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It is also possible to gather statistics on indexes explicitly These statistics (additional to those gathered along with the table statistics) are displayed in the INDEX_STATS view; they include
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The number of index entries referring to extant rows The number of index entries referring to deleted rows
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This information is of value because of the manner in which indexes are maintained: when rows are deleted, the index keys remain After a prolonged period, indexes can become inefficient due to the amount of space occupied by these references to deleted rows
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In addition to the table statistics shown in DBA_TABLES, the index and column statistics in DBA_INDEXES and
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DBA_TAB_COLUMNS are also gathered whenever you analyze a table Analyzing an index populates INDEX_STATS
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Object statistics are not real-time; they are static, which means that they become out of date as DML operations are applied to the tables It is therefore necessary to gather statistics regularly, to ensure that the optimizer always has access to statistics that reflect reasonably accurately the current state of the database Statistics can be gathered manually, or the process can be automated Manual statistics gathering can done with either the ANALYZE command or by executing procedures in the DBMS_STATS package, as in Figure 13-1, or through Database Control In Figure 13-1, first the table REGIONS is analyzed, using the ANALYZE command The keywords COMPUTE STATISTICS instruct Oracle to analyze the whole table, not just a proportion of it The following query shows that there are four rows in the table Then there is a row inserted, but the statistics haven t been updated in real time; the number of rows is incorrect, until the table is analyzed again this time with the DBMS_STATSGATHER_TABLE_STATS procedure Numerous arguments can be supplied to the GATHER_TABLE_STATS procedure to control what it does this is the simplest form of its use The ANALYZE command is easy to use but has limited functionality DBMS_ STATS has many more options and is the recommended tool
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FIGURE 13-1
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Gathering statistics from the SQL*Plus prompt
Gathering statistics will improve performance, but the actual gathering may impose a strain on the database that will have a noticeable effect on performance while the Object statistics are not analysis is in progress This paradoxical situation real time; they are static until refreshed raises two questions First, how frequently by a new analysis If this is not done should statistics be gathered The more with suf cient frequency, they will be frequently this is done, the better performance seriously out of date and the optimizer may be but if it is done more frequently than may consequently develop inappropriate necessary, performance will suffer needlessly execution plans Second, what proportion of an object needs to be analyzed to gain an accurate picture of it Analyzing a huge table will be a long and resource-intensive process it may well be that analyzing a representative sample of the object would be enough for the optimizer and would not impose such a strain on the database
13:
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The DBMS_STATS procedures can take many arguments to influence the depth of the analysis, far more than the older ANALYZE command You can also manually gather statistics through Database Control From the database home page, take the Server tab and then the Manage Optimizer Statistics link in the Query Optimizer section Then take the Gather Optimizer Statistics link to run a wizard that will take you through the process The first window of the wizard includes this information message: For 11, Oracle recommends you enable automated maintenance task (Optimizer Statistics Gathering) to generate optimizer statistics regularly within maintenance windows This wizard should only be used for cases where the task is inappropriate or disabled For example, you may want to gather optimizer statistics immediately, or the task failed to execute within a maintenance window, or you want to customize options to gather optimizer statistics This is a strong hint that Oracle Corporation believes you should automate the gathering of statistics, rather than gathering them on an ad hoc basis If the database was created with the Database Configuration Assistant, automatic statistics gathering will have been configured, as a job managed by the Scheduler The wizard prompts you through the process of setting the various options for what and how to analyze, and when to run the task Figure 13-2 shows a procedure call generated by the wizard, which can be seen by clicking the Show SQL button on the Review page The example in Figure 13-2 shows the use of the GATHER_SCHEMA_STATS procedure, which will analyze all the objects belonging to one user Taking the arguments in turn:
OWNNAME specifies the schema to be analyzed CASCADE will analyze indexes as well as tables The setting given lets
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