Tom Lane wrote:
> I wanted to enumerate my concerns while yesterday's
> events are still fresh in mind. (Andres or Robert might have more.)
>
> * I do not understand why this feature is on-by-default in the first
> place. It can only be a win for expression indexes that are many-to-one
> mappings; for indexes that are one-to-one or few-to-one, it's a pretty
> big loss. I see no reason to assume that most expression indexes fall
> into the first category. I suggest that the design ought to be to use
> this optimization only for indexes for which the user has explicitly
> enabled recheck_on_update. That would allow getting rid of the cost check
> in IsProjectionFunctionalIndex, about which we'd otherwise have to have
> an additional fight: I do not like its ad-hoc-ness, nor the modularity
> violation (and potential circularity) involved in having the relcache call
> cost_qual_eval.
That was my impression too when I had a closer look at this feature.
What about an option "hot_update_check" with values "off" (default),
"on" and "always"?
Yours,
Laurenz Albe