System Architecture 2-123

Data Row Format and Rowid

Data Row Format and Rowid
OnLinecan store rows that are longer than a page. OnLine also supports the
VARCHAR data type, which results in rows of varying length.
Asa 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 toIBM I nformix Guide to SQL: Tutorial for instructions about how to
estimate the length of fixed-length and variable-length data rows.
The termrowid refers to a unique 4-byte integer that is a combination of a
pageidentification 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 page 2-121 for a definition of the slot table and how it stores the
locationof a data row on a page.) The page that contains the first byte of the
datarow is the page that is specified by the rowid. This page is called the data
row home page.