Editing Surrogate key (section)

===Query optimization===
Relational databases assume a unique [[Index (database)|index]] is applied to a table's primary key. The unique index serves two purposes: (i) to enforce entity integrity, since primary key data must be unique across rows and (ii) to quickly search for rows when queried. Since surrogate keys replace a table's identifying attributes—the [[natural key]]—and since the identifying attributes are likely to be those queried, then the query optimizer is forced to perform a full table scan when fulfilling likely queries. The remedy to the full table scan is to apply indexes on the identifying attributes, or sets of them. Where such sets are themselves a [[candidate key]], the index can be a unique index.

These additional indexes, however, will take up disk space and slow down inserts and deletes.