Please excuse my absence from this thread.
On 2 March 2018 at 12:34, Konstantin Knizhnik <k.knizhnik@postgrespro.ru> wrote:
>
>
> On 01.03.2018 22:48, Andres Freund wrote:
>>
>> Hi,
>>
>> I still don't think, as commented upon by Tom and me upthread, that we
>> want this feature in the current form.
The performance benefit of this patch is compelling. I don't see a
comment from Tom along those lines, just a queston as to whether the
code is in the right place.
>> Your arguments about layering violations seem to be counteracted by the
>> fact that ProjectionIsNotChanged() basically appears to do purely
>> executor work inside inside heapam.c.
>
>
> ProjectionIsNotChanged doesn't perform evaluation itslef, is calls
> FormIndexDatum.
> FormIndexDatum is also called in 13 other places in Postgres:
> analyze.c, constraint.c, index.c, tuplesort, execIndexing.c
>
> I still think that trying to store somewhere result odf index expression
> evaluation to be able to reuse in index update is not so good idea:
> it complicates code and add extra dependencies between different modules.
> And benefits of such optimization are not obvious: is index expression
> evaluation is so expensive, then recheck_on_update should be prohibited for
> such index at all.
Agreed. What we are doing is trying to avoid HOT updates.
We make the check for HOT update dynamically when it could be made
statically in some cases. Simplicity says just test it dynamically.
We could also do work to check for HOT update for functional indexes
earlier but for the same reason, I don't think its worth adding.
The patch itself is about as simple as it can be, so the code is in
the right place. My problems with it have been around the actual user
interface to request it.
Index option handling has changed (and this needs rebase!), but other
than that I think we want this and am planning to commit something
early next week.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
