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2-122 IBM Informix OnLine Database Server Administrator’s Guide
Structure of a Dbspace Page
For example, in a data page, the slot table entry would describe the page
offset where the data row (or portion of a data row) starts and the length of
the row (or portion of a row). (Refer to the discussion of data row storage,
which begins on page 2-125, for more details about the function of the slot
table.)
The number of the slot table entry is stored as part of the data row rowid. The
data row rowid is a unique identifier for each data row. It is composed of the
page number where the row is stored and the number of the slot table entry
that points to that data row.
As part of a rowid, the number of the slot table entry is stored as a 1-byte
unsigned integer. Since the rowid cannot store a slot table entry greater than
255, this is the upper limit on the number of rows than can be stored in a
single data page.
(Refer to page 2-123 for more detailed information about the data row rowid
and the rowid structure.)
The slot table is the only OnLine structure that points to a specific location in
a data page. For this reason, OnLine can initiate page compression whenever
required, according to internal algorithms. Typically, page compression
changes the location of the data row in the page and, therefore, generates a
new page offset that is written into the slot table entry. However, the number
of the slot table entry remains fixed. Thus all forwarding pointers and
descriptor values that rely on a rowid value remain accurate. Refer to
page 2-132 for more information about page compression.