oracle in query efficiency

Oracle database is one of the most widely used commercial databases at present. It has the characteristics of high efficiency, scalability, security and stability. Query statements are one of the most used functions of the database, and their efficiency directly affects the performance of the entire system.

Among database query statements, in query is a relatively common one. It can return multiple values ​​that need to be matched in one query. However, there are some efficiency issues that need to be paid attention to when using in query.

1. When using in query, you need to pay attention to the following points:

1. Avoid using too many in subqueries

In subqueries It is essentially a nested query. Too many nested query statements will greatly reduce query efficiency. For example, the following query statement:

select * from table1 where id in (select id from table2 where name = 'Tom');

This query statement is a nested query. If There is a large amount of data in table2, and nested queries can become very time-consuming. If you must use an in subquery, you can query the subquery results in advance and store them in a temporary table, and then use this temporary table in the main query statement.

2. Create an index for the field in the in subquery

In the in subquery, if the queried field is not indexed, the query efficiency will be very low. Therefore, when using in query, you should create indexes for the query fields in the subquery, which can greatly improve query efficiency.

3. Use exists instead of in subquery

In some cases, using exists subquery is more efficient than in subquery. The exists query can avoid retrieval of duplicate records. It only determines whether the query results exist and does not need to return specific query results. Therefore, when the query result is only whether it exists, you should use the exists subquery.

2. Optimize the efficiency of in query

In actual use, in order to improve the efficiency of in query, you can start from the following aspects:

1. Use Simple in query

In the in statement, you can use a numerical list to replace the query clause, which can greatly improve the efficiency of the query. For example, the following query statement:

select * from table1 where id in (1,2,3,4,5);

This query statement uses a numerical list to replace the subquery , which can effectively improve query efficiency.

2. Query statement optimization

Tuning the in query statement can improve query efficiency. Query statements can be analyzed and optimized, unnecessary subqueries and conditions can be deleted, and the execution time of query statements can be reduced.

3. Database table optimization

Optimizing the database table can also improve the efficiency of in queries. Table fields can be reasonably divided and indexes created, and large tables can be split and distributed to multiple physical devices to improve the system's concurrent processing capabilities.

4. Use partition table

Use partition table to divide a large table into multiple small tables. Each small table can be managed and maintained independently, which can improve Query efficiency and concurrent processing capabilities of the system. Especially in large-scale Internet applications, where billions of data need to be processed every day, the use of partition tables is a necessary means to improve query efficiency.

Summary:

in query is a relatively common query method, and its efficiency directly affects the performance of the entire system. Optimizing the efficiency of in query can start from reducing nested queries, using exists instead of in query, optimizing in query statements, optimizing database tables, using partition tables, etc. to improve query efficiency and the system's concurrent processing capabilities.

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