RE: Initial Schema Sync for Logical Replication

Hi Everyone, based on internal discussion with Masahiko
I have implemented concurrent DDL support for initial schema sync.

Concurrent Patch workflow

1. When TableSync worker creates a replicaton slot, It will
save the slot lsn into pg_subscription_rel with
SUBREL_SYNC_SCHEMA_DATA_SYNC state, and it will wait for
its state to be SUBREL_STATE_DATASYNC.

2. Applier process will apply DDLs till tablesync lsn, and then
it will change pg_subscription_rel state to SUBREL_STATE_DATASYNC.

3. TableSync will continue applying pending DML/DDls till it catch up.

This patch needs DDL replication to apply concurrent DDLs, I have cherry-
picked this DDL patch [0]

Issues
1) needs testing for concurrent DDLs, Not sure how to make tablesync process wait so that
concurrent DDLs can be issued on publisher.
2) In my testing created table does not appear on the same conenction on subscriber,
I have to reconnect to see table.
3) maybe different chars for SUBREL_SYNC_SCHEMA_DATA_INIT and SUBREL_SYNC_SCHEMA_DATA_SYNC,
currently they are 'x' and 'y'.
4) I need to add SUBREL_SYNC_SCHEMA_DATA_INIT and SUBREL_SYNC_SCHEMA_DATA_SYNC to
pg_subscription_rel_d.h to make it compile succesfully.
5) It only implement concurrent alter as of now

[0] = https://www.postgresql.org/message-id/OS0PR01MB57163E6487EFF7378CB8E17C9438A%40OS0PR01MB5716.jpnprd01.prod.outlook.com