Apple Newton Utilities Guide de l'utilisateur télécharger pdf (Page 444)

100

101

CHAPTER 11

Data Storage and Retrieval

11-12 About Data Storage on Newton Devices

Indexes sort key values in ascending order unless the index spec frame used to

create a particular index speciﬁes descending order.

Begin Keys and End Keys 11

Because index keys are sorted by value, you can improve the speed of an index

query signiﬁcantly by limiting the range of index key values it tests. One way to do

this is to eliminate from the search any index key values that fall outside speciﬁed

minimum or maximum values. For example, you can specify a minimum index key

value used to select the ﬁrst entry to be tested, causing the query to “skip over” all

lesser-valued index keys. A minimum value used in this way is deﬁned in the query

spec as a

beginKey value.

Similarly, you can specify a maximum index key value to be used in selecting the

last entry to be tested, causing the query to ignore entries having index keys of

greater value. A maximum value used in this way is deﬁned in the query spec as an

endKey value.

You can use these optional

beginKey and endKey values together to specify a

subrange of index key values, as shown in Figure 11-3. Note that if an endrange

value is not speciﬁed, it is unbounded; for example, if you don’t specify an

endKey

value the query result potentially includes all entries through the end of the index.

Figure 11-3 Using beginKey and endKey values to specify an index subrange

You can also deﬁne a special kind of key that is itself excluded from the valid

subrange of index values. These keys are deﬁned as

beginExclKey and

endExclKey values in the query spec. Figure 11-4 depicts the use of

beginExclKey and endExclKey values to define the same index subrange

shown in Figure 11-3. Note that you cannot specify both the inclusive and

Ascending key order

Descending key order

beginKey endKey

subrange of index key values

3 4 5

1 2 ... 439 440 441 442 443 444 445 446 447 448 449 ... 941 942

Commentaires sur ces manuels

Pas de commentaire

Apple Newton Utilities Guide de l'utilisateur Page 444

Commentaires sur ces manuels

Produits connexes et manuels pour Logiciel Apple Newton Utilities