Let's talk about the order of queries in Oracle database

Oracle database querying is a broad topic, especially when it comes to query optimization. When performing query optimization, it is very important to understand the order of queries in Oracle, because the order of queries has a great impact on query performance and results. In this article, we will introduce the order of queries in Oracle database and provide some useful tips to optimize the performance of queries.

1. Parsing SQL statements

Parsing SQL statements is the first step in querying. Its purpose is to convert SQL statements into an internal format that can be executed. This process includes the following steps:

a. Lexical analysis: Decompose the SQL statement into keywords, identifiers, symbols, etc.
b. Syntax analysis: Check the correctness of the SQL statement according to SQL grammar rules
c. Generate parse tree: Convert the SQL statement into a parse tree for subsequent execution

2. Optimize the query

After parsing the SQL statement, Oracle will automatically optimize the query , to find the optimal query plan. The query optimization process includes the following steps:

a. Select the optimal query plan: Oracle will select an optimal query plan based on factors such as table size, index availability, and query complexity. .
b. Estimate cost: Oracle will estimate the cost of each possible query plan and select the one with the lowest cost.
c. Generate query plan: Oracle will generate a query plan and use it to obtain the results when executing the query.

3. Execute the query

Before executing the query, Oracle must determine the query plan and the data source for the query. After determining the data source, Oracle begins to execute the query. The process includes the following steps:

a. Open the table: Oracle will open the tables involved in the query and prepare to access these tables.
b. Accessing data: The way you access a table depends on the query plan and how the table is physically stored. If an index is used, Oracle will use the index to retrieve the data. If no index is used, Oracle scans the entire table.
c. Calculate the query: Once Oracle obtains all the data, it can start calculating the query results.
d. Return results: After the calculation is completed, Oracle returns the results to the client.

4. Tips for optimizing query performance

a. Use indexes: Using indexes is one of the best ways to improve query performance. Complex queries can be accelerated using multiple indexes.
b. Reduce table joins: Table joins are one of the most resource-consuming operations in queries. The use of table joins must be carefully considered and the number of tables to be joined must be minimized.
c. Optimize subqueries in queries: Optimizing subqueries in queries can greatly improve query performance. You can use inner or outer joins to improve performance.
d. Good database design: A good database design can make queries more efficient. For example, use appropriate data types, avoid duplicate data, etc.

Conclusion

Understanding the order of queries in Oracle database can help developers better optimize queries to maximize database performance. By optimizing queries, query costs can be greatly reduced and the accuracy of query results can be improved. Therefore, it is very important to carefully consider query order and optimization techniques when performing Oracle database design and query development.

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