IBM 000-8697 Manuale Utente

System Architecture

OnLine can store rows that are longer than a page. OnLine also supports the

VARCHAR

 data type, which results in rows of varying length.

As a result, rows do not conform to a single format. The following facts about
rows must be considered when OnLine stores data rows in a page:

■

Rows within a table are not necessarily the same length.

■

The length of a row may change when it is modified.

■

The length of a row can be greater than a page.

■

Blobs are not stored within the data row. Instead, the data row
contains a 56-byte descriptor that points to the location of the blob.
(The descriptor can point to either a dbspace blob page or a
blobspace blobpage.)

Refer to

utorial for instructions about how to

estimate the length of fixed-length and variable-length data rows.

The term rowid refers to a unique 4-byte integer that is a combination of a
page identification number (the logical page number) and the number of an
entry in the slot table on that page. The rowid defines the location of a data
row. (Refer to

for a definition of the slot table and how it stores the

location of a data row on a page.) The page that contains the first byte of the
data row is the page that is specified by the rowid. This page is called the data
row home page.