Re: [HACKERS] Another nasty cache problem

> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Now, if we did proper locking, no SI message could arrive for such an
> > entry.
> 
> > My assumption is that these are mostly system cache entries, and they
> > rarely change, right?  If someone is operating on a table that gets an
> > SI entry, odds are that later on the system will fail because the table
> > is changed in some way, right?
> 
> If the tuple is actually *changed* then that's true (and locking should
> have prevented it anyway).  But we also issue cache flushes against
> whole system tables in order to handle VACUUM of a system table.  There,
> the only thing that's actually been modified is the tuple's physical
> location (ctid).  We don't want to blow away transactions that are just
> looking at cache entries when a VACUUM happens.
> 
> Perhaps the caches shouldn't store ctid?  Not sure.

I am guilt of that.  There are a few place where I grab the tuple from
the cache, then use that to update the heap.  I thought it was a nifty
solution at the time.  I thought I used the CacheCopy calls for that,
but I am not positive.  Even if I did, that doesn't help because the
copy probably has an invalid tid at that point, thought I have opened
the table. Maybe I have to make sure I open the table before geting the
tid from the cache.

Is it only the tid that is of concern.  If so, that can probably be
fixed somehow.


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