| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Julien Rouhaud <julien(dot)rouhaud(at)dalibo(dot)com> |
| Cc: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Marko Tiikkaja <marko(at)joh(dot)to>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Removing Functionally Dependent GROUP BY Columns |
| Date: | 2016-01-23 15:39:20 |
| Message-ID: | 15746.1453563560@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Julien Rouhaud <julien(dot)rouhaud(at)dalibo(dot)com> writes:
> I wonder if in remove_useless_groupby_columns(), in the foreach loop you
> could change the
> + if (bms_subset_compare(pkattnos, relattnos) == BMS_SUBSET1)
> + {
> by something like
> + if (bms_num_members(relattnos) <= bms_num_members(pkattnos))
> + continue;
> +
> + if (bms_is_subset(pkattnos, relattnos))
> + {
> which may be cheaper.
FWIW, I really doubt that would be cheaper. The various flavors of
subset comparison are word-at-a-time bitmasking operations, but
bms_num_members has to grovel over individual bits; it's certain to
be more expensive than a subset test.
regards, tom lane
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