On Tue, Mar 06, 2012 at 07:07:41AM +0100, Boszormenyi Zoltan wrote: > 2012-03-05 19:56 keltez?ssel, Noah Misch ?rta: > >> Or how about a new feature in the backend, so ECPG can do > >> UPDATE/DELETE ... WHERE OFFSET N OF cursor > >> and the offset of computed from the actual cursor position and the > >> position known > >> by the application? This way an app can do readahead and do work on rows > >> collected > >> by the cursor with WHERE CURRENT OF which gets converted to WHERE OFFSET OF > >> behind the scenes. > > That's a neat idea, but I would expect obstacles threatening our ability to > > use it automatically for readahead. You would have to make the cursor a > > SCROLL cursor. We'll often pass a negative offset, making the operation > > fail > > if the cursor query used FOR UPDATE. Volatile functions in the query will > > get > > more calls. That's assuming the operation will map internally to something > > like MOVE N; UPDATE ... WHERE CURRENT OF; MOVE -N. You might come up with > > innovations to mitigate those obstacles, but those innovations would > > probably > > also apply to MOVE/FETCH. In any event, this would constitute a substantive > > patch in its own right. > > I was thinking along the lines of a Portal keeping the ItemPointerData > for each tuple in the last FETCH statement. The WHERE OFFSET N OF cursor > would treat the offset value relative to the tuple order returned by FETCH. > So, OFFSET 0 OF == CURRENT OF and other values of N are negative. > This way, it doesn't matter if the cursor is SCROLL, NO SCROLL or have > the default behaviour with "SCROLL in some cases". Then ECPGopen() > doesn't have to play games with the DECLARE statement. Only ECPGfetch() > needs to play with MOVE statements, passing different offsets to the backend, > not what the application passed.
That broad approach sounds promising. The main other consideration that comes to mind is a plan to limit resource usage for a cursor that reads, say, 1B rows. However, I think attempting to implement this now will significantly decrease the chance of getting the core patch features committed now. > > One way out of trouble here is to make WHERE CURRENT OF imply READHEAD > > 1/READHEAD 0 (incidentally, perhaps those two should be synonyms) on the > > affected cursor. If the cursor has some other readahead quantity declared > > explicitly, throw an error during preprocessing. > > I played with this idea a while ago, from a different point of view. > If the ECPG code had the DECLARE mycur, DML ... WHERE CURRENT OF mycur > and OPEN mycur in exactly this order, i.e. WHERE CURRENT OF appears in > a standalone function between DECLARE and the first OPEN for the cursor, > then ECPG disabled readahead automatically for that cursor and for that > cursor only. But this requires effort on the user of ECPG and can be very > fragile. Code cleanup with reordering functions can break previously > working code. Don't the same challenges apply to accurately reporting an error when the user specifies WHERE CURRENT OF for a readahead cursor? -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
