Many times I have came across this with PostgreSQL if an aggregate like sum, count function is used on a single column of the table the limit clause makes the query real faster.
Just try this
select count(*) from my_table limit 1;
This would be more faster than just
select count(*) from my_table;
The reason limits decide the scan of data pages and reduces the I/O for the interface program (in my Case psql). What do you people say.