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

2012/10/25 Kevin Grittner <kgrittn@mail.com>:
> Tom Lane wrote:
>> "Kevin Grittner" <kgrittn@mail.com> writes:
>> > Pavel Stehule wrote:
>> >> 2012/10/22 Tom Lane <tgl@sss.pgh.pa.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'
limit10;
 
>>
>> >> 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