| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Amit Langote <amitlangote09(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Emre Hasegeli <emre(at)hasegeli(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, keisuke kuroda <keisuke(dot)kuroda(dot)3862(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: In PG12, query with float calculations is slower than PG11 |
| Date: | 2020-02-06 18:31:07 |
| Message-ID: | CA+TgmobeuwC_3Lru88qZ04s_+OPsE9ZZfM8e_x5urT+FYs_hYA@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Feb 6, 2020 at 11:04 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> So it appears to me that what commit 6bf0bc842 did in this area was
> not just wrong, but disastrously so. Before that, we had a macro that
> evaluated isinf(val) before it evaluated the inf_is_valid condition.
> Now we have check_float[48]_val which do it the other way around.
> That would be okay if the inf_is_valid condition were cheap to
> evaluate, but in common code paths it's actually twice as expensive
> as isinf().
Well, if the previous coding was a deliberate attempt to dodge this
performance issue, the evidence seems to be well-concealed. Neither
the comments for that macro nor the related commit messages make any
mention of it. When subtle things like this are performance-critical,
good comments are pretty critical, too.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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