On Fri, Feb 29, 2008 at 05:28:29PM +0530, Gurjeet Singh wrote:
> On Fri, Feb 29, 2008 at 3:55 PM, Martijn van Oosterhout <kleptog@xxxxxxxxx> wrote:
> > Except if you have an index on the column you're ordering by. Then thePostgres will always try to do the smallest amount of work possible.
> > server can really return the first row quickly.
>
> Quickly for sure... but I don't think 'without processing all the rows that
> qualify'.
Putting a LIMIT in will cause the planner to pick a plan that returns an
appropriate number of rows quickly, sometimes it can be a very different
plan.
How do you explain that when you run:
> I think it will still process all the rows and return just one.
SELECT 1/v FROM (VALUES (1),(0)) c(v);
Without a limit you get a "division by zero" exception, and when you
have a limit of one row you get a result back.
You are correct, and repeating what I said in the first mail. Your query does not involve an ORDER BY (or other clauses), hence the first row that the executor encounters, satisfies what the query asked for. But if it had an ORDER BY you will again get the division ERROR.
Above I was referring to the case where the planner chooses an Index access.
Best regards,
gurjeet[.singh]@EnterpriseDB.com
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