Database tables and indexes may be stored on disk in one of a number of forms, including ordered/unordered flat files, ISAM, heap files, hash buckets, or B+ trees.
Such forms or structures are one aspect of the overall schema used by a database engine to store information.
Unordered storage typically stores the records in the order they are inserted.
[citation needed] Ordered storage typically stores the records in order and may have to rearrange or increase the file size when a new record is inserted, resulting in lower insertion efficiency.
However, ordered storage provides more efficient retrieval as the records are pre-sorted, resulting in a complexity of
[citation needed] Heap files are lists of unordered records of variable size.
Most conventional relational databases use "row-oriented" storage, meaning that all data associated with a given row is stored together.