Information Technology Reference
In-Depth Information
For example, the allocation of a new page q as part of a heap file structure could
be logged as the following system-generated
structure-modification transaction
:
h
S; B
i
.
h
S;
allocate-page-for-heap
;s;f;p;q
i
.
h
S; C
i
.
Here S is a new transaction identifier generated by the database management
system. When the structure modification is recorded as committed, it can no longer
be rolled back, that is, it behaves as a committed transaction.
However, for reasons of efficiency, we do not require full transaction semantics
from a structure modification. The full semantics would require forcing the log onto
disk, which we want to avoid.
If all of the actual physical database operations in a structure modification can
be logged into one log record, it is not necessary to produce a transaction identifier
or to log the begin or finish of the transaction. Thus, only the following record is
logged for a structure modification that allocates a new page in a heap structure:
n
Wh
allocate-page-for-heap
;s;f;p;q
i
:
(3.14)
The
LSN
n of the log record is stamped into the
P
AGE
-LSN
fields of all the affected
pages, s; f; p; q, while holding those pages write-latched.
Example 3.3
Assume that the actions of the transactions
T
1
D
BI Œx;
v
AI
1
Œx;
v
C
and
T
2
D
BI Œy;
w
C
are executed in the left-to-right order
AI
1
Œx;
v
C
T
1
W
BI Œx;
v
T
2
W
BI Œy;
w
C
Also assume that the action IŒx;
v
causes a new page to be allocated where the
tuple .x;
v
/ is inserted. Then the following will be written to the log:
n
1
:
h
T
1
;B
i
.
n
2
:
h
allocate-page-for-heap
;s;f;p;q
i
.
n
3
:
h
T
1
;I;q;x;
v
;n
1
i
.
n
4
:
h
T
2
;B
i
.
n
5
:
h
T
2
;I;q;y;
w
;n
4
i
.
n
6
:
h
T
2
;C
i
.
n
7
:
h
T
1
;A
i
.
n
8
:
h
T
1
;I
1
;q;x;n
1
i
.
n
9
:
h
T
1
;C
i
.
The log record with
LSN
n
2
does not belong to any transaction. It is a redo-only log
record that is only used to redo the effect of the allocation on one or more of the
four pages s; f; p; q affected should the effect of the allocation, due to a failure, be
missing from some of those pages. The
U
NDO
-N
EXT
-LSN
value n
1
in T
1
's log record
with
LSN
n
3
skips over that with LSN n
2
to T
1
's previous log record (with
LSN
n
1
).
