Usually, the database system can use several different search strategies to access data. Among other things, these search strategies differ in the costs that they incur when they are executed.
The simplest search strategy is the sequential search (table scan), in which the database system searches through the entire table, row-by-row. However, for large quantities of data, the sequential search is often the most complex and expensive search strategy. For this reason, the SQL optimizer evaluates other search strategies that scan smaller areas of the table.
The search strategies that are available depend on the search condition, the structure of the SQL statement and the design of the database (primary key etc.).