IBM 000-8697 Manual do Utilizador descarregar pdf (Página 217)

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System Architecture 2-123

Data Row Format and Rowid

OnLine canstore rows thatare longer than apage. OnLine also supports the

VARCHAR data type, which results in rows of varying length.

Asaresult,rowsdo not conformtoasingleformat.Thefollowingfactsabout

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 to IBM Informix Guide to SQL: Tutorial for instructions about how to

estimate the length of ﬁxed-length and variable-length data rows.

The term rowid refers to a unique 4-byte integer that is a combination of a

page identiﬁcation 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.(Refertopage 2-121foradeﬁnitionoftheslottableandhow itstoresthe

location of a data row on a page.) The page that contains the ﬁrst byte of the

datarowisthepagethatisspeciﬁedbytherowid.Thispageiscalledthedata

row home page.

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