How does Oracle determine if an index is effective?

To determine if indexes are effective in Oracle, there are several methods available. One option is to use Explain Plan, where you can execute the query command before obtaining the query plan using the EXPLAIN PLAN command. The query plan will show the indexes used by the Oracle database when executing the query. If the expected index is shown in the query plan, then it can be determined that the index is effective. Another method is to use Autotrace, where you can get the execution plan and statistics of the query using the AUTOTRACE command. By entering “SET AUTOTRACE ON” in the SQL*Plus command line before executing the query command, once the query is executed, the execution plan and statistics, including the index used, will be displayed. If the expected index is shown, then it can be determined that the index is effective. Additionally, you can utilize SQL Trace to track the execution of query statements by enabling SQL Trace at the session level. Beginning at the start of the session, use the ALTER SESSION command to enable SQL Trace, then execute the query statement. After execution, detailed execution information, including the usage of indexes, can be obtained by examining the TRACE file. If the TRACE file shows the expected index being used, then it can be determined that the index is effective. Lastly, monitoring performance metrics can also be used to assess the effectiveness of indexes in Oracle. By examining metrics such as index hit ratio, I/O wait time, response time, and other performance indicators, the impact of indexes on performance can be evaluated. If there is a noticeable improvement in these metrics, then it can be determined that the index is effective. In conclusion, to determine the effectiveness of indexes in Oracle, methods such as query plans, Autotrace, SQL Trace, and performance metrics can be utilized to evaluate the usage of indexes.