IBM 000-8697 Instrukcja Użytkownika Pobierz pdf (Strona 223)

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System Architecture 2-129

Data Pages and Data Row Storage

Storage of Modiﬁed Rows

When a row is modiﬁed, OnLine attempts to return the modiﬁed row to its

current location. If the row size is unchanged, no changes are needed in the

slot table. If the row is smaller than before, OnLine changes the slot table

entry for this row to reﬂect the new row length. If the row no longer ﬁts,

OnLine attempts to store the row in another location on the same page. If

OnLine can do this, the slot table entry is changed to reﬂect both the new

starting offset and the new length of the row.

If the modiﬁeddata row is shorterthan a page butcannot beaccommodated

on the current page, a 4-byte forwarding pointer(containing thenew rowid)

is stored on the home page. The data row retains its originalrowid, which is

stored in the index page. The data is moved to the new page and the space

freed by the move is available for other rows. Figure 2-30 illustrates data

storage if the updated row is too large for the home page but shorter than a

whole page.

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