oracle sql execution process

Oracle is a feature-rich relational database management system (RDBMS) that is widely used in enterprise-level applications and network systems. In Oracle, SQL language is the main operating language. When we execute a SQL statement in Oracle, it will go through the following steps:

1. grammar analysis

Before executing the SQL statement, Oracle first needs to perform the syntax analysis process , In this stage, Oracle will perform syntactic and semantic analysis of the SQL statement to check the correctness of the statement and determine the execution plan. If the SQL statement has syntax errors or logic errors, execution will stop, otherwise continue to the next step.

2. Query Optimization

After completing the syntax analysis, Oracle will perform the query optimization process, which is to determine how to execute this SQL statement as quickly as possible. In order to decide how to execute the query, Oracle will compare different execution plans through the query optimizer, and then select the optimal execution plan. The optimizer can use cost-based optimization or rule-based optimization, but cost optimization is more common.

3. Execution plan generation

After completing query optimization, Oracle will generate an execution plan to guide how the query is executed. An execution plan is an interpreter that tells Oracle information about how tables, indexes, and other objects will be used, as well as any associated cost information required to perform the operation.

4. Data reading

Once the execution plan is generated, the query will start accessing the data on disk and in memory to perform the operation. This operation may involve reading from one or more tables, possibly using indexes to speed up read operations, which will greatly help improve query performance. Oracle implements cache management and data reading through Oracle Buffer Cache.

5. Query results returned

Finally, the query results will be returned to the user. When a SQL statement is executed, the system caches all rows in the result set into memory until the entire result set is in place, and then transmits it back to the client in one go. This caching mechanism improves performance when processing large result sets.

Summary

The Oracle SQL execution process has gone through multiple steps such as syntax analysis, query optimization, execution plan generation, data reading and query result return. Each step requires the system to perform a series of complex calculations and operations, but the execution of these steps is very fast. At each step, Oracle uses a variety of optimization techniques and algorithms to improve query efficiency. Therefore, writing optimized SQL statements and running SQL queries in Oracle is always a key challenge, and it is also a huge opportunity to improve your skills and development capabilities through continuous learning and practice.

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