PostgreSQL 12 High Availability Cookbook - Third Edition

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Not all DDL statements are equally safe. Say that we want to alter the event column and change the type to TEXT. Watch what happens if we attempt that directly:

The risk here is that certain type conversions would cause a full table rewrite on not just the node being altered, but every node in the cluster. This could lead to a long-duration exclusive lock everywhere, and could eventually cause major problems.

As an alternative, BDR recommends this process:

1. Add a new column:

ALTER TABLE event_log 
  ADD event_new TEXT; 

2. Create a trigger function that keeps the columns synchronized:

CREATE FUNCTION f_copy_event() RETURNS TRIGGER ...