Re: SIREAD lock versus ACCESS EXCLUSIVE lock

"Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
> Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> If you don't believe that a catcache lookup will ever fail, I will
>> contract to break the patch.
> As you probably know by now by reaching the end of the thread, this
> code is going away based on Heikki's arguments; but for my
> understanding, so that I don't make a bad assumption in this area
> again, what could cause the following function to throw an exception
> if the current process is holding an exclusive lock on the relation
> passed in to it?  (I could be a heap or an index relation.)  It
> seemed safe to me, and I can't spot the risk on a scan of the called
> functions.  What am I missing?

Out-of-memory.  Query cancel.  The attempted catalog access failing
because it results in a detected deadlock.  I could probably think of
several more if I spent ten minutes on it; and that's not even
considering genuine problem conditions such as a corrupted catalog
index, which robustness demands that we not fall over completely for.

You should never, ever assume that an operation as complicated as a
catalog lookup can't fail.
        regards, tom lane