Database index

A database index is an auxiliary structure that maps selected column values to locations of matching records. It speeds up retrieval at the cost of storage and maintenance during data changes.

B-tree: A balanced search tree that keeps keys ordered and supports efficient lookup, insertion, and deletion. B-trees organize many database indexes into searchable pages suited to storage devices.
Database table: A database structure that organizes related records into rows and named columns. An index points into a table to identify rows matching indexed values.
Clustered index: An index whose key order determines the physical organization of a table's records. Clustering can make range retrieval efficient by placing nearby key values together.
Query performance: The speed and resource consumption of executing a database query. Indexes can reduce reads and latency, but poor choices can add overhead.
Hash index: An index that uses a hash function to map keys to buckets containing matching records. Hash indexes trade ordered traversal for efficient equality lookups.
Database key: A column or set of columns used to identify, relate, or order database records. Index entries are organized around values from one or more key columns.
Composite index: An index built over multiple columns, with entries ordered by a defined column sequence. Column order determines which combinations and prefixes of query conditions it can serve.
Database transaction: A logical unit of database work governed by properties such as atomicity and consistency. Transactions must keep index entries consistent with table changes.
Bitmap index: An index that represents the rows matching each value with a bit vector. Bitmap indexes combine compactly when queries filter several low-cardinality columns.
Database page: A fixed-size unit of data that a database reads from or writes to storage. Index traversal typically reads pages, so page layout affects lookup cost.

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