On Fri, 2012-08-24 at 09:35 +0800, Craig Ringer wrote: > Hi all > > I've noticed that FETCH doesn't seem to be supported in subqueries or in > CTEs. > > Is there a specific reason for that, beyond "nobody's needed it and > implemented it"? I'm not complaining at all, merely curious. 1. Cursors have their own snapshot, so it would be kind of like looking at two snapshots of data at the same time. That would be a little strange. 2. For regular subqueries, it would also be potentially non-deterministic, because the FETCH operation has the side effect of advancing the cursor. So, if you had something like "SELECT * FROM (FETCH 1 FROM mycursor) x WHERE FALSE", it's not clear whether the FETCH would execute or not. After the query, it may have advanced the cursor or may not have, depending on whether the optimizer decided it didn't need to compute the subquery. 3. Cursors are really meant for a more effective interaction with the client, it's not really meant as an operator (and it doesn't change the results, anyway). You can already do LIMIT/OFFSET in a subquery if you need that kind of thing. All that being said, there may be some use case for something like what you are describing, if you get creative. Regards, Jeff Davis -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
