Sekundærindeksering

Sekundærindeksering is a database concept that refers to the creation and use of indexes on columns that are not part of the primary key of a table. While a primary index, usually built on the primary key, uniquely identifies each row and allows for fast retrieval, secondary indexes provide alternative paths to access data. These indexes are built on one or more columns that are frequently used in WHERE clauses, JOIN conditions, or ORDER BY clauses.

The purpose of secondary indexes is to speed up query performance for specific search criteria. When a query involves filtering or sorting based on a column that has a secondary index, the database can use that index to locate the relevant rows much more efficiently than performing a full table scan. This is particularly beneficial in large tables where scanning every row would be time-consuming.

Common implementations of secondary indexes include B-trees and hash indexes. B-trees are widely used due to their balanced structure, which ensures efficient search, insertion, and deletion operations. Hash indexes can offer very fast lookups for exact matches but are less suitable for range queries.

However, secondary indexes also come with trade-offs. They consume additional storage space and can slow down data modification operations such as INSERT, UPDATE, and DELETE. This is because whenever the data in an indexed column changes, the corresponding secondary index must also be updated. Therefore, the decision to create a secondary index should be based on a careful analysis of query patterns and the anticipated performance benefits versus the overhead costs. Database administrators often monitor query performance and analyze execution plans to determine which columns would benefit most from secondary indexing.