MultiKey

Multikey is a term used in database indexing to describe an index that contains multiple keys for a single document when the indexed field holds an array value. It is commonly discussed in the context of document-oriented databases, especially MongoDB, where indexing array fields creates multikey indexes.

How it works: When a document includes an array in an indexed field, the index stores an entry for each element of the array. For example, a document with tags: ["mongodb","database"] and an index on tags yields two index entries for that document. In the case of a compound index that includes multiple array fields, the index may contain multiple key combinations corresponding to the elements of the arrays, which can significantly increase the number of index entries and the overall index size.

Implications: Multikey indexes enable efficient queries that search for documents whose array fields contain specific values. They can improve query performance for array-based predicates but also increase index size and the cost of writes, since the system must maintain index entries for every array element. Some query patterns or index shapes may not fully utilize a multikey index, depending on how the query is written.

Example: Document { _id: 1, title: "Example", tags: ["mongodb","indexing"] }. An index on { tags: 1 } creates separate index entries for "mongodb" and "indexing," allowing queries like { tags: "mongodb" } to use the index.

See also: MongoDB, database indexing, arrays in databases, single-key index, compound index.