Re: Patch: Write Amplification Reduction Method (WARM)

On Tue, Mar 14, 2017 at 7:16 PM, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
wrote:

> Pavan Deolasee wrote:
> > On Tue, Mar 14, 2017 at 7:17 AM, Alvaro Herrera <
> alvherre(at)2ndquadrant(dot)com>
> > wrote:
>
> > > I have already commented about the executor involvement in btrecheck();
> > > that doesn't seem good. I previously suggested to pass the EState down
> > > from caller, but that's not a great idea either since you still need to
> > > do the actual FormIndexDatum. I now think that a workable option would
> > > be to compute the values/isnulls arrays so that btrecheck gets them
> > > already computed.
> >
> > I agree with your complaint about modularity violation. What I am unclear
> > is how passing values/isnulls array will fix that. The way code is
> > structured currently, recheck routines are called by index_fetch_heap().
> So
> > if we try to compute values/isnulls in that function, we'll still need
> > access EState, which AFAIU will lead to similar violation. Or am I
> > mis-reading your idea?
>
> You're right, it's still a problem. (Honestly, I think the whole idea
> of trying to compute a fake index tuple starting from a just-read heap
> tuple is a problem in itself;

Why do you think so?

> I just wonder if there's a way to do the
> recheck that doesn't involve such a thing.)
>

I couldn't find a better way without a lot of complex infrastructure. Even
though we now have ability to mark index pointers and we know that a given
pointer either points to the pre-WARM chain or post-WARM chain, this does
not solve the case when an index does not receive a new entry. In that
case, both pre-WARM and post-WARM tuples are reachable via the same old
index pointer. The only way we could deal with this is to mark index
pointers as "common", "pre-warm" and "post-warm". But that would require us
to update the old pointer's state from "common" to "pre-warm" for the index
whose keys are being updated. May be it's doable, but might be more complex
than the current approach.

>
> > I wonder if we should instead invent something similar to IndexRecheck(),
> > but instead of running ExecQual(), this new routine will compare the
> index
> > values by the given HeapTuple against given IndexTuple. ISTM that for
> this
> > to work we'll need to modify all callers of index_getnext() and teach
> them
> > to invoke the AM specific recheck method if xs_tuple_recheck flag is set
> to
> > true by index_getnext().
>
> Yeah, grumble, that idea does sound intrusive, but perhaps it's
> workable. What about bitmap indexscans? AFAICS we already have a
> recheck there natively, so we only need to mark the page as lossy, which
> we're already doing anyway.
>

Yeah, bitmap indexscans should be ok. We need recheck logic only to avoid
duplicate scans and since a TID can only occur once in the bitmap, there is
no risk for duplicate results.

Thanks,
Pavan

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