Re: Patch: Write Amplification Reduction Method (WARM)

On Fri, Mar 24, 2017 at 6:46 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:

>
>
> I was worried for the case if the index is created non-default
> collation, will the datumIsEqual() suffice the need. Now again
> thinking about it, I think it will because in the index tuple we are
> storing the value as in heap tuple. However today it occurred to me
> how will this work for toasted index values (index value >
> TOAST_INDEX_TARGET). It is mentioned on top of datumIsEqual() that it
> probably won't work for toasted values. Have you considered that
> point?
>
>
No, I haven't and thanks for bringing that up. And now that I think more
about it and see the code, I think the naive way of just comparing index
attribute value against heap values is probably wrong. The example of
TOAST_INDEX_TARGET is one such case, but I wonder if there are other
varlena attributes that we might store differently in heap and index. Like
index_form_tuple() ->heap_fill_tuple seem to some churning for varlena.
It's not clear to me if index_get_attr will return the values which are
binary comparable to heap values.. I wonder if calling index_form_tuple on
the heap values, fetching attributes via index_get_attr on both index
tuples and then doing a binary compare is a more robust idea. Or may be
that's just duplicating efforts.

While looking at this problem, it occurred to me that the assumptions made
for hash indexes are also wrong :-( Hash index has the same problem as
expression indexes have. A change in heap value may not necessarily cause a
change in the hash key. If we don't detect that, we will end up having two
hash identical hash keys with the same TID pointer. This will cause the
duplicate key scans problem since hashrecheck will return true for both the
hash entries. That's a bummer as far as supporting WARM for hash indexes is
concerned, unless we find a way to avoid duplicate index entries.

Thanks,
Pavan

--
Pavan Deolasee http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services