Re: In PG12, query with float calculations is slower than PG11

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Amit Langote <amitlangote09(at)gmail(dot)com>
Cc: 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 16:03:51
Views: Raw Message | Whole Thread | Download mbox | Resend email
Lists: pgsql-hackers

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().

Andres seems to be of the opinion that the compiler should be willing
to ignore the semantic requirements of the C standard in order
to rearrange the code back into the cheaper order. That sounds like
wishful thinking to me ... even if it actually works on his compiler,
it certainly isn't going to work for everyone.

The patch looks unduly invasive to me, but I think that it might be
right that we should go back to a macro-based implementation, because
otherwise we don't have a good way to be certain that the function
parameter won't get evaluated first. (Another reason to do so is
so that the file/line numbers generated for the error reports go back
to being at least a little bit useful.) We could use local variables
within the macro to avoid double evals, if anyone thinks that's
actually important --- I don't.

I think the current code is probably also misusing unlikely(),
and that the right way would be more like

if (unlikely(isinf(val) && !(inf_is_valid)))

regards, tom lane

In response to


Browse pgsql-hackers by date

  From Date Subject
Next Message Fujii Masao 2020-02-06 16:16:31 Re: table partitioning and access privileges
Previous Message Alvaro Herrera 2020-02-06 15:59:09 Re: Do not check unlogged indexes on standby