Data Management

Achieve faster SQL performance with dbms_stats

Oracle SQL optimization relies on accurate database statistics. The dbms_stats utility makes the gathering of useful data easier. Examine the options available with this utility and form your best plan of execution to increase SQL performance.


When a SQL statement is executed, the database must convert the query into an execution plan and choose the best way to retrieve the data. For Oracle, each SQL query has many choices for execution plans, including which index to use to retrieve table row, what order in which to join multiple tables together, and which internal join methods to use (Oracle has nested loop joins, hash joins, star joins, and sort merge join methods). These execution plans are computed by the Oracle cost-based SQL optimizer commonly known as the CBO.

The choice of executions plans made by the Oracle SQL optimizer is only as good as the Oracle statistics. To always choose the best execution plan for a SQL query, Oracle relies on information about the tables and indexes in the query.

Starting with the introduction of the dbms_stats package, Oracle provides a simple way for the Oracle professional to collect statistics for the CBO. The old-fashioned analyze table and dbms_utility methods for generating CBO statistics are obsolete and somewhat dangerous to SQL performance because they don't always capture high-quality information about tables and indexes. The CBO uses object statistics to choose the best execution plan for all SQL statements.

The dbms_stats utility does a far better job in estimating statistics, especially for large partitioned tables, and the better stats result in faster SQL execution plans.

Listing A shows a sample execution of dbms_stats with the options clause.
execdbms_stats.gather_schema_stats( -
ownname          => 'SCOTT', -
options          => 'GATHER AUTO', -
estimate_percent => dbms_stats.auto_sample_size, -
method_opt       => 'for all columns size repeat', -
degree           => 15 -
)


To fully appreciate dbms_stats, you need to examine each of the major directives. Let’s take a close look at each directive and see how it is used to gather top-quality statistics for the cost-based SQL optimizer.

The options parameter
Using one of the four provided methods, this option governs the way Oracle statistics are refreshed:
  • gather—Reanalyzes the whole schema
  • gather empty—Only analyzes tables that have no existing statistics
  • gather stale—Only reanalyzes tables with more than 10% modifications (inserts, updates, deletes).
  • gather auto—Reanalyzes objects which currently have no statistics and objects with stale statistics (Using gather auto is like combining gather stale and gather empty.)

Note that both gather stale and gather auto require monitoring. If you issue the alter table xxx monitoring command, Oracle tracks changed tables with the dba_tab_modifications view, which allows you to see the exact number of inserts, updates, and deletes tracked since the last analysis of statistics.

The estimate percent option
The following estimate_percent argument is a new way to allow Oracle’s dbms_stats to automatically estimate the best percentage of a segment to sample when gathering statistics:
estimate_percent => dbms_stats.auto_sample_size

You can verify the accuracy of the automatic statistics sampling by looking at the dba_tables sample_size column. It is interesting to note that Oracle chooses between 5 and 20 percent for a sample size when using automatic sampling. Remember, the better the quality of your statistics, the better the decision of the CBO.

The method_opt option
The method_opt parameter for dbms_stats is very useful for refreshing statistics when the table and index data change. The method_opt parameter is also very useful for determining which columns require histograms.

In some cases, the distribution of values within an index will effect the CBOs decision to use an index versus perform a full-table scan. This happens when a where clause has a disproportional amount of values, making a full-table scan cheaper than index access.

Oracle histograms statistics can be created when you have a highly skewed index, where some values have a disproportional number of rows. In the real world, this is quite rare, and one of the most common mistakes with the CBO is the unnecessary introduction of histograms in the CBO statistics. As a general rule, histograms are used when a column's values warrant a change to the execution plan.

To aid in intelligent histogram generation, Oracle uses the method_opt parameter of dbms_stats. There are also important new options within the method_opt clause, namely skewonly, repeat and auto:
method_opt=>'for all columns size skewonly'
method_opt=>'for all columns size repeat'
method_opt=>'for all columns size auto'


The skewonly option is very time-intensive because it examines the distribution of values for every column within every index.

If dbms_stats discovers an index whose columns are unevenly distributed, it will create histograms for that index to aid the cost-based SQL optimizer in making a decision about index versus full-table scan access. For example, if an index has one column that is in 50 percent of the rows, as shown in Listing B, a full-table scan is faster than an index scan to retrieve these rows.
—*************************************************************
— SKEWONLY option—Detailed analysis

— Use this method for a first-time analysis for skewed indexes
— This runs a long time because all indexes are examined
—*************************************************************
 
begin
  dbms_stats.gather_schema_stats(
     ownname          => 'SCOTT',
     estimate_percent => dbms_stats.auto_sample_size,
     method_opt       => 'for all columns size skewonly',
      degree           => 7
   );
end;


If you need to reanalyze your statistics, the reanalyze task will be less resource intensive with the repeat option. Using the repeat option (Listing C) will only reanalyze indexes with existing histograms, and will not search for other histograms opportunities. This is the way that you will reanalyze you statistics on a regular basis.
—**************************************************************
— REPEAT OPTION - Only reanalyze histograms for indexes
— that have histograms

— Following the initial analysis, the weekly analysis
— job will use the “repeat” option. The repeat option
— tells dbms_stats that no indexes have changed, and
— it will only reanalyze histograms for
— indexes that have histograms.
—**************************************************************
begin
   dbms_stats.gather_schema_stats(
      ownname          => 'SCOTT',
      estimate_percent => dbms_stats.auto_sample_size,
      method_opt       => 'for all columns size repeat',
      degree           => 7
   );
end;


The auto option within dbms_stats is used when Oracle table monitoring is implemented using the alter table xxx monitoring; command. The auto option, shown in Listing D, creates histograms based upon data distribution and the manner in which the column is accessed by the application (e.g., the workload on the column as determined by monitoring). Using method_opt=>’auto’ is similar to using the gather auto in the option parameter of dbms_stats.
begin
  dbms_stats.gather_schema_stats(
     ownname          => 'SCOTT',
      estimate_percent => dbms_stats.auto_sample_size,
      method_opt       => 'for all columns size auto',
      degree           => 7
   );
end;


Parallel collection
Oracle allows for parallelism when collecting CBO statistics, which can greatly speed up the time required to collect statistics. A parallel statistics collection requires an SMP server with multiple CPUs.

Better execution speed
The dbms_stats utility is a great way to improve SQL execution speed. By using dbms_stats to collect top-quality statistics, the CBO will usually make an intelligent decision about the fastest way to execute any SQL query. The dbms_stats utility continues to improve and the exciting new features of automatic sample size and automatic histogram generation greatly simplify the job of the Oracle professional.

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