In addition to ISAM, it is possible to imagine further keyed access methods which can better fulfill more specialized requirements. With the aid of PAM, BS2000 offers users the facility of implementing their own access methods. The following example of a hash access method shows that this is possible with a minimum of overhead.
The following characteristics are either specified or required:
90% of accesses should be possible with a single I/O operation
certain frequently used blocks should be able to be read without an I/O operation even if DAB is not used
the frequency with which changes are performed is high, but it is only very rare that records are deleted, created or significantly extended
the distribution of the keys is known and follows simply formulated rules
the file is not read sequentially
sufficient memory is available.
Given these conditions, the number of the block containing the record can be determined from the key with the aid of a simple algorithm. The extent to which the blocks are filled is so limited that the vast majority of updates do not cause it to overflow. In the unlikely event that a block should overflow, the block contains a pointer to the next block, which in turn contains a pointer to the next block should it also overflow. Buffering frequently used blocks does not cause any problems because the user implements the algorithm himself.
Although a special conversion algorithm for converting the key to the block number must be implemented every time that a key is allocated, this process can, compared with the global ISAM procedure, produce considerable increases in performance with large files.