  isam stands for indexed sequential access method, a method for indexing data for fast retrieval. isam was originally developed by ibm for mainframe computers. today the term is used for several related concepts: * specifically, the ibm isam product and the algorithm it employs. * a database system where an application developer directly uses an application programming interface to search indexes in order to locate records in data files. in contrast, a relational database uses a query optimizer which automatically selects indexes. * an indexing algorithm that allows both sequential and keyed access to data. most databases now  use some variation of the b-tree for this purpose, although the original ibm isam and vsam implementations did not do so. * most generally, any index for a database. indexes are used by almost all databases, both relational and otherwise. in an isam system, data is organized into records which are composed of fixed length fields. records are stored sequentially, originally to speed access on a tape system. a secondary set of hash tables known as indexes contain "pointers" into the tables, allowing individual records to be retrieved without having to search the entire data set. this is a departure from the contemporaneous navigational databases, in which the pointers to other data were stored inside the records themselves. the key improvement in isam is that the indexes are small and can be searched quickly, thereby allowing the database to access only the records it needs. additionally modifications to the data do not require changes to other data, only the table and indexes in question. when an isam file is created, index nodes are fixed, and their pointers do not change during inserts and deletes that occur later (only content of leaf nodes change afterwards). as a consequence of this, if inserts to some leaf node exceed the node's capacity, new records are stored in overflow chains. if there are many more inserts than deletions from a table, these overflow chains can gradually become very large, and this affects the time required for retrieval of a record.ramakrishnan raghu,
Computer_file_formats Computer_file_systems