How to lock entire Postgres table but allow reads

Question

Sorry for the noob question, but what is the correct table level lock mode to allow reading from a table but not writing to it?

The scenario that I have is that I want to prevent two tables from being written to while a third table is receiving an update. As soon as that update completes it's okay for the other two tables to unlock and receive additional updates.

If it isn't possible to prevent those tables from being written to while still allowing them to be read from then I could also just lock them entirely so they can't be written to or read from during the lock.

Here is the documentation page, I just don't understand what it's saying: https://www.postgresql.org/docs/current/explicit-locking.html

Answer 1

I agree the docs are not the easiest to understand when you don't already know what they are saying. But once you know that the answer is lock table foobar in exclusive mode, I think it is pretty clear in hindsight.

EXCLUSIVE

Conflicts with the ROW SHARE, ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE, and ACCESS EXCLUSIVE lock modes. This mode allows only concurrent ACCESS SHARE locks, i.e., only reads from the table can proceed in parallel with a transaction holding this lock mode.

Although there are probably better ways to do whatever it is you are trying to do, like maybe increasing your isolation level. Although this may be the easiest one to understand, and that has some value.

Answer 2

Don't do that. Explicitly locking tables is almost always the wrong thing to do. If you describe the use case in more detail, I am confident that a better solution can be found.

Locking tables frequently will keep autovacuum from doing its important work on the table, and eventually an anti-wraparound autovacuum job will come along and block you, bringing processing to a standstill.