Proposal: UPDATE/DELETE ... WHERE OFFSET n OF cursor_name, was: Re: New ECPG idea, was: Re: ECPG FETCH readahead

Hi,

2013-08-17 13:02 keltezéssel, Boszormenyi Zoltan írta:
[snip, discussion of WHERE CURRENT OF in the ECPG client lib]

I had a second thought about it and the client side caching and
parser behind the application's back seems to be an overkill.

Instead, I propose a different solution, which is a logical extension of
FETCH { FORWARD | BACKWARD } N, which is a PostgreSQL extension.

The proposed solution would be:

UPDATE / DELETE ... WHERE OFFSET SignedIconst OF cursor_name

I imagine that FETCH would keep the array of TIDs/ItemPointerDatas
of the last FETCH statement.

The argument to OFFSET would be mostly in negative terms,
with 0 being equivalent of WHERE CURRENT OF.

E.g.:

FETCH 2 FROM mycur; -- fetches two rows
UPDATE mytab SET ... WHERE OFFSET -1 OF mycur; -- updates the first row
UPDATE mytab SET ... WHERE OFFSET 0 OF mycur; -- updates current row

or

FETCH 3 FROM mycur; -- fetches two rows, reaches end of the cursor
UPDATE mytab SET ... WHERE OFFSET -2 OF mycur; -- updates the first row
UPDATE mytab SET ... WHERE OFFSET -1 OF mycur; -- updates the second row
UPDATE mytab SET ... WHERE OFFSET 0 OF mycur; -- throws an error like WHERE CURRENT OF

or

FETCH 3 FROM mycur; -- fetches two rows, reaches end of the cursor
MOVE BACKWARD 2 IN mycur;
UPDATE mytab SET ... WHERE OFFSET 0 OF mycur; -- updates the first row (now current)
UPDATE mytab SET ... WHERE OFFSET 1 OF mycur; -- updates the second row

The cached array can be kept valid until the next FETCH statement,
even if moves out of the interval of the array, except in case the
application changes the sign of the cursor position, e.g. previously it used
MOVE ABSOLUTE with positive numbers and suddenly it switches to
backward scanning with MOVE ABSOLUTE <negative number> or vice-versa.

This would solve the only source of slowdown in the client side cursor caching
in ECPG present in my current ECPG cursor readahead patch, there would be
no more MOVE + UPDATE/DELETE WHERE CURRENT OF. On the other hand,
exploiting this proposed feature in ECPG would make it incompatible with older
servers unless it detects the server version it connects to and uses the current
method.

Comments?

Best regards,
Zoltán Böszörményi

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Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
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