Re: tablecmds.c and lock hierarchy

Hi all,

As mentioned in the thread related to lowering locks of autovacuum
reloptions in ALTER TABLE SET
(http://www.postgresql.org/message-id/CAFcNs+oX7jVENC_3i54fDQ3ibmOGmknc2tMevdSmvojbSXGbGg@mail.gmail.com),
I have noticed the following code in
AlterTableGetLockLevel@tablecmds.c:
                /*
                 * Take the greatest lockmode from any subcommand
                 */
                if (cmd_lockmode > lockmode)
                        lockmode = cmd_lockmode;

The thing is that, as mentioned by Alvaro and Andres on this thread,
we have no guarantee that the different relation locks compared have a
monotone hierarchy and we may finish by taking a lock that does not
behave as you would like to. We are now lucky enough that ALTER TABLE
only uses ShareUpdateExclusiveLock, ShareRowExclusiveLock and
AccessExclusiveLock that actually have a hierarchy so this is not a
problem yet.
However it may become a problem if we add in the future more lock
modes and that are used by ALTER TABLE.