Re: Distinct + Limit

Hi list,

I have the following table with millions of rows:

CREATE TABLE table1
(
col1 text,
col2 text,
col3 text,
col4 text,
col5 text,
col6 text
)

select col1 from table1 group by col1 limit 1;
select distinct on (col1) col1 from table1 limit 1;

select col1 from table1 group by col1 limit 2;
select distinct on (col1) col1 from table1 limit 2;

Performing any of these following queries results in a full sequential scan, followed by a hash aggregate, and then the limit. An optimization could be to stop the sequential scan as soon as the limit of results has been reached. Am I missing something?

Limit (cost=2229280.06..2229280.08 rows=2 width=8)
-> HashAggregate (cost=2229280.06..2229280.21 rows=15 width=8)
-> Seq Scan on table1 (cost=0.00..2190241.25 rows=15615525 width=8)

Similarly, the following query results in a sequential scan:

select * from table1 where col1 <> col1;

This query is generated by the Sequel library abstraction layer in Ruby when filtering record based on a empty array of values. We fixed this by handling that case on the client side, but originally thought the server would have rewritten it and sent a empty result set.

I would greatly appreciate any help on speeding up these without having to rewrite the queries on the client side.

Thanks,

Francois