2012/10/22 Tom Lane <tgl@sss.pgh.pa.us>:
> Pavel Stehule <pavel.stehule@gmail.com> writes:
>> but using DISTINCT breaks KNN searching optimization
>
>> postgres=# explain select distinct nazobce, nazobce <-> 'Benešov' from
>> obce order by nazobce <-> 'Benešov' limit 10
>
> Don't hold your breath. There are two ways the system could implement
> the DISTINCT clause: either sort and uniq, or hashaggregate.
> hashaggregate will destroy any input ordering, so there's no value in
> using the index as input. sort and uniq requires the input to be sorted
> by *all* the columns being distinct'ed, not just one, so again this
> index isn't useful. You could get a plan using the index if you only
> wanted the <-> output column, eg
>
> contrib_regression=# explain select distinct t <-> 'foo' from test_trgm order by t <-> 'foo' limit 10;
> QUERY PLAN
> -------------------------------------------------------------------------------------
> Limit (cost=0.00..0.87 rows=10 width=12)
> -> Unique (cost=0.00..86.75 rows=1000 width=12)
> -> Index Scan using ti on test_trgm (cost=0.00..84.25 rows=1000 width=12)
> Order By: (t <-> 'foo'::text)
> (4 rows)
>
> 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;
> QUERY PLAN
> -------------------------------------------------------------------------------------
> Limit (cost=0.00..0.87 rows=10 width=12)
> -> Unique (cost=0.00..86.75 rows=1000 width=12)
> -> Index Scan using ti on test_trgm (cost=0.00..84.25 rows=1000 width=12)
> Order By: (t <-> 'foo'::text)
> (4 rows)
>
> regards, tom lane
good tip - it's working
thank you
Regards
Pavel
