Re: More FOR UPDATE/FOR SHARE problems

On Mon, 2009-01-26 at 14:31 -0600, Kevin Grittner wrote:
> > Do you re-run the query to find new tuples that might now satisfy
> > the search condition that didn't before?
>  
> There can't be any.  Blocks taken during the reading of rows so far
> have not been released, and would preclude the update from changing
> results read so far.

Let's say the sequence is:

Data:i  j
--------1  102  103  204  20

Session1:
BEGIN;
UPDATE a SET j = (j - 10) WHERE i = 2 OR i = 3;

Session2:
BEGIN;
UPDATE a SET j = j + 100 WHERE j = 10;

Session1:
COMMIT;

Session2:
COMMIT;

In PostgreSQL, the result is:
i |  j  
---+-----4 |  202 |   03 |  101 | 110
(4 rows)

Which cannot be obtained by any serial execution. What is the result in
Sybase, Oracle, etc.?

It seems like it would be a challenge to know that the tuple with i=3
would be updated to a value that matches the search condition j=10. So
can you tell me a little more about the mechanism by which Sybase solves
this problem?

Regards,Jeff Davis