# 进行PostgreSQL增量DDL同步需要做哪些准备？
通过在源库中创建事件触发器和函数来获取源库的DDL信息，然后在Migration的增量实时同步阶段实现DDL操作的同步。
#### 前提条件
- 当前支持的DDL操作包含如下： 表级同步支持：DROP TABLE 、ALTER TABLE（包含ADD COLUMN、DROP COLUMN、ALTER COLUMN、RENAME COLUMN）。
  
- 不支持使用Using进行类型强转的DDL。
- 执行如下操作步骤前，请检查待同步的源数据库public模式下，是否存在名为hw_migration_ddl_info的表、名为hw_migration_ddl_function()的函数、名为hw_migration_ddl_event的触发器。如存在，请将其删除。
 
#### 操作步骤
1. 使用拥有创建事件触发器权限的用户连接要同步的数据库。
2. 执行如下语句，创建存储DDL信息的表。 
   ```
   DROP TABLE IF EXISTS public.hw_migration_ddl_info;
   DROP SEQUENCE IF EXISTS public.hw_migration_ddl_info_id_seq;
   CREATE TABLE public.hw_migration_ddl_info(  
   id                             bigserial primary key,  
   ddl                            text,  
   username                       varchar(64) default current_user,    
   txid                           varchar(16) default txid_current()::varchar(16),  
   tag                            varchar(64),    
   database                       varchar(64) default current_database(),    
   schema                         varchar(64) default current_schema,  
   client_address                 varchar(64) default inet_client_addr(),  
   client_port                    integer default inet_client_port(),  
   event_time                     timestamp default current_timestamp
   );
   ```
   
   
3. 执行如下语句，创建函数。 
   ```
   CREATE OR REPLACE FUNCTION public.hw_migration_ddl_function()
       RETURNS event_trigger
       LANGUAGE plpgsql
       SECURITY INVOKER
   AS $$
       declare ddl text;
       declare real_num int;
       declare max_num int := 50000;
   begin
     if (tg_tag in ('ALTER TABLE','DROP TABLE')) then
         select current_query() into ddl; 
         insert into public.hw_migration_ddl_info(ddl, username, txid, tag, database, schema, client_address, client_port, event_time)
         values (ddl, current_user, cast(txid_current() as varchar(16)), tg_tag, current_database(), current_schema,  inet_client_addr(), inet_client_port(), current_timestamp);
         select count(id) into real_num from public.hw_migration_ddl_info;
         if real_num > max_num then
           if current_setting('server_version_num')::int<100000 then
             delete from public.hw_migration_ddl_info where id<(select min(id)+1000 from public.hw_migration_ddl_info) and not exists (select 0 from pg_locks l join pg_database d on l.database=d.oid where d.datname=current_catalog and pid<>pg_backend_pid() and locktype='relation' and relation=to_regclass('public.hw_migration_ddl_info_pkey')::oid and mode='RowExclusiveLock');
           else 
             delete from public.hw_migration_ddl_info where id<(select min(id)+1000 from public.hw_migration_ddl_info) and (xmax=0 or coalesce(txid_status(xmax::text::bigint), '')<>'in progress');
           end if;
         end if;
     end if;
   end;
   $$;
   ```
   
   
4. 执行以下语句，为2和3中创建的对象赋予必要权限。 
   ```
   GRANT USAGE ON SCHEMA public TO public; GRANT SELECT,INSERT,DELETE ON TABLE public.hw_migration_ddl_info TO public; GRANT SELECT,USAGE ON SEQUENCE public.hw_migration_ddl_info_id_seq TO public; GRANT EXECUTE ON FUNCTION public.hw_migration_ddl_function() TO public;
   ```
   
   
5. 执行以下语句，创建DDL事件触发器。 
   ```
   CREATE EVENT TRIGGER hw_migration_ddl_event ON ddl_command_end EXECUTE PROCEDURE public.hw_migration_ddl_function();
   ```
   
   
6. 执行以下语句，将创建的事件触发器设置为enable。 
   ```
   ALTER EVENT TRIGGER hw_migration_ddl_event ENABLE ALWAYS;
   ```
   
   
7. 创建PostgreSQL为源的同步任务。
8. 待同步任务结束后，如果不需要再执行PostgreSQL为源的同步任务，请执行下语句删除创建的表、函数、触发器。 
   ```
   DROP EVENT trigger hw_migration_ddl_event;
   DROP FUNCTION public.hw_migration_ddl_function();
   DROP TABLE public.hw_migration_ddl_info;
   ```