Re: Patch: Write Amplification Reduction Method (WARM)

On Wed, Apr 5, 2017 at 8:42 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Tue, Apr 4, 2017 at 10:21 PM, Pavan Deolasee
> <pavan(dot)deolasee(at)gmail(dot)com> wrote:
> > On Thu, Mar 30, 2017 at 7:55 PM, Robert Haas <robertmhaas(at)gmail(dot)com>
> wrote:
> >> but
> >> try to access the TOAST table would be fatal; that probably would have
> >> deadlock hazards among other problems.
> >
> > Hmm. I think you're right. We could make a copy of the heap tuple, drop
> the
> > lock and then access TOAST to handle that. Would that work?
>
> Yeah, but it might suck. :-)

Well, better than causing a deadlock ;-)

Lets see if we want to go down the path of blocking WARM when tuples have
toasted attributes. I submitted a patch yesterday, but having slept over
it, I think I made mistakes there. It might not be enough to look at the
caller supplied new tuple because that may not have any toasted values, but
the final tuple that gets written to the heap may be toasted. We could look
at the new tuple's attributes to find if any indexed attributes are
toasted, but that might suck as well. Or we can simply block WARM if the
old or the new tuple has external attributes i.e. HeapTupleHasExternal()
returns true. That could be overly restrictive because irrespective of
whether the indexed attributes are toasted or just some other attribute is
toasted, we will block WARM on such updates. May be that's not a problem.

We will also need to handle the case where some older tuple in the chain
has toasted value and that tuple is presented to recheck (I think we can
handle that case fairly easily, but its not done in the code yet) because
of a subsequent WARM update and the tuples updated by WARM did not have any
toasted values (and hence allowed).

Thanks,
Pavan

--
Pavan Deolasee http://www.2ndQuadrant.com/
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