Overview of SQL Processing

 This section explains how Oracle Database processes SQL statements. Specifically, the section explains the way in which the database processes DDL statements to create objects, DML to modify data, and queries to retrieve data.

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Stages of SQL Processing

The general stages of SQL processing are parsing, optimization, row source generation, and execution. Depending on the statement, the database may omit some of these steps.

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The following figure depicts the general stages:

Figure 7-3 Stages of SQL Processing

Description of "Figure 7-3 Stages of SQL Processing"

SQL Parsing

The first stage of SQL processing is SQL parsing. This stage involves separating the pieces of a SQL statement into a data structure that can be processed by other routines.

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When an application issues a SQL statement, the application makes a parse call to the database to prepare the statement for execution. The parse call opens or creates a cursor, which is a handle for the session-specific private SQL area that holds a parsed SQL statement and other processing information. The cursor and private SQL area are in the PGA.

During the parse call, the database performs the following checks:

• Syntax check

• Semantic check

• Shared pool check

The preceding checks identify the errors that can be found before statement execution. Some errors cannot be caught by parsing. For example, the database can encounter a deadlock or errors in data conversion only during statement execution.

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