ddl_command(id,ddl_text,tag,database,schema,object_type, objid, top_objid, object_identity,top_object_identity,top_schema,all_children_table)
          VALUES (default,ddl_text, TG_TAG,current_database(),current_schema,record_object.object_type, record_object.objid, top_objid ,record_object.object_identity,top_object_identity,top_schema,all_children_table);
          END LOOP;
          EXCEPTION 
          WHEN OTHERS THEN
              RAISE LOG 'OMS ddl trigger error during command process: %', SQLERRM;
          END
          $BODY$;
     

   • Drop event trigger function

     CREATE OR REPLACE FUNCTION public.oms_capture_ddl_for_dropped()
     RETURNS event_trigger
     LANGUAGE 'plpgsql'
     COST 100
     VOLATILE NOT LEAKPROOF SECURITY DEFINER
     AS $BODY$
     DECLARE ddl_text text;
     DECLARE record_object record;
     BEGIN
     SELECT current_query() INTO ddl_text;
     FOR record_object in (select * from pg_event_trigger_dropped_objects() WHERE TG_TAG NOT IN ('ALTER TABLE','DROP PUBLICATION','ALTER PUBLICATION')) LOOP
     IF record_object.object_type = 'type' THEN
     ELSE
     INSERT INTO public.oms_dropped_ddl_command(id,ddl_text,tag,database,schema, objid, object_type, object_name)
     VALUES (default,ddl_text, TG_TAG,current_database(),current_schema, record_object.objid, record_object.object_type, record_object.object_name);
     END IF;
     END LOOP;
     EXCEPTION
     WHEN OTHERS THEN
     RAISE LOG 'OMS drop ddl trigger error during command process: %', SQLERRM;
     END
     $BODY$;
     

4. Change the owner of the created functions to the account for connecting OMS to the source database. Assume that the account name is postgres:

   ALTER FUNCTION public.oms_capture_ddl_for_non_dropped()
   OWNER TO postgres;
   ALTER FUNCTION public.oms_capture_ddl_for_dropped()
   OWNER TO postgres;
   

5. Execute the following statements to create global event triggers.

   CREATE EVENT TRIGGER oms_capture_ddl_for_non_dropped
   ON ddl_command_end
   WHEN TAG IN ('CREATE TABLE', 'ALTER TABLE','CREATE INDEX','CREATE SCHEMA','COMMENT')
   EXECUTE PROCEDURE public.oms_capture_ddl_for_non_dropped();
   
   CREATE EVENT TRIGGER oms_capture_ddl_for_dropped ON sql_drop EXECUTE PROCEDURE public.oms_capture_ddl_for_dropped();
   

What to do next

After you end a data migration task, log in to the source database and execute the following statements to delete the trigger functions and corresponding tables.

drop EVENT trigger oms_capture_ddl_for_dropped;
drop EVENT trigger oms_capture_ddl_for_non_dropped;
drop function public.oms_capture_ddl_for_non_dropped();
drop function public.oms_capture_ddl_for_dropped();
drop table public.oms_non_dropped_ddl_command;
drop table public.oms_dropped_ddl_command;