Oracle® Database Performance Tuning Guide 10g Release 1 (10.1) Part Number B1075201 


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This chapter explains why statistics are important for the query optimizer and how to gather and use optimizer statistics with the DBMS_STATS
package.
The chapter contains the following sections:
Optimizer statistics are a collection of data that describe more details about the database and the objects in the database. These statistics are used by the query optimizer to choose the best execution plan for each SQL statement. Optimizer statistics include the following:
The optimizer statistics are stored in the data dictionary. They can be viewed using data dictionary views. See "Viewing Statistics".
Because the objects in a database can be constantly changing, statistics must be regularly updated so that they accurately describe these database objects. Statistics are maintained automatically by Oracle or you can maintain the optimizer statistics manually using the DBMS_STATS
package. For a description of the automatic and manual processes, see "Automatic Statistics Gathering" or "Manual Statistics Gathering".
The DBMS_STATS
package also provides procedures for managing statistics. You can save and restore copies of statistics. You can export statistics from one system and import those statistics into another system. For example, you could export statistics from a production system to a test system. In addition, you can lock statistics to prevent those statistics from changing. The lock methods are described in "Locking Statistics for a Table or Schema".
The recommended approach to gathering statistics is to allow Oracle to automatically gather the statistics. Oracle gathers statistics on all database objects automatically and maintains those statistics in a regularlyscheduled maintenance job. Automated statistics collection eliminates many of the manual tasks associated with managing the query optimizer, and significantly reduces the chances of getting poor execution plans because of missing or stale statistics.
Optimizer statistics are automatically gathered with the job GATHER_STATS_JOB
. This job gathers statistics on all objects in the database which have:
This job is created automatically at database creation time and is managed by the Scheduler. This Scheduler runs this job when the maintenance window is opened. By default, the maintenance window opens every night from 10 P.M. to 6 A.M. and all day on weekends. The GATHER_STATS_JOB
continues until it finishes, even if it exceeds the allocated time for the maintenance window. The default behavior of the maintenance window can be changed.
See Also:
Oracle Database Administrator's Guide for information on the Scheduler and maintenance windows tasks 
The GATHER_STATS_JOB
job gathers optimizer statistics by calling the DBMS_STATS.GATHER_DATABASE_STATS_JOB_PROC
procedure. The GATHER_DATABASE_STATS_JOB_PROC
procedure collects statistics on database objects when the object has no previously gathered statistics or the existing statistics are stale because the underlying object has been modified significantly (more than 10% of the rows).The DBMS_STATS.GATHER_DATABASE_STATS_JOB_PROC
is an internal procedure, but its operates in a very similar fashion to the DBMS_STATS.GATHER_DATABASE_STATS
procedure using the GATHER
AUTO
option. The primary difference is that the DBMS_STATS.GATHER_DATABASE_STATS_JOB_PROC
procedure prioritizes the database objects that require statistics, so that those objects which most need updated statistics are processed first. This ensures that the mostneeded statistics are gathered before the maintenance window closes.
Automatic statistics gathering is enabled by default when a database is created, or when a database is upgraded from an earlier database release. You can verify that the job exists by viewing the DBA_SCHEDULER_JOBS
view:
SELECT * FROM DBA_SCHEDULER_JOBS WHERE JOB_NAME = 'GATHER_STATS_JOB';
In situations when you want to disable automatic statistics gathering, the most direct approach is to disable the GATHER_STATS_JOB
as follows:
BEGIN DBMS_SCHEDULER.DISABLE('GATHER_STATS_JOB'); END; /
Automatic statistics gathering relies on the modification monitoring feature, described in "Determining Stale Statistics". If this feature is disabled, then the automatic statistics gathering job is not able to detect stale statistics. This feature is enabled when the STATISTICS_LEVEL
parameter is set to TYPICAL
or ALL
. TYPICAL
is the default value.
This section discusses:
Automatic statistics gathering should be sufficient for most database objects which are being modified at a moderate speed. However, there are cases where automatic statistics gathering may not be adequate. Because the automatic statistics gathering runs during an overnight batch window, the statistics on tables which are significantly modified during the day may become stale. There are typically two types of such objects:
For highly volatile tables, there are two approaches:
OPTIMIZER_DYNAMIC_SAMPLING
parameter, and this parameter should be set to a value of 2 or higher. The default value is 2. The statistics can set to NULL by deleting and then locking the statistics:
BEGIN DBMS_STATS.DELETE_TABLE_STATS('OE','ORDERS'); DBMS_STATS.LOCK_TABLE_STATS('OE','ORDERS'); END; /
See "Dynamic Sampling Levels" for information about the sampling levels that can be set.
This is more effective than the GATHER_STATS_JOB
, because any statistics generated on the table during the overnight batch window may not be the most appropriate statistics for the daytime workload.
For tables which are being bulkloaded, the statisticsgathering procedures should be run on those tables immediately following the load process, preferably as part of the same script or job that is running the bulk load.
For external tables, statistics are not collected during GATHER_SCHEMA_STATS
, GATHER_DATABASE_STATS
, and automatic statistics gathering processing. However, you can collect statistics on an individual external table using GATHER_TABLE_STATS
. Sampling on external tables is not supported so the ESTIMATE_PERCENT
option should be explicitly set to NULL
. Because data manipulation is not allowed against external tables, it is sufficient to analyze external tables when the corresponding file changes.
If the monitoring feature is disabled by setting STATISTICS_LEVEL
to BASIC
, automatic statistics gathering cannot detect stale statistics. In this case statistics need to be manually gathered. See "Determining Stale Statistics" for information on the automatic monitoring facility.
Another area in which statistics need to be manually gathered are the system statistics. These statistics are not automatically gathered. See "System Statistics" for more information.
Statistics on fixed objects, such as the dynamic performance tables, need to be manually collected using GATHER_FIXED_OBJECTS_STATS
procedure. Fixed objects record current database activity; statistics gathering should be done when database has representative activity.
Whenever statistics in dictionary are modified, old versions of statistics are saved automatically for future restoring. Statistics can be restored using RESTORE
procedures of DBMS_STATS
package. See "Restoring Previous Versions of Statistics" for more information.
In some cases, you may want to prevent any new statistics from being gathered on a table or schema by the DBMS_STATS_JOB
process, such as highly volatile tables discussed in "When to Use Manual Statistics". In those cases, the DBMS_STATS
package provides procedures for locking the statistics for a table or schema. See "Locking Statistics for a Table or Schema" for more information.
If you choose not to use automatic statistics gathering, then you need to manually collect statistics in all schemas, including system schemas. If the data in your database changes regularly, you also need to gather statistics regularly to ensure that the statistics accurately represent characteristics of your database objects.
Statistics are gathered using the DBMS_STATS
package. This PL/SQL package package is also used to modify, view, export, import, and delete statistics.
The DBMS_STATS
package can gather statistics on table and indexes, and well as individual columns and partitions of tables. It does not gather cluster statistics; however, you can use DBMS_STATS
to gather statistics on the individual tables instead of the whole cluster.
When you generate statistics for a table, column, or index, if the data dictionary already contains statistics for the object, then Oracle updates the existing statistics. The older statistics are saved and can be restored later if necessary. See "Restoring Previous Versions of Statistics".
When gathering statistics on system schemas, you can use the procedure DBMS_STATS.GATHER_DICTIONARY_STATS
. This procedure gather statistics for all system schemas, including SYS
and SYSTEM,
and other optional schemas, such as CTXSYS
and DRSYS
.
When statistics are updated for a database object, Oracle invalidates any currently parsed SQL statements that access the object. The next time such a statement executes, the statement is reparsed and the optimizer automatically chooses a new execution plan based on the new statistics. Distributed statements accessing objects with new statistics on remote databases are not invalidated. The new statistics take effect the next time the SQL statement is parsed.
Table 151 lists the procedures in the DBMS_STATS
package for gathering statistics on database objects:
See Also:
PL/SQL Packages and Types Reference for syntax and examples of all 
When using any of these procedures, there are several important considerations for statistics gathering:
The statisticsgathering operations can utilize sampling to estimate statistics. Sampling is an important technique for gathering statistics. Gathering statistics without sampling requires full table scans and sorts of entire tables. Sampling minimizes the resources necessary to gather statistics.
Sampling is specified using the ESTIMATE_PERCENT
argument to the DBMS_STATS
procedures. While the sampling percentage can be set to any value, Oracle Corporation recommends setting the ESTIMATE_PERCENT
parameter of the DBMS_STATS
gathering procedures to DBMS_STATS
.AUTO_SAMPLE_SIZE
to maximize performance gains while achieving necessary statistical accuracy. AUTO_SAMPLE_SIZE
lets Oracle determine the best sample size necessary for good statistics, based on the statistical property of the object. Because each type of statistics has different requirements, the size of the actual sample taken may not be the same across the table, columns, or indexes. For example, to collect table and column statistics for all tables in the OE
schema with autosampling, you could use:
EXECUTE DBMS_STATS.GATHER_SCHEMA_STATS('OE',DBMS_STATS.AUTO_SAMPLE_SIZE);
When the ESTIMATE_PERCENT
parameter is manually specified, the DBMS_STATS
gathering procedures may automatically increase the sampling percentage if the specified percentage did not produce a large enough sample. This ensures the stability of the estimated values by reducing fluctuations.
The statisticsgathering operations can run either serially or in parallel. The degree of parallelism can be specified with the DEGREE
argument to the DBMS_STATS
gathering procedures. Parallel statistics gathering can be used in conjunction with sampling. Oracle Corporation recommends setting the DEGREE
parameter to DBMS_STATS.AUTO_DEGREE
. This setting allows Oracle to choose an appropriate degree of parallelism based on the size of the object and the settings for the parallelrelated init.ora parameters.
Note that certain types of index statistics are not gathered in parallel, including cluster indexes, domain indexes, and bitmap join indexes.
For partitioned tables and indexes, DBMS_STATS
can gather separate statistics for each partition, as well as global statistics for the entire table or index. Similarly, for composite partitioning, DBMS_STATS
can gather separate statistics for subpartitions, partitions, and the entire table or index.The type of partitioning statistics to be gathered is specified in the GRANULARITY
argument to the DBMS_STATS
gathering procedures.
Depending on the SQL statement being optimized, the optimizer can choose to use either the partition (or subpartition) statistics or the global statistics. Both types of statistics are important for most applications, and Oracle Corporation recommends setting the GRANULARITY
parameter to AUTO
to gather both types of partition statistics.
When gathering statistics on a table, DBMS_STATS
gathers information about the data distribution of the columns within the table. The most basic information about the data distribution is the maximum value and minimum value of the column. However, this level of statistics may be insufficient for the optimizer's needs if the data within the column is skewed. For skewed data distributions, histograms can also be created as part of the column statistics to describe the data distribution of a given column. Histograms are described in more details in "Viewing Histograms".
Histograms are specified using the METHOD_OPT
argument of the DBMS_STATS
gathering procedures. Oracle Corporation recommends setting the METHOD_OPT
to FOR
ALL
COLUMNS
SIZE
AUTO
. With this setting, Oracle automatically determines which columns require histograms and the number of buckets (size) of each histogram. You can also manually specify which columns should have histograms and the size of each histogram.
Statistics must be regularly gathered on database objects as those database objects are modified over time. In order to determine whether or not a given database object needs new database statistics, Oracle provides a table monitoring facility. This monitoring is enabled by default when STATISTICS_LEVEL
is set to TYPICAL
or ALL
. Monitoring tracks the approximate number of INSERT
s, UPDATE
s, and DELETE
s for that table, as well as whether the table has been truncated, since the last time statistics were gathered. The information about changes of tables can be viewed in the USER_TAB_MODIFICATIONS
view. Following a datamodification, there may be a few minutes delay while Oracle propagates the information to this view. Use the DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO
procedure to immediately reflect the outstanding monitored information kept in the memory.
The GATHER_DATABASE_STATS
or GATHER_SCHEMA_STATS
procedures gather new statistics for tables with stale statistics when the OPTIONS
parameter is set to GATHER
STALE
or GATHER
AUTO
. If a monitored table has been modified more than 10%, then these statistics are considered stale and gathered again.
You can create userdefined optimizer statistics to support userdefined indexes and functions. When you associate a statistics type with a column or domain index, Oracle calls the statistics collection method in the statistics type whenever statistics are gathered for database objects.
You should gather new column statistics on a table after creating a functionbased index, to allow Oracle to collect column statistics equivalent information for the expression. This is done by calling the statisticsgathering procedure with the METHOD_OPT
argument set to FOR
ALL
HIDDEN
COLUMNS
.
See Also:
Oracle Data Cartridge Developer's Guide for details about implementing userdefined statistics 
When gathering statistics manually, you not only need to determine how to gather statistics, but also when and how often to gather new statistics.
For an application in which tables are being incrementally modified, you may only need to gather new statistics every week or every month. The simplest way to gather statistics in these environment is to use a script or job scheduling tool to regularly run the GATHER_SCHEMA_STATS
and GATHER_DATABASE_STATS
procedures. The frequency of collection intervals should balance the task of providing accurate statistics for the optimizer against the processing overhead incurred by the statistics collection process.
For tables which are being substantially modified in batch operations, such as with bulk loads, statistics should be gathered on those tables as part of the batch operation. The DBMS_STATS
procedure should be called as soon as the load operation completes.
For partitioned tables, there are often cases in which only a single partition is modified. In those cases, statistics can be gathered only on those partitions rather than gathering statistics for the entire table. However, gathering global statistics for the partitioned table may still be necessary.
See Also:
PL/SQL Packages and Types Reference for more information about the 
System statistics describe the system's hardware characteristics, such as I/O and CPU performance and utilization, to the query optimizer. When choosing an execution plan, the optimizer estimates the I/O and CPU resources required for each query. System statistics enable the query optimizer to more accurately estimate I/O and CPU costs, enabling the query optimizer to choose a better execution plan.
When Oracle gathers system statistics, it analyzes system activity in a specified period of time. The statistics are collected using the DBMS_STATS.GATHER_SYSTEM_STATS
procedure. Oracle Corporation highly recommends that you gather system statistics.
Table 152 lists the optimizer system statistics gathered by the DBMS_STATS
package and the options for gathering or manually setting specific system statistics.
Unlike table, index, or column statistics, Oracle does not invalidate already parsed SQL statements when system statistics get updated. All new SQL statements are parsed using new statistics.
See Also:
PL/SQL Packages and Types Reference for detailed information on the procedures in the 
This section discusses:
Whenever statistics in dictionary are modified, old versions of statistics are saved automatically for future restoring. Statistics can be restored using RESTORE
procedures of DBMS_STATS
package. These procedures use a time stamp as an argument and restore statistics as of that time stamp. This is useful in case newly collected statistics leads to some suboptimal execution plans and the administrator wants to revert to the previous set of statistics.
There are dictionary views that display the time of statistics modifications. These views are useful in determining the time stamp to be used for statistics restoration.
DBA_OPTSTAT_OPERATIONS
contain history of statistics operations performed at schema and database level using DBMS_STATS
.*_TAB_STATS_HISTORY
views (ALL
, DBA
, or USER
) contain a history of table statistics modifications.The old statistics are purged automatically at regular intervals based on the statistics history retention setting and the time of the recent analysis of the system. Retention is configurable using the ALTER_STATS_HISTORY_RETENTION
procedure of DBMS_STATS
. The default value is 31 days, which means that you would be able to restore the optimizer statistics to any time in last 31 days.
Automatic purging is enabled when STATISTICS_LEVEL
parameter is set to TYPICAL
or ALL
. If automatic purging is disabled, the old versions of statistics need to be purged manually using the PURGE_STATS
procedure.
The other DBMS_STATS
procedures related to restoring and purging statistics are:
PURGE_STATS
: This procedure can be used to manually purge old versions beyond a time stamp.GET_STATS_HISTORY_RENTENTION
: This function can be used to get the current statistics history retention value.GET_STATS_HISTORY_AVAILABILTY
: This function gets the oldest time stamp where statistics history is available. Users cannot restore statistics to a time stamp older than the oldest time stamp.When restoring previous versions of statistics, the following limitations apply:
RESTORE
procedures cannot restore userdefined statistics.ANALYZE
command has been used for collecting statistics.Statistics can be exported and imported from the data dictionary to userowned tables. This enables you to create multiple versions of statistics for the same schema. It also enables you to copy statistics from one database to another database. You may want to do this to copy the statistics from a production database to a scaleddown test database.
Note: Exporting and importing statistics is a distinct concept from the EXP and IMP utilities of the database. The 
Before exporting statistics, you first need to create a table for holding the statistics. This statistics table is created using the procedure DBMS_STATS.CREATE_STAT_TABLE
. After this table is created, then you can export statistics from the data dictionary into your statistics table using the DBMS_STATS.EXPORT_*_STATS
procedures. The statistics can then be imported using the DBMS_STATS.IMPORT_*_STATS
procedures.
Note that the optimizer does not use statistics stored in a userowned table. The only statistics used by the optimizer are the statistics stored in the data dictionary. In order to have the optimizer use the statistics in a userowned tables, you must import those statistics into the data dictionary using the statistics import procedures.
In order to move statistics from one database to another, you must first export the statistics on the first database, then copy the statistics table to the second database, using the EXP and IMP utilities or other mechanisms, and finally import the statistics into the second database.
The functionality for restoring statistics is similar in some respects to the functionality of importing and exporting statistics. In general, you should use the restore capability when:
You should use EXPORT/IMPORT_*_STATS
procedures when:
Statistics for a table or schema can be locked. Once statistics are locked, no modifications can be made to those statistics until the statistics have been unlocked. These locking procedures are useful in a static environment in which you want to guarantee that the statistics never change.
The DBMS_STATS
package provides two procedures for locking and two procedures for unlocking statistics:
You can set table, column, index, and system statistics using the SET_*_STATISTICS
procedures. Setting statistics in the manner is not recommended, because inaccurate or inconsistent statistics can lead to poor performance.
The purpose of dynamic sampling is to improve server performance by determining more accurate estimates for predicate selectivity and statistics for tables and indexes. The statistics for tables and indexes include table block counts, applicable index block counts, table cardinalities, and relevant join column statistics. These more accurate estimates allow the optimizer to produce better performing plans.
You can use dynamic sampling to:
This dynamic sampling feature is controlled by the OPTIMIZER_DYNAMIC_SAMPLING
parameter. For dynamic sampling to automatically gather the necessary statistics, this parameter should be set to a value of 2 or higher. The default value is 2. See "Dynamic Sampling Levels" for information about the sampling levels that can be set.
The primary performance attribute is compile time. Oracle determines at compile time whether a query would benefit from dynamic sampling. If so, a recursive SQL statement is issued to scan a small random sample of the table's blocks, and to apply the relevant single table predicates to estimate predicate selectivities. The sample cardinality can also be used, in some cases, to estimate table cardinality. Any relevant column and index statistics are also collected.
Depending on the value of the OPTIMIZER_DYNAMIC_SAMPLING
initialization parameter, a certain number of blocks are read by the dynamic sampling query.
See Also:
Oracle Database Reference for details about this initialization parameter 
For a query that normally completes quickly (in less than a few seconds), you will not want to incur the cost of dynamic sampling. However, dynamic sampling can be beneficial under any of the following conditions:
Dynamic sampling can be applied to a subset of a single table's predicates and combined with standard selectivity estimates of predicates for which dynamic sampling is not done.
You control dynamic sampling with the OPTIMIZER_DYNAMIC_SAMPLING
parameter, which can be set to a value from 0
to 10
. The default is 2
.
0
means dynamic sampling will not be done.Dynamic sampling is repeatable if no rows have been inserted, deleted, or updated in the table being sampled. The parameter OPTIMIZER_FEATURES_ENABLE
turns off dynamic sampling if set to a version prior to 9.2.0.
The sampling levels are as follows if the dynamic sampling level used is from a cursor hint or from the OPTIMIZER_DYNAMIC_SAMPLING
initialization parameter:
The sampling levels are as follows if the dynamic sampling level used is from a table hint:
See Also:
"DYNAMIC_SAMPLING" for information about setting the sampling levels with the 
When Oracle encounters a table with missing statistics, Oracle dynamically gathers the necessary statistics needed by the optimizer. However, for certain types of tables, Oracle does not perform dynamic sampling. These include remote tables and external tables. In those cases and also when dynamic sampling has been disabled, the optimizer uses default values for its statistics, shown in Table 153 and Table 154.
Index Statistic  Default Value Used by Optimizer 


1 

25 

1 

1 

100 

800 
This section discusses:
Statistics on tables, indexes, and columns are stored in the data dictionary. To view statistics in the data dictionary, query the appropriate data dictionary view (USER
, ALL
, or DBA
). These DBA_*
views include the following:
See Also:
Oracle Database Reference for information on the statistics in these views 
Column statistics may be stored as histograms. These histograms provide accurate estimates of the distribution of column data. Histograms provide improved selectivity estimates in the presence of data skew, resulting in optimal execution plans with nonuniform data distributions.
Oracle uses two types of histograms for column statistics: heightbalanced histograms and frequency histograms. The type of histogram is stored in the HISTOGRAM
column of the *TAB_COL_STATISTICS
views (USER
and DBA
). This column can have values of HEIGHT
BALANCED
, FREQUENCY
, or NONE
.
In a heightbalanced histogram, the column values are divided into bands so that each band contains approximately the same number of rows. The useful information that the histogram provides is where in the range of values the endpoints fall.
Consider a column C with values between 1 and 100 and a histogram with 10 buckets. If the data in C is uniformly distributed, then the histogram looks similar to Figure 151, where the numbers are the endpoint values.
Text description of the illustration pfgrf208.gif
The number of rows in each bucket is one tenth the total number of rows in the table. Fourtenths of the rows have values that are between 60 and 100 in this example of uniform distribution.
If the data is not uniformly distributed, then the histogram might look similar to Figure 152.
Text description of the illustration pfgrf209.gif
In this case, most of the rows have the value 5 for the column. Only 1/10 of the rows have values between 60 and 100.
Heightbalanced histograms can be viewed using the *TAB_HISTOGRAMS
tables, as shown in Example 151.
BEGIN DBMS_STATS.GATHER_table_STATS (OWNNAME => 'OE', TABNAME => 'INVENTORIES', METHOD_OPT => 'FOR COLUMNS SIZE 10 quantity_on_hand'); END; / SELECT column_name, num_distinct, num_buckets, histogram FROM USER_TAB_COL_STATISTICS WHERE table_name = 'INVENTORIES' AND column_name = 'QUANTITY_ON_HAND'; COLUMN_NAME NUM_DISTINCT NUM_BUCKETS HISTOGRAM     QUANTITY_ON_HAND 237 10 HEIGHT BALANCED SELECT endpoint_number, endpoint_value FROM USER_HISTOGRAMS WHERE table_name = 'INVENTORIES' and column_name = 'QUANTITY_ON_HAND' ORDER BY endpoint_number; ENDPOINT_NUMBER ENDPOINT_VALUE   0 0 1 27 2 42 3 57 4 74 5 98 6 123 7 149 8 175 9 202 10 353
In the query output, one row corresponds to one bucket in the histogram.
In a frequency histogram, each value of the column corresponds to a single bucket of the histogram. Each bucket contains the number of occurrences of that single value. Frequency histograms are automatically created instead of heightbalanced histograms when the number of distinct values is less than or equal to the number of histogram buckets specified. Frequency histograms can be viewed using the *TAB_HISTOGRAMS
tables, as shown in Example 152.
BEGIN DBMS_STATS.GATHER_table_STATS (OWNNAME => 'OE', TABNAME => 'INVENTORIES', METHOD_OPT => 'FOR COLUMNS SIZE 20 warehouse_id'); END; / SELECT column_name, num_distinct, num_buckets, histogram FROM USER_TAB_COL_STATISTICS WHERE table_name = 'INVENTORIES' AND column_name = 'WAREHOUSE_ID'; COLUMN_NAME NUM_DISTINCT NUM_BUCKETS HISTOGRAM     WAREHOUSE_ID 9 9 FREQUENCY SELECT endpoint_number, endpoint_value FROM USER_HISTOGRAMS WHERE table_name = 'INVENTORIES' and column_name = 'WAREHOUSE_ID' ORDER BY endpoint_number; ENDPOINT_NUMBER ENDPOINT_VALUE   36 1 213 2 261 3 370 4 484 5 692 6 798 7 984 8 1112 9