Francois Deliege <fdeliege@xxxxxxxxx> 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 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. 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. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
