Information Technology Reference
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Thus, the first tuple, .x;
u
/,insertedbyT
2
is not among the tuples read by T
1
, while
the second tuple, .y;
v
/
D
.x
j
;
v
j
/,is.
We also note that the phantom phenomenon is not prevented by the simple
locking protocol in which transactions obtain commit-duration shared locks (read
locks) on the keys of tuples read and commit-duration exclusive locks (write locks)
on the keys of tuples inserted, deleted, or updated.
t
1.9
Savepoints and Partial Rollbacks
The transaction model of
SQL
allows for
partial rollbacks
of transactions: a subse-
quence of the update actions performed by a forward-rolling transaction is rolled
back without aborting and rolling back the entire transaction. After performing a
partial rollback, the transaction remains in the forward-rolling phase and can thus
perform any new forward-rolling actions.
The actions to be rolled back in a partial rollback constitute a sequence of actions
from the latest update action by the transaction back to preset
savepoint
. Savepoints
are set in the application program using the
SQL
statement
set savepoint
P
where P is a unique name for the savepoint. The
SQL
statement
rollback to savepoint
P
executes a partial rollback to savepoint P : all forward-rolling update actions
performed by the transaction after setting P that are not yet undone are undone.
Example 1.10
Partial rollbacks can be nested (Fig.
1.2
).
insert into
r
values
.x
1
;
v
1
/;
set savepoint
P
1
;
insert into
r
values
.x
2
;
v
2
/;
set savepoint
P
2
;
insert into
r
values
.x
3
;
v
3
/;
rollback to savepoint
P
2
;
insert into
r
values
.x
4
;
v
4
/;
rollback to savepoint
P
1
;
insert into
r
values
.x
5
;
v
5
/;
commit
.
The statement
rollback to savepoint
P
2
deletes from relation r the inserted tuple
.x
3
;
v
3
/. The statement
rollback to savepoint
P
1
deletes from r the inserted tuples
.x
4
;
v
4
/ and .x
2
;
v
2
/. At the end of the transaction, an initially empty relation r
contains only the tuples .x
1
;
v
1
/ and .x
5
;
v
5
/.
t
We now add partial rollbacks to our transaction model. For that purpose, we
define the following actions:
