Re: Lock contention, docs vs. reality

On Sun, 22 Apr 2007, Roland Turner wrote:

> I'm working with 7.4, but the 8.2 docs[1] have the same apparent error:
>
> ROW SHARE
>
> Conflicts with the EXCLUSIVE and ACCESS EXCLUSIVE lock modes.
>
> The SELECT FOR UPDATE and SELECT FOR SHARE commands acquire a
> lock of this mode on the target table(s) (in addition to ACCESS
> SHARE locks on any other tables that are referenced but not
> selected FOR UPDATE/FOR SHARE).
>
> If that conflict list were correct, then ROW SHARE wouldn't conflict
> with itself, much less with ROW EXCLUSIVE (required to prevent
> INSERT/UPDATE/DELETE); commonsense dictates that it should conflict with
> both, and experiment demonstrates that it actually does so.

The list in question revolves around table-level locks. Those statements
mentioned also take out locks on affected rows. You should be able select
for update one row while updating a different row in the same table.