On Wed, Mar 28, 2012 at 9:13 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Francois Deliege <fdeliege@gmail.com> writes:
>> 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.
>
> Well, if you had an index on the column, you would get a significantly
> better plan ...
>
>> 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
ofvalues. We fixed this by handling that case on the client side, but originally thought the server would have
rewrittenit and sent a empty result set.
>
> It does not, and never will, because that would be an incorrect
> optimization. "col1 <> col1" isn't constant false, it's more like
> "col1 is not null". I'd suggest "WHERE FALSE", or "WHERE 1 <> 1"
> if you must, to generate a provably false constraint.
'col1 is distinct from col1' could be optimized like that. all though
it would be pretty hard to imagine a use case for it.
merlin