Re: Query Planner not taking advantage of HASH PARTITION

On Wed, Apr 20, 2022 at 07:11:37PM -0700, Benjamin Tingle wrote:
> @ the first point about write locks
> I think I had/have a misconception about how inserts work in postgres. It's
> my understanding that postgres will never draft a parallel insert plan for
> any query (except maybe CREATE TABLE AS?)

It's correct that DML (INSERT/UPDATE/DELETE) currently is not run in parallel.
https://www.postgresql.org/docs/current/when-can-parallel-query-be-used.html

> because the process needs to acquire an exclusive access write lock to the
> table it is inserting on.

But this is incorrect - DML acquires a relation lock, but not a strong one.
Multiple processes can insert into a table at once (because the row-excl lock
level is not self-conflicting, to be technical).
https://www.postgresql.org/docs/current/explicit-locking.html

In fact, that's a design requirement. It's understood that many people would
be unhappy if only one client were able to run UPDATEs at once, and that only a
toy system would acquire a strong lock for DML.

--
Justin