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Providing Statistics to the CBO
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The CBO can follow optimal execution paths only if it has detailed knowledge of the database objects. In Oracle Database 10g, the recommended way to provide these statistics is by letting the database automatically collect statistics for you. This is known as the Automatic Optimizer Statistics Collection feature, which I explained in 17. You can also manually provide statistics to the Optimizer with the DBMS_STATS package. Note that whether you rely on automatic collection of statistics or collect them yourself manually, Oracle uses the DBMS_STATS package to collect statistics.
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Using DBMS_STATS to Collect Statistics
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Although letting the database automatically collect Optimizer statistics is the recommended approach under Oracle Database 10g, you can still manually collect Optimizer statistics using the DBMS_STATS package.
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For large tables, Oracle recommends just sampling the data, rather than looking at all of it. Oracle lets you specify row or block sampling, and it sometimes seems to recommend sampling sizes as low as 5 percent. The default sampling size for an estimate is low too. Oracle also recommends using the DBMS_STATS automatic sampling procedure. However, statistics gathered with sampled data aren t reliable. The difference between collecting Optimizer statistics with the estimate at 30 percent and 50 percent is startling at times in terms of performance. Always choose the option of collecting full statistics for all your objects, even if the frequency is not as high as it could be if you just sampled the data.
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CHAPTER 21 IMPROVING DATABASE PERFORMANCE: SQL QUERY OPTIMIZATION
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As I explained in 17, you must manually collect Optimizer statistics under the following conditions: When you use external tables When you need to collect system statistics To collect statistics on fixed objects, such as the dynamic performance tables (for dynamic tables, you should use the GATHER_FIXED_OBJECTS_STATS procedure to collect Optimizer statistics) Immediately after you run a bulk load job, because this makes your automatically collected statistics unrepresentative The following sections show you how to make use of the DBMS_STATS package to gather statistics.
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Note Oracle recommends that you not use the older ANALYZE statement to collect statistics for the Optimizer, but rather use the DBMS_STATS package. The ANALYZE command is retained for backward compatibility, and you must use it for non-Optimizer statistics collection tasks, such as verifying the validity of an object (using the VALIDATE clause), or identifying migrated and chained rows in a table (using the LIST CHAINED ROWS clause).
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Storing the Optimizer Statistics
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You use various DBMS_STATS package procedures to collect Optimizer statistics. Most of these procedures have three common attributes STATOWN, STATTAB, and STATID which enable you to save the collected statistics in a database table owned by a user. By default, these attributes are null, and you shouldn t provide a value for any of these attributes if your goal is to collect statistics for the Optimizer. When you ignore these attributes, Optimizer statistics you collect are stored in the data dictionary tables by default, where they re accessible to the Oracle Optimizer.
Collecting the Statistics
The DBMS_STATS package has several procedures that let you collect data at different levels. The main data collection procedures for database table and index data are as follows: GATHER_DATABASE_STATISTICS gathers statistics for all objects in the database. GATHER_SCHEMA_STATISTICS gathers statistics for an entire schema. GATHER_TABLE_STATISTICS gathers statistics for a table and its indexes. GATHER_INDEX_STATISTICS gathers statistics for an index. Let s use the DBMS_STATS package to collect statistics first for a schema, and then for an individual table. Collecting statistics at the schema level: SQL> EXECUTE DBMS_STATS.GATHER_SCHEMA_STATS (ownname => 'hr'); PL/SQL procedure successfully completed. SQL> Collecting statistics at the table level: SQL> EXECUTE DBMS_STATS.GATHER_TABLE_STATS ('hr','employees'); PL/SQL procedure successfully completed. SQL>
CHAPTER 21 IMPROVING DATABASE PERFORMANCE: SQL QUERY OPTIMIZATION
The GATHER_DATABASE_STATISTICS procedure collects Optimizer statistics for the entire database. This is probably the most common way of using the DBMS_STATS package, as you can use this procedure to collect statistics for all database objects with a single statement. Here s an example: SQL> EXECUTE dbms_stats.gather_database_stats (> ESTIMATE_PERCENT => NULL, > METHOD_OPT => 'AUTO', > GRANULARITY => 'ALL', > CASCADE => 'TRUE',> OPTIONS => 'GATHER AUTO'); PL/SQL procedure successfully completed. SQL>
Although you can use the ESTIMATE_PERCENT attribute to collect Optimizer statistics for a sample ranging Tip from 0.000001 to 100 percent of the rows in a table, you should strive to collect statistics for all the rows (by using null as the value for this attribute). Collecting statistics based on a sample is fraught with dangers. Unless the tables are so huge that you can t collect all statistics within your maintenance window, strive to collect full statistics on all objects, especially those that have heavy DML changes.
Let me explain the preceding GATHER_DATABASE_STATS procedure briefly here: The example shows only some of the various attributes or parameters that you can specify. You can see the complete list of attributes by typing in this command: SQL> DESCRIBE DBMS_STATS.GATHER_DATABASE_STATS If you don t specify any of the attributes, Oracle uses the default values for those attributes. Even when I use a default value, I list the attribute here, for exposition purposes. The ESTIMATE_PERCENT attribute refers to the percentage of rows that should be used to estimate the statistics. I chose null as the value. Null here, contrary to intuition, means that Oracle collects statistics based on all rows in a table. This is the same as using the COMPUTE STATISTICS option in the traditional ANALYZE command. The default for this attribute is to let Oracle estimate the sample size for each object, using the DBMS_STATS.AUTO_SAMPLE_SIZE procedure. You can use the METHOD_OPT attribute to specify several things, including whether histograms should be collected. Here, I chose AUTO. This means Oracle will collect histograms based on the distribution of column data and the workload of the columns. The GRANULARITY attribute applies only to tables. The ALL value collects statistics for subpartitions, partitions, and global statistics for all tables. The CASCADE=>YES option specifies that statistics be gathered on all indexes, along with the table statistics. The OPTIONS attribute is critical. The most important values for this attribute are as follows: GATHER gathers statistics for all objects, regardless of whether they have stale or fresh statistics. GATHER_AUTO collects statistics for only those objects that Oracle deems necessary. GATHER_EMPTY collects statistics only for objects without statistics. GATHER_AUTO results in collection of statistics for only stale objects, the determination as to the object staleness being made by checking the DBA_TAB_MODIFICATIONS view.
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