Re: [HACKERS] DISTINCT ON: speak now or forever hold your peace

At 13:41 25/01/00 +1100, Chris Bitmead wrote:
>Tom Lane wrote:
>>
>> If I don't hear loud hollers very soon, I'm going to eliminate the
>> DISTINCT ON "feature" for 7.0. As previously discussed, this feature
>> is not standard SQL and has no clear semantic interpretation.
>
>I don't feel overly strongly about this, but if I remember right you can
>do some pretty cool things with this feature, provided you do define
>some semantics clearly. Like I think you can find the first tuple
>(given some ORDER BY clause) that fulfills some criteria. I think it is
>
>SELECT DISTINCT ON name name, age ORDER BY age;
>
>will get the youngest person. This might not be clearly specified now,
>but
>as long as it's useful, how about clearly defining it? I don't know that
>there is an easy way of doing this in standard SQL.

I don't know about PGSQL, but in other systems, I use:

Select <whatever> from <wherever> order by age asc limit to 1 row;

I *think* the PGSQL syntax is:

Select <whatever> from <wherever> order by age asc limit 1;

I have no idea if the optimizer does 'fast first' optimizations, so I don't
know how quick this would be on a large table.

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