Indexed File
An indexed file is a type of file organization where data records are stored in an index structure ordered by the primary key; data can be retrieved using index structures ordered by primary or alternate keys or sequentially. Indexed files can be created using the CREATE/FDL command with a valid FDL file. Important considerations when designing an indexed file are the depth of an index and bucket size.
Sequential Access
When a program sequentially accesses an indexed file, RMS uses one or more indexes to determine the order in which to process the file records. Because index entries are ordered by key values, an index represents a logical ordering of the records in the file. If you define more than one key for the file, each index associated with a key represents a different logical ordering of the records in the file. Your program can then use the sequential access mode to retrieve records in the logical order represented by any index.
To retrieve records sequentially from an indexed file, your program must first specify a key of reference (for example, primary key, first alternate key, second alternate key, and so on). For successive retrievals, RMS uses the appropriate index to retrieve records based on how the records are ordered in the index.
If RMS accesses the index in ascending sort order, it returns the record with a key value equal to or higher than the key value in the previously accessed record. Conversely, if RMS accesses records in descending order, it accesses the next record having a key value equal to or lower than the key value in the previously accessed record. In contrast to a request to retrieve data sequentially from an indexed file, a request to store data sequentially in an indexed file does not require a key of reference. Rather, RMS uses the definition of the primary key to place the record in the primary index and, where applicable, uses the definition of the appropriate alternate key to place a record pointer in the alternate index.
Random Access
To randomly access a record from an indexed file, your program must specify both a key value and the index that RMS must search (for example, primary index, first alternate key index, and so on). When RMS finds a record with a matching key value, it passes the record to your program.
