SQL Internal Processing Oracle Tips by Burleson Consulting

This chapter is concerned with providing a detailed understanding of how SQL is processed within the Oracle database instance. We will be exploring the Oracle library cache, and you will see how you can write queries to see exactly what is happening to your SQL statements. We will also explore techniques for investigating SQL sort activity and see how you can quickly find SQL statements that are invoking disk sorts. Best of all, I will show you how to get the execution plan for all SQL statements in the library cache and create reports that show you exactly what your SQL is doing. This chapter will include the following sections:

• Oracle shared SQL and private SQL areas

• SGA statistics for SQL

• Monitoring and tuning Oracle sorting

• Identifying high-impact SQL in the library cache

• Reporting on SQL in the library cache

Let’s begin with an in-depth overview of the SQL areas.

Shared and Private SQL Areas

Within the library cache, Oracle stores the source for your SQL statement as well as the execution plan for the SQL. Within an individual SQL statement, Oracle partitions the SQL into several areas, a shared area and a private area (see Figure 5-1). In order to make all SQL statements reusable, Oracle segments the parts of the SQL statement that are generic into the shared area, specifically the original parse tree for the SQL and the execution plan. The private SQL area, which includes SQL information that is user-specific, is divided into a persistent areas and a run-time area. The persistent area remains in memory unless the corresponding cursor has been closed. Hence, you should always close all open cursors that will not be used again for better memory utilization within the private SQL area. The run-time area is freed after the statement is executed.

Figure 1: The SQL shared and private areas

The private area includes binding data, run-time buffers, cursors, host variables, and other control structures that are specific to the user. The separation of the SQL areas allows Oracle SQL to remain fully reentrant and reusable while allowing simultaneous execution of any SQL statement.

Next, let’s look at some system-wide SGA statistics that can give us insight into our SQL behavior.

This is an excerpt from "Oracle High-Performance SQL Tuning" by Donald K. Burleson, published by Oracle Press.