| From: | Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Simon Riggs <simon(at)2ndquadrant(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>, Stephen Frost <sfrost(at)snowman(dot)net>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Christoph Berg <myon(at)debian(dot)org>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: [HACKERS] Surjective functional indexes |
| Date: | 2018-11-08 12:23:19 |
| Message-ID: | a9b352e807b53ef04b307e1453399fd601bdf076.camel@cybertec.at |
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| Lists: | pgsql-hackers |
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
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