Re: ToDo: KNN Search should to support DISTINCT clasuse?

2012/10/25 Kevin Grittner <kgrittn(at)mail(dot)com>:
> Tom Lane wrote:
>> "Kevin Grittner" <kgrittn(at)mail(dot)com> writes:
>> > Pavel Stehule wrote:
>> >> 2012/10/22 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
>> >>> Perhaps it would be close enough to what you want to use DISTINCT ON:
>> >>> contrib_regression=# explain select distinct on( t <-> 'foo') *,t <-> 'foo' from test_trgm order by t <-> 'foo' limit 10;
>>
>> >> good tip - it's working
>>
>> > If two or more values happen to be at exactly the same distance,
>> > wouldn't you just get one of them?
>>
>> Yeah, that is a hazard. I'm not sure whether <->'s results are
>> sufficiently quantized to make that a big problem in practice.
>
> It doesn't seem too far-fetched for trigram queries:
>
> test=# select nm, nm <-> 'anders' from (values ('anderson'),('andersen'),('andersly')) x(nm);
> nm | ?column?
> ----------+----------
> anderson | 0.4
> andersen | 0.4
> andersly | 0.4
> (3 rows)
>
> test=# select distinct on (nm <-> 'anders') nm, nm <-> 'anders' from (values ('anderson'),('andersen'),('andersly')) x(nm) order by nm <-> 'anders' limit 3;
> nm | ?column?
> ----------+----------
> anderson | 0.4
> (1 row)

yes it is issue - but I am thinking about simple "fuzzy" searching, so
exact result is not strongly expected. On second hand if SELECT
DISTINCT * will be supported it should be nice.

Pavel

>
> -Kevin