Information Technology Reference
In-Depth Information
6.
P
AGE
-LSN
: The log sequence number (
LSN
) of the log record written for the latest
update onto the page. When a buffered page is updated, the
P
AGE
-LSN
of the page
is set to the LSN of the log record written for the update. In restart recovery,
the
P
AGE
-LSN
in the disk version of a page is needed in determining how far
the updates onto the page had reached at the time of the system failure (see
Chap.
4
). During normal processing, the
P
AGE
-LSN
s of the current versions of
pages are utilized for accelerating undo actions (see Sect.
4.3
) and traversals of
index structures (see Chap.
7
).
7.
Next page
: The page identifier of the “next page” in a linked structure of pages
(depending on the graph structure).
The
record area
of a page contains the actual data for the page. In the case of a
data page, the contents are the tuples of the logical database that are placed on the
page. The record area is filled from beginning to end (starting after the header).
The
record index
of a page is an array m at the end of the page. An element mŒi
of the array contains a byte offset pointing to the i th record in the record area. The
record index grows upward from the end: mŒ1 is at the end of the page and mŒ2
just before that. Figure
2.1
shows a tuple in page 924 at position mŒ3.
The pages of a database are usually grouped into
files
or
segments
, each of which
consists of extents of one or more consecutive pages. Typically the grouping is based
on the types of the records. One relation in a relational database is often placed in
its own file. In some systems, the tuples of two or more relations that are related via
a foreign-key constraint can be stored in the same file—this organization is often
called a
cluster file
.
It is possible to map a file in a database management system directly to a file
in the file system provided by the operating system. Then, the database system can
use the services provided by the operating system, and the database management
software becomes smaller and usually simpler.
However, the file system does not always provide all the features that are
important for implementing database operations efficiently. A database management
system places stricter demands on the buffering of pages in the main memory than
a normal file system: the database management system has to ensure that, in certain
(924, 3)
...
924
tuple
...
Fig. 2.1
Referencing a tuple
in a data page
