oracle sql execution process

Oracle SQL execution process

Oracle SQL is a widely used relational database management system. Its execution process is mainly divided into four stages: PARSE, BIND, EXECUTE and FETCH. In this article, we will detail how each stage works and what it does.

1. PARSE phase

The PARSE phase refers to the process of analyzing and interpreting SQL statements into syntax trees. During this stage, Oracle will identify the different parts of the SQL statement and convert them into internal data structures.

First, Oracle will check whether the syntax of the SQL statement is correct. If the SQL statement format is incorrect, an error message will be returned. For example, if a SELECT statement does not provide the necessary FROM clause, Oracle will return a syntax error.

Once the SQL syntax is correct, Oracle will interpret the SQL statement as a syntax tree. A syntax tree is a data structure in which each node represents a part of a SQL statement, such as the SELECT clause, FROM clause, and WHERE clause. Syntax trees allow Oracle to accurately process SQL statements during query execution.

2. BIND phase

The BIND phase refers to the process of binding SQL statements to database objects. At this stage, Oracle will determine the parameters required to execute the SQL statement and bind these parameters to the database objects.

For example, if a SELECT statement requires some WHERE conditions, Oracle will determine the parameters required for these conditions and bind them to the database object. Binding these parameters is to prevent them from being modified during the execution of the SQL statement, thereby ensuring the correctness of the query.

3. EXECUTE phase

The EXECUTE phase refers to the process of actually executing the SQL statement. At this stage, Oracle will use the bound parameters and syntax tree to execute the query.

During execution, Oracle uses internal algorithms to generate query plans. A query plan is a data structure that contains the sequence of instructions and steps required to execute a query. Query plans allow Oracle to execute queries quickly because it executes various parts of the query efficiently.

For example, if a SELECT statement requires access to multiple tables, Oracle will use the query plan to determine how the query results should be assembled. This involves selecting appropriate access paths and assembling the results.

4. FETCH phase

The FETCH phase refers to the process of transmitting data in blocks. During this phase, Oracle will retrieve the data blocks from the database and transfer them to the client computer.

A block is a group of records, usually 32K or 64K in size. When a client requests a result set, Oracle retrieves the records from the database and organizes them into chunks. When the client requests more records, Oracle continues to retrieve data blocks and transfer them to the client.

Summary

The above are the four stages of Oracle SQL execution process, including PARSE, BIND, EXECUTE and FETCH. These stages cooperate with each other and together constitute the query execution process of Oracle SQL. Understanding the working principle and role of each stage can help us better understand the query execution mechanism of Oracle SQL, thereby providing guidance for us to write more efficient SQL statements.

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