IBM 000-8697 Bedienungsanleitung PDF herunterladen (Seite 221)

100

101

System Architecture 2-127

Data Pages and Data Row Storage

Multipage Storage

When OnLine receives a row that is longer than a page, the row is stored in

asmany whole pagesaspossible. The trailingportionis less thanafull page.

The page that contains the ﬁrst byte of the row is the row home page. The

number of thehome pagebecomes the logicalpage numbercontained in the

rowid. Each full page that follows the home page is referred to as a big-

remainder page. If the trailing portion of the row is less than a full page, it is

storedonaremainderpage.Figure 2-28illustrates the concepts ofhomepage,

big-remainder page, and remainder page.

When a row is longer than one page but less than two pages, the home row

contains a forward pointer to a remainder page. The forward pointer is

always stored as the ﬁrst four bytes in the data portion of the page. The

forward pointer contains the rowid of the next portion of the row. A ﬂag

valueis added to theslottableentryofthe data rowtoindicatethat a pointer

exists.

When a rowis longer thantwopages, the homerowandeach big-remainder

pagecontainforwardpointerstothenextportionofthedatarow.Figure 2-25

illustrates data storage for rows that are longer than two pages.

Figure 2-28

Data rows are

originally stored as

whole-page sized

segments,

measured from the

leading end of the

data row.

Data row represented in whole-page sized segments

Home page

Big-remainder page

Remainder page

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