Create a trigger

This topic describes how to use a trigger to capture DDL events when a DDL change (such as adding a table field) occurs in the source database of a data migration task. This ensures that DML changes are synchronized properly.

Background information

When you use OceanBase Migration Service (OMS) Community Edition to synchronize incremental data from a source PostgreSQL database, by default, only DML statements (such as INSERT, DELETE, and UPDATE) are synchronized. If a DDL change (such as adding a table field) occurs in the source database and OMS cannot obtain the new schema information, subsequent DML synchronizations related to the object will fail, causing the task to be interrupted. To avoid this, you need to create a DDL record table and a trigger in the source PostgreSQL database to capture DDL information, ensuring that DML changes are synchronized properly.

Limitations

The source PostgreSQL database must be of V10.x or later.

Procedure

1. Log in to the source database and switch to the database to be migrated.

   For example, use the PSQL tool and execute the \c <database name> command to switch to the database to be migrated.

2. Create the following two tables to record DDL event changes.

   • Table for non-DROP DDL event information

     ----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 for DROP DDL event information

     ----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 event trigger functions.

   • Function for non-DROP DDL events

       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')) 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
         $BODY$;
     

   • Function for DROP DDL events

     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 OMS account for connecting to the source database. Here is an example using the postgres account:

   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 command to create a global event trigger.

       CREATE EVENT TRIGGER oms_capture_ddl_for_non_dropped
       ON ddl_command_end
       WHEN TAG IN ('CREATE TABLE', 'ALTER TABLE','CREATE INDEX','CREATE SCHEMA')
       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 the data migration task is completed, you need to log in to the source database and execute the following command 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;