Martin Marques <[EMAIL PROTECTED]> writes:
> I need to know if there is at least one row in the relation that comes from a
> determinated query. Which is the best way to do this:
> (a) Build the query with "SELECT * ..." and after executing the query see if
> numRows()>0
> (b) Build the query with "SELECT count(*) ...", fetch the row and see if
> count>0
Either of these implies computing the entire query result set, which is
much more computation than you want. Instead do
SELECT * .... LIMIT 1
and then see if you got a row or not. Aside from not computing useless
rows, the LIMIT will bias the optimizer in favor of fast-start plans.
regards, tom lane
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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
