ISAM

ISAM ( English Indexed Sequential Access Method - an index-sequential access method) - a way to store data for quick access to them. The method was developed by IBM for mainframes in 1963, it is currently the main way of presenting data in almost all databases.

ISAM separately stores data records and indexes (service data) for quick access to records. Data is stored sequentially (originally, ISAM was used to store data on tape drives that provide only sequential read / write). The second data set - a hash table - indexes containing pointers that will allow you to retrieve specific records without searching the entire database. This is somewhat different from indexes in modern search databases, since indexes are stored directly in records in them. The key feature of ISAM is that indexes are small and quick to search. A change in records does not require changing all records; it is only necessary to rebuild the index.

Relational databases can be built on the basis of ISAM data storage with added logic to preserve the integrity of the relationships between tables. Typically, the field used for communication ( foreign key ) is indexed for quick search. Of course, this is slower than just storing pointers to the required records in another table directly in the records, but changes at the physical level of data storage will not require changing pointers. (need to improve the explanation).

ISAM is easy to implement and it is a cheap method. The fee for this is that each client machine must keep its own connections with each file that is being accessed. This can lead to conflicts when several clients are working at the same time when trying to change or insert new values ​​and lead to data loss. Usually this problem is solved by adding a client-server application that serves and manages user requests in order to maintain data integrity. This is the basic concept of a DBMS that creates a client tier over the proper data.

ISAM has been superseded by IBM's methodology called VSAM (Virtual Storage Access Method). Later, IBM developed DB2 , which became the main DBMS from IBM. VSAM is a way to physically store data in DB2.

MySQL has implemented the ISAM extension - MyISAM .