CREATE DATABASE inside transaction

Question

According to postgresql docs;

CREATE DATABASE cannot be executed inside a transaction block.

Is there a technical reason for this?

Answer 1

When you try it, you get the error:

ERROR:  CREATE DATABASE cannot run inside a transaction block

This comes from src/backend/access/transam/xact.c (line 3023 on my sources, but varies by version), in PreventTransactionChain(...).

The comment there explains that:

This routine is to be called by statements that must not run inside a transaction block, typically because they have non-rollback-able side effects or do internal commits.

For CREATE DATABASE it's called from src/backend/tcop/utility.c in standard_ProcessUtility under the case for T_CreatedbStmt, but unfortunately there isn't any informative comment that says why specifically CREATE DATABASE isn't safe to run in a transaction.

Looking at the sources, I can see that for one thing it forces a checkpoint.

Overall, though, I don't see anything that really screams out "we can't do this transactionally". It's more "we haven't implemented the functionality to do this transactionally".