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CHAPTER 21 IMPROVING DATABASE PERFORMANCE: SQL QUERY OPTIMIZATION
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SELECT r.emp_id, select_emp(r.emp_id), r.emp_type, select_emp_desc(r.emp_type), COUNT(*) FROM emp_records r GROUP BY r.emp_id, r.emp_type;
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The parsing stage of query processing consumes resources, and ideally you should parse just once and use the same parsed version of the statement for repeated executions. Parsing is a much more expensive operation than executing the statement. You should use bind variables in SQL statements instead of literal values to reduce the amount of parsing in the database. Bind variables should be identical in terms of their name, data type, and length. Failure to use bind variables leads to heavy use of the shared pool area and, more often than not, contention for latches and a general slowing down of the database when a large number of queries are being processed. Sometimes your application may not be changeable into a form where bind variables are used. In 22, you ll see how to use Oracle configuration parameters to force statements that fail to use bind variables to do so.
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Views have several benefits to offer, but faster performance may not necessarily be one of them. Views are useful when you want to present only the relevant portions of a table to an application or a user. Whenever you query a view, it has to be instantiated at that time. Because the view is just a SQL query, it has to perform this instantiation if you want to query the view again. If your query uses joins on views, it could lead to substantial time for executing the query.
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Full table scans can occur sometimes, even when you have indexed a table. The use of functions on indexed columns is a good example for when you unwittingly can cause Oracle to skip indexes and go to a full table scan. You should avoid the use of inequality and the greater than or equal to predicates, as they may also bypass indexes.
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How the DBA Can Help Improve SQL Processing
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Performance tuning involves the optimization of SQL code and the calibration of the resources used by Oracle. The developers generally perform SQL tuning, and the DBA merely facilitates their tuning efforts by setting the relevant initialization parameters, turning tracing on, and so on. Nevertheless, the DBA can implement several strategies to help improve SQL processing in his or her database. In some cases, you and the developers might be working together to optimize the application. What if you can t modify the code, as is the case when you re dealing with packaged applications Alternatively, what if even the developers are aware that major code changes are needed to improve performance, but time and budget constraints make the immediate revamping of the application difficult There are several ways you can help without having to change the code itself. It s common for DBAs to bemoan the fact that the response times are slow because of poorly written SQL. I ve heard this in every place I ve worked, so I assume this is a universal complaint of DBAs who have to manage the consequences of bad code. A perfectly designed and coded application with all the right joins and smart indexing strategies would be nice, but more often than not,
CHAPTER 21 IMPROVING DATABASE PERFORMANCE: SQL QUERY OPTIMIZATION
that perfect state of affairs doesn t happen. The theory of the next best option dictates that you should do everything you can to optimize within the limitations imposed by the application design. That said, let s look at some of the important ways in which you can help improve query performance in an application, even when you can t change the code right away.
Using Partitioned Tables
Partitioned tables usually lead to tremendous improvements in performance, and they re easy to administer. By partitioning a table into several subpartitions, you re in essence limiting the amount of data that needs to be examined to satisfy your queries. If you have large tables, running into tens of millions of rows, consider partitioning them. Five table partitioning schemes are available to you in Oracle Database 10g, and they re explained in 5. You can index partitioned tables in a variety of ways, depending on the needs of the application. Partition maintenance is also easy, and it s well worth the additional effort when you consider the tremendous gains partitioned tables provide.
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