PostgreSQL 10 High Performance - Third Edition

Tuning Hot Standby

Imagine the case where the standby is replaying the block changes resulting from a DROP TABLE on the master. If you're using Hot Standby to run queries on the standby, it's possible there's one using that table already running when these changes arrive. The server has two choices at this point, it can either apply the changes, or it can continue to let the query run; it can't do both.

While getting rid of a table is a rare situation, this is just the easiest to understand example from a class of problems where Hot Standby cannot replay some blocks without cancelling queries that expect them to be there. One of the most common sources for such changes happens when you vacuum the master database.

What the server should do ...