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CHAPTER 11

Data Storage and Retrieval

Using Newton Data Storage Objects 11-49

A multiple-slot index solves this problem by sorting entries according to multiple

key values. The key values are extracted from up to six index paths speciﬁed by the

path array of the index specification frame. For example, the following code

fragment speciﬁes a multiple-slot index that sorts entries according to the values

each holds in its

'last, 'first, and 'num slots:

// multiple-slot index on data from three slots

myMultiSlotSpec :=

{structure:'multiSlot,

path: ['last,'first,'num],

type: ['string, 'string, 'int }

The ﬁrst key in the path array is called the primary key; subsequent lower-order

keys, if they are present, are the secondary key, tertiary key, and so on, up to a total

of six keys per array.

The primary key speciﬁes a minimum criterion for inclusion in the index and

provides a value used to sort the indexed entries initially. In the example, only

entries having a

last slot are indexed, and the value of the last slot is used to

impose an initial ordering on the indexed entries. Thus, the multiple-slot index in

the previous example sorts the

"Bates" entry before all of the "Perry" entries.

The secondary key, if it is present, is used to sort entries having identical primary

keys. In the previous example, the multiple-slot index imposes a secondary

ordering on all

"Perry" entries, according to the value each holds in its first

slot. Similarly, the tertiary key, if present, is used to sort further any entries having

identical secondary key values. Because none of the entries in the example have

identical secondary key values (none of the

first slots hold identical values), the

value of each entry’s

num slot has no effect on how the index sorts the entries.

Thus, the multiple-slot index shown previously sorts the set of sample entries in the

following order:

{last: "Bates", first: "Carol", num: 5}

{last: "Perry", first: "Barbara", num: 3}

{last: "Perry", first: "Bruce", num: 1}

{last: "Perry", first: "Daphne", num: 7}

{last: "Perry", first: "John", num: 4}

{last: "Perry", first: "Ralph", num: 2}

Now that you’re familiar with the manner in which multiple-slot indexes sort entries,

let’s look at the way the

Query method uses a multiple-slot index to select entries.

Missing slots in a multiple-slot query spec are treated as

nil values, just as they

are when querying on single-slot indexes. For example, if the query spec is missing

endKey slot, the upper end of the range of entries examined by the query is

unbounded, just as it would be for a query on a single-slot index.

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