IBM 000-8697 Bedienungsanleitung PDF herunterladen (Seite 216)

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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.)

Thenumberoftheslottableentryisstoredaspartofthedatarowrowid.The

data row rowid is aunique identiﬁerfor eachdata row. Itis composedof 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 therowid cannot store aslot table entrygreater than

255, this is the upper limit on the number of rows than can be stored in a

single data page.

(Refer to page 2-123for more detailed informationabout thedata row rowid

and the rowid structure.)

The slot tableis the onlyOnLine structure that pointsto aspeciﬁc location in

adatapage.For this reason,OnLinecan initiate page compressionwhenever

required, according to internal algorithms. Typically, page compression

changes the location of the data row in the page and, therefore, generates a

newpage offset that iswritten into theslottable entry. However, thenumber

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.

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