IBM 000-8697 Manual do Utilizador descarregar pdf (Página 229)

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101

System Architecture 2-135

Structure of an Index Page

The byte partof the indexkey value isas long asneeded to containthe value

of the index key. If the indexed data is a VARCHAR data type, the calculated

length ofthe index key is the maximum length plus1. The additional byte is

a required-length byte, which precedes the VARCHAR data when it is stored

in the database. Therefore, the maximum VARCHAR that can be indexed is

254 bytes.

As an example of an index key value, consider row 101 in the table

stores5:customer. Thelname valueof row 101 is Pauli. The index key value

for this lname value is composed of a byte entry, Pauli, and a 4-byte rowid

entry for data row 101.

If twoor more rows share thesame lname value, the rowid part of the index

keyvalueisa list of rowidsforrowsthatsharethiskey value. In thisway,the

bytes part remains unique within the index. Figure 2-32 illustrates the

concept of the index key value.

When the ﬁrst row of data is inserted into a table, the root node index page

receives one index key value and one 2-byte slot table entry. The root node

page serves as a leaf node page until it becomes full.

Figure 2-32

Index key

values contain two

parts: a byte part

and one or more

rowids.

Index key values

rowid

rowid rowid

bytes

n 3

3 07n

1 2 ... 224 225 226 227 228 229 230 231 232 233 234 ... 701 702

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