Abstract: In database application systems, performance becomes one of their main problems needed to be resolved with the increase of data in it. The optimization of SQL sentence is a main way to promote the performance of the system under the same hardware condition. This paper analyzes the execution processes of SQL sentence in Oracle database, compares the execution time and number of data blocks called in before and after of optimization to determine the optimal effect. Finally, the paper gets the SQL statement that time consuming less and data block calling less.

Key words: Oracle database, optimization, optimizer, index