Re: Choosing values for multivariate MCV lists
От | Tomas Vondra |
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Тема | Re: Choosing values for multivariate MCV lists |
Дата | |
Msg-id | 20190623202319.4lcqsi5zc53hktda@development обсуждение исходный текст |
Ответ на | Re: Choosing values for multivariate MCV lists (Dean Rasheed <dean.a.rasheed@gmail.com>) |
Ответы |
Re: Choosing values for multivariate MCV lists
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Список | pgsql-hackers |
On Sun, Jun 23, 2019 at 08:48:26PM +0100, Dean Rasheed wrote: >On Sat, 22 Jun 2019 at 15:10, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote: >> One annoying thing I noticed is that the base_frequency tends to end up >> being 0, most likely due to getting too small. It's a bit strange, though, >> because with statistic target set to 10k the smallest frequency for a >> single column is 1/3e6, so for 2 columns it'd be ~1/9e12 (which I think is >> something the float8 can represent). >> > >Yeah, it should be impossible for the base frequency to underflow to >0. However, it looks like the problem is with mcv_list_items()'s use >of %f to convert to text, which is pretty ugly. > Yeah, I realized that too, eventually. One way to fix that would be adding %.15f to the sprintf() call, but that just adds ugliness. It's probably time to rewrite the function to build the tuple from datums, instead of relying on BuildTupleFromCStrings. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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