Use DISTINCT in "Total" queries about sqid HOT 3 CLOSED

Added in 518a878

I wanted to look into local duplicate elimination because I had a case where adding DISTINCT would make an otherwise working query timeout. But actually I haven't been able to replicate this since then.

Question: Does DISTINCT impact performance when added to a query that wouldn't produce duplicates anyway? Or would it be a bad idea to just include DISTINCT by default for any other reasons?

from sqid.

Yes, DISTINCT has a big impact on performance even if there are no duplicates, since you don't know that there are no duplicates until you have done all the extra computation required for DISTINCT. In some cases, the query engine could be smart enough to see that there are no duplicates from the query, so as to drop the DISTINCT again, but it is always better if we omit it whenever possible.

from sqid.

887562b

from sqid.