Re: Choosing values for multivariate MCV lists

On Mon, Jun 24, 2019 at 01:42:32AM +0200, Tomas Vondra wrote:
>On Sun, Jun 23, 2019 at 10:23:19PM +0200, Tomas Vondra wrote:
>>On Sun, Jun 23, 2019 at 08:48:26PM +0100, Dean Rasheed wrote:
>>>On Sat, 22 Jun 2019 at 15:10, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)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.
>>
>
>OK, attached is a patch doing this. It's pretty simple, and it does
>resolve the issue with frequency precision.
>
>There's one issue with the signature, though - currently the function
>returns null flags as bool array, but values are returned as simple
>text value (formatted in array-like way, but still just a text).
>
>In the attached patch I've reworked both to proper arrays, but obviously
>that'd require a CATVERSION bump - and there's not much apetite for that
>past beta2, I suppose. So I'll just undo this bit.
>

Meh, I forgot to attach the patch, of couse ...

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Tomas Vondra http://www.2ndQuadrant.com
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