oracle in does not use index

Oracle is one of the most commonly used database management systems in the industry. Its excellent performance and powerful functions have been widely recognized and used. However, even such a powerful system can encounter some performance issues in some situations. Among them, one of the most common problems is not using the index in the query.

Essentially, an index is a data structure used to speed up finding data in a table. In Oracle, there are two main types of indexes: B-tree indexes and bitmap indexes. When querying, the database system will access the index first and then the table, so good index design means faster query response time. However, in some cases, Oracle may choose not to index, which will result in a significant decrease in query performance.

There are many reasons for this situation. Here are some common situations:

1. The query condition does not use the index field: If the condition column in a query statement does not create a corresponding index , Oracle cannot use indexes to speed up queries. For example, if there is a column named "age" in the table but not in the query statement, Oracle will not use any index. At this time, the query conditions should be optimized so that the columns in the conditions use indexes as much as possible.

2. Index failure: Index failure means that the index cannot provide sufficient selectivity for the query, causing Oracle to choose to use a full table scan instead of using the index query. For example, if the table has a column named "gender" but there are only two possible values: "M" and "F", then index failure will occur. At this time, it can be solved by adding more distinctive columns as indexes, or changing query conditions.

3. Index expiration: Index expiration means that the data in the index is out of date, and Oracle has not found a suitable time to update the index. This usually occurs when table data is modified frequently. For example, if the value of a column in the table changes frequently and Oracle does not change the index key corresponding to the column, the index will expire. In order to avoid index expiration, you can use the automatic index maintenance function provided by Oracle, which will ensure that the index data and table data are synchronized.

4. Too much data in the table: If a table contains a large amount of data, the index may not be suitable for all queries, so Oracle may choose to use a full table scan. At this time, you should consider using partitioned tables or creating multiple indexes to speed up query performance.

When facing these problems, you can optimize query performance through the following methods:

1. Use the optimizer to determine the best execution path: Oracle's optimizer can automatically select the best execution path. Optimal execution path for fast response to queries. Users can manually intervene in the optimizer's operation to improve query performance by setting various database parameters.

2. Establish appropriate indexes: Creating indexes is the most basic means to speed up queries. Proper indexing can increase query speed and reduce system burden. But be careful not to abuse indexes, as too many indexes will burden system performance.

3. Use partitioned tables: Partitioning table data can improve query performance, especially when there is a lot of data in the table.

4. Modify the SQL statement: By modifying the query conditions in the SQL statement or using the prompt command, you can force the optimizer to use the specified index and improve the query speed.

In short, it is relatively common for "no indexing" to occur in Oracle, but some methods can be used to optimize query performance and improve system response speed. For those large database systems that require frequent queries, optimizing query performance is crucial, which will directly affect the availability and response speed of the system.

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