iX V6.5 (30391-90011)

474 Chapter10

Internal Structures and Techniques

Data Set Internal Structures

Figure 10-3. Media Record for Secondary Entry

When more than one detail chain head is present, they are physically ordered left-to-right

in the order that the associated paths are speciﬁed in the schema.

Primary Entries

Selection of record addresses for master entries begins with a calculated address

determined by a hashing algorithm applied to the value of each entry's key item. The

algorithm is described later in this chapter. Each such calculated address is known as a

primary address and each entry residing at its primary address is called a primary

entry.

Secondary Entries

A new entry with a unique key item value will be assigned the same primary address as an

existing primary entry whenever the key item values of both entries generate the same

calculated address. When this occurs,the entries are considered synonyms of one another.

TurboIMAGE/XL assigns the new entry a secondary address obtained from unused

records in the vicinity of the primary entry. For master sets which are designated for

dynamic expansion, a secondary address may also be obtained from the expanded area.

Each entry residing at a secondary address is called a secondary entry.

Synonym Chains

When multiple data entries "arrive" at the same primary area, they are linked together to

form a synonym chain. A synonym chain consists of the primary entry and all of the data

entries with key values that hash to the same key location. Each synonym chain is

maintained by a 10-byte chain head in the media record of the primary entry and 10-byte

links in the media records of the secondary entries. A master data set entry can contain

both a synonym chain head and multiple detail chain heads. These are two distinct types

of chain heads.

If no secondary entries are present, the synonym chain count is one (for the total number

of entries that hash to this location) and the pointers to the ﬁrst and last secondary entries

are zeros. If one or more secondaries are present, the synonym chain count is equal to the

total number of entries that hash to the same primary address and the pointers reference

the ﬁrst and last secondary entries.