Re: Multi-column distinctness.

On 20 October 2015 at 16:48, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
wrote:

> and I can get this to work too
>>
>> | DROP STATISTICS ON '(' name_list ')'
>>
>> if I expand out the "DROP opt_column" productions to be two separate
>> productions with and without COLUMN, as was done long ago for ADD
>> COLUMN.
>>
>> These also work without ON, actually, though it does not work to
>> leave out the parens. (I think that has to do with the conflict
>> against multiple ALTER TABLE subcommands, not against ADD COLUMN.)
>>
>> It doesn't look like we could easily stick a "name" in between
>> STATISTICS and ON, but I'm unconvinced that that's necessary.
>>
>
> Well, it's definitely easier to reference the statistics by name (e.g. in
> the DROP command).
>
> Can't we just insist that there be only one statistics declaration
>> for a given column list?
>>
>
> I would rather not, because I envision preferring different types of
> statistics for different types of queries. For example the statistics
> discussed in this particular thread only supports equality clauses, so this
> restriction would mean we can't also define histogram, we'll be unable to
> estimate queries with ranges.

Can we do something like this...

ADD STATISTICS ON (col list) USING (histogram, MFV)

so we have Types/Methods of statistic, rather than specific names for the
statistic entry?

Since this command doesn't actually ADD the statistics, it just creates a
specification used by the next ANALYZE, it would seem better to use a
different keyword than ADD, perhaps DECLARE STATISTICS ON... and DROP
STATISTICS ON

--
Simon Riggs http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/>
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services