Statistics are used to quantify the data distribution and storage characteristics of tables, columns, indexes and partitions. The COB uses these statistics to estimate how much I/O and memory are required to execute a SQL statement using a particular execution plan. Statistics are stored in the data dictionary, and they can be exported from one database and imported into another. Situations in where you would want to perform. this, might be to transfer production statistics to a test system to simulate the real environment, even though the test system may only have small samples of the data.

In order to give the Oracle cost-based optimizer the most up-to-date information about schema objects (and the best chance for choosing a good execution plan) all application tables and indexes to be accessed must be analyzed. New statistics should be gathered on schema objects that are out of date. After loading or deleting large amounts of data would obviously change the number of rows. Other changes like updating a large amount of rows would not effect the number of rows, but may effect the average row length.

Statistics can be generated with the ANALYZE statement or with the package DBMS_STATS (introduced in Oracle8i). The DBMS_STATS package is great for DBA's in managing database statistics only for use by the COB. The package itself allows the DBA to create, modify, view and delete statistics from a standard, well-defined set of package procedures. The statistics can be gathered on tables, indexes, columns, partitions and schemas, but note that it does not generate statistics for clusters.

DBMS_STATS provides a mechanism for you to view and modify optimizer statistics gathered for database objects.The statistics can reside in two different locations:

• The dictionary.
• A table created in the user's schema for this purpose.

Only statistics stored in the dictionary itself have an impact on the cost-based optimizer.

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. Oracle also invalidates any currently parsed SQL statements that access the object.

The next time such a statement executes, the optimizer automatically chooses a new execution plan based on the new statistics. Distributed statements issued on remote databases that access the analyzed objects use the new statistics the next time Oracle parses them.

When you associate a statistics type with a column or domain index, Oracle calls the statistics collection method in the statistics type if you analyze the column or domain index.

• Analyze
  • The only method available for collecting statistics in Oracle 8.0 and lower.
  • ANALYZE can only run serially.
  • ANALYZE cannot overwrite or delete certain types of statistics that where generated by DBMS_STATS.
  • ANALYZE calculates global statistics for partitioned tables and indexes instead of gathering them directly. This can lead to inaccuracies for some statistics, such as the number of distinct values.
    • For partitioned tables and indexes, ANALYZE gathers statistics for the individual partitions and then calculates the global statistics from the partition statistics.
    • For composite partitioning, ANALYZE gathers statistics for the subpartitions and then calculates the partition statistics and global statistics from the subpartition statistics.
  • ANALYZE can gather additional information that is not used by the optimizer, such as information about chained rows and the structural integrity of indexes, tables, and clusters. DBMS_STATS does not gather this information.
  • No easy way of knowing which tables or how much data within the tables have changed. The DBA would generally re-analyze all of their tables on a semi-regular basis.

• DBMS_STATS
  • Only available for Oracle 8i and higher.
  • Statistics can be generated to a statistics table and can then be imported or exported between databases and re-loaded into the data dictionary at any time. This allows the DBA to experiment with various statistics.
  • DBMS_STATS routines have the option to run via parallel query or operate serially.
  • Can gather statistics for sub-partitions or partitions.
  • Certain DDL commands (ie. create index) automatically generate statistics, therefore eliminating the need to generate statistics explicitly after DDL command.
  • DBMS_STATS does not generate information about chained rows and the structural integrity of segments.
  • The DBA can set a particular table, a whole schema or the entire database to be automatically monitored when a modification occurs. When enabled, any change (insert, update, delete, direct load, truncate, etc.) that occurs on a table will be tracked in the SGA. This information is incorporated into the data dictionary by the SMON process at a pre-set interval (every 3 hours in Oracle 8.1.x, and every 15 minutes in Oracle 9i). The information collected by this monitoring can be seen in the DBA_TAB_MODIFICATIONS view. Oracle 9i introduced a new function in the DBMS_STATS package called: FLUSH_DATABASE_MONITORING_INFO. The DBA can make use of this function to flush the monitored table data more frequently. Oracle 9i will also automatically call this procedure prior to executing DBMS_STATS for statistics gathering purposes. Note that this function is not included with Oracle 8i.
  • DBMS_STATS provides a more efficient, scalable solution for statistics gathering and should be used over the traditional ANALYZE command which does not support features such as parallelism and stale statistics collection.
  • Use of table monitoring in conjunction with DBMS_STATS stale object statistics generation is highly recommended for environments with large, random and/or sporadic data changes. These features allow the database to more efficiently determine which tables should be re-analyzed versus the DBA having to force statistics collection for all tables. Including those that have not changed enough to merit a re-scan)

Oracle collects the following statistics for a table. Statistics marked with an asterisk are always computed exactly. Table statistics, including the status of domain indexes, appear in the data dictionary views USER_TABLES, ALL_TABLES, and DBA_TABLES in the columns shown in parentheses.

• Number of rows (NUM_ROWS)

• * Number of data blocks below the high water mark (that is, the number of data blocks that have been formatted to receive data, regardless whether they currently contain data or are empty) (BLOCKS)

• * Number of data blocks allocated to the table that have never been used (EMPTY_BLOCKS)

• Average available free space in each data block in bytes (AVG_SPACE)

• Number of chained rows. [Not collected by DBMS_STATS] (CHAIN_COUNT)

• Average row length, including the row's overhead, in bytes (AVG_ROW_LEN)

Index Statistics

Oracle collects the following statistics for an index. Statistics marked with an asterisk are always computed exactly. For conventional indexes, the statistics appear in the data dictionary views USER_INDEXES, ALL_INDEXES, and DBA_INDEXES in the columns in parentheses.

• * Depth of the index from its root block to its leaf blocks (BLEVEL)

• Number of leaf blocks (LEAF_BLOCKS)

• Number of distinct index values (DISTINCT_KEYS)

• Average number of leaf blocks per index value (AVG_LEAF_BLOCKS_PER_KEY)

• Average number of data blocks per index value (for an index on a table) (AVG_DATA_BLOCKS_PER_KEY)

• Clustering factor (how well ordered the rows are about the indexed values) (CLUSTERING_FACTOR)

These views are prefixed with DBA_ or ALL_ or USER_. For ease of reading, we will use DBA_% views, but ALL_% views or USER_% views could be used as well.

Conventions Used

- Statistics available only since 8.0.X rdbms release         : (*)
- Statistics available only since 8.1.X rdbms release         : (**)
- Statistics not available at partition or subpartition level : (G)
- Statistics not available at subpartition level              : (GP)

Table level statistics can be retrieved from:

DBA_ALL_TABLES - (8.X onwards)

DBA_OBJECT_TABLES - (8.X onwards

DBA_TABLES - (all versions)

DBA_TAB_PARTITIONS - (8.X onwards)

DBA_TAB_SUBPARTITIONS - (8.1 onwards)

Columns to look at are:

  NUM_ROWS                         : Number of rows (always exact even when computed 
                                     with ESTIMATE method) 
  BLOCKS                           : Number of blocks which have been used even  
                                     if they are empty due to delete statements 
  EMPTY_BLOCKS                     : Number of empty blocks (these blocks have  
                                     never been used) 
  AVG_SPACE                        : Average amount of FREE space in bytes in blocks  
                                     allocated to the table : Blocks + Empty Blocks 
  CHAIN_CNT                        : Number of chained or migrated rows     
  AVG_ROW_LEN                      : Average length of rows in bytes 
  AVG_SPACE_FREELIST_BLOCKS (*)(G) : Average free space of blocks in the freelist 
  NUM_FREELIST_BLOCKS       (*)(G) : Number of blocks in the freelist 
  SAMPLE_SIZE                      : Sample defined in ESTIMATE method (0 if COMPUTE) 
  LAST_ANALYZED                    : Timestamp of last analysis 
  GLOBAL_STATS             (**)    : For partitioned tables, YES means statistics  
                                     are collected for the TABLE as a whole 
                                     NO means statistics are estimated from statistics  
                                     on underlying table partitions or subpartitions 
  USER_STATS               (**)    : YES if statistics entered directly by the user

Index level statistics can be retrieved from:

DBA_INDEXES - (all versions )

DBA_IND_PARTITIONS - (8.X onwards)

DBA_IND_SUBPARTITIONS - (8.1 onwards )

Columns to look at are:

  BLEVEL                       : B*Tree level : depth of the index from its root  
                                 block to its leaf blocks 
  LEAF_BLOCKS                  : Number of leaf blocks 
  DISTINCT_KEYS                : Number of distinct keys 
  AVG_LEAF_BLOCKS_PER_KEY      : Average number of leaf blocks in which each 
                                 distinct key appears (1 for a UNIQUE index) 
  AVG_DATA_BLOCKS_PER_KEY      : Average number of data blocks in the table that  
                                 are pointed to by a distinct key 
  CLUSTERING_FACTOR            : - if near the number of blocks, then the table is  
                                   ordered : index entries in a single leaf block  
                                   tend to point to rows in same data block 
                                 - if near the number of rows, the table is  
                                   randomly ordered : index entries in a single  
                                   leaf block are unlikely to point to rows in  
                                   same data block 
  SAMPLE_SIZE                  : Sample defined in ESTIMATE method (0 if COMPUTE) 
  LAST_ANALYZED                : Timestamp of last analysis 
  GLOBAL_STATS            (**) : For partitioned indexes, YES means statistics  
                                 are collected for the INDEX as a whole 
                                 NO means statistics are estimated from statistics  
                                 on underlying index partitions or subpartitions 
  USER_STATS              (**) : YES if statistics entered directly by the user 
  PCT_DIRECT_ACCESS   (**)(GP) : For secondary indexes on IOTs, percentage of  
                                 rows with VALID guess

Column level statistics can be retrieved from:

DBA_TAB_COLUMNS - (all versions)

DBA_TAB_COL_STATISTICS - (Version 8.X onwards)

DBA_PART_COL_STATISTICS - (Version 8.X onwards)

DBA_SUBPART_COL_STATISTICS - (Version 8.1 onwards)

The last three views extract statistics data from DBA_TAB_COLUMNS.

Columns to look at are:

  NUM_DISTINCT                 : Number of distinct values 
  LOW_VALUE                    : Lowest value  
  LOW_VALUE                    : Highest value  
  DENSITY                      : Density 
  NUM_NULLS                    : Number of columns having a NULL value 
  AVG_COL_LEN                  : Average length in bytes 
  NUM_BUCKETS                  : Number of buckets in histogram for the column    
  SAMPLE_SIZE                  : Sample defined in ESTIMATE method (0 if COMPUTE) 
  LAST_ANALYZED                : Timestamp of last analysis 
  (**)GLOBAL_STATS             : For partitioned tables, YES means statistics  
                                 are collected for the TABLE as a whole 
                                 NO means statistics are estimated from statistics 
                                 on underlying table partitions or subpartitions 
  (**)USER_STATS               : YES if statistics entered directly by the user 

COMPUTE STATISTICS

COMPUTE STATISTICS instructs Oracle to compute exact statistics about the analyzed object and store them in the data dictionary.

When computing statistics, an entire object is scanned to gather data about the object. This data is used by Oracle to compute exact statistics about the object. Slight variances throughout the object are accounted for in these computed statistics. Because an entire object is scanned to gather information for computed statistics, the larger the size of an object, the more work that is required to gather the necessary information.

To perform. an exact computation, Oracle requires enough space to perform. a scan and sort of the table. If there is not enough space in memory, then temporary space may be required. For estimations, Oracle requires enough space to perform. a scan and sort of only the rows in the requested sample of the table. For indexes, computation does not take up as much time or space, so it is best to perform. a full computation.

Some statistics are always computed exactly, such as the number of data blocks currently containing data in a table or the depth of an index from its root block to its leaf blocks.

Use estimation for tables and clusters rather than computation, unless you need exact values. Because estimation rarely sorts, it is often much faster than computation, especially for large tables.

ESTIMATE STATISTICS

ESTIMATE STATISTICS instructs Oracle to estimate statistics about the analyzed object and stores them in the data dictionary.

When estimating statistics, Oracle gathers representative information from portions of an object. This subset of information provides reasonable, estimated statistics about the object. The accuracy of estimated statistics depends upon how representative the sampling used by Oracle is. Only parts of an object are scanned to gather information for estimated statistics, so an object can be analyzed quickly. You can optionally specify the number or percentage of rows that Oracle should use in making the estimate.

To estimate statistics, Oracle selects a random sample of data. You can specify the sampling percentage and whether sampling should be based on rows or blocks.

• Row sampling reads rows without regard to their physical placement on disk. This provides the most random data for estimates, but it can result in reading more data than necessary. For example, in the worst case a row sample might select one row from each block, requiring a full scan of the table or index.

• Block sampling reads a random sample of blocks and uses all of the rows in those blocks for estimates. This reduces the amount of I/O activity for a given sample size, but it can reduce the randomness of the sample if rows are not randomly distributed on disk. Block sampling is not available for index statistics.

Notes on estimating statistics

• The default estimate of the analyze command reads the first approx 1064 rows of the table so the results often leave a lot to be desired.

• The general consensus is that the default value of 1064 is not sufficient for accurate statistics when dealing with tables of any size. Many claims have shown that estimating statistics on 30 percent produces very accurate results. I personally have been running estimate 35 percent. This seems to produce very accurate numbers. It also saves a lot of time over full scans.

• Note that if an estimate does 50% or more of a table Oracle converts the estimate to a full compute statistics.

The stattab parameter specifies the name of a table in which to hold statistics, and it is assumed that it resides in the same schema as the object for which statistics are collected (unless the statown parameter is specified). Users may create multiple tables with different stattab identifiers to hold separate sets of statistics.

Additionally, users can maintain different sets of statistics within a single stattab by using the statid parameter, which can help avoid cluttering the user's schema.

For all of the SET or GET procedures, if stattab is not provided (i.e., NULL), then the operation works directly on the dictionary statistics; therefore, users do not need to create these statistics tables if they only plan to modify the dictionary directly. However, if stattab is not NULL, then the SET or GET operation works on the specified user statistics table, and not the dictionary.

Create Stats Table

DBMS_STATS.CREATE_STAT_TABLE (
  ownname  VARCHAR2, 
  stattab  VARCHAR2,
  tblspace VARCHAR2 DEFAULT NULL);

• ownname : Name of the schema.

• stattab : Name of the table to create. This value should be passed as the stattab parameter to other procedures when the user does not want to modify the dictionary statistics directly.

• tblspace : Tablespace in which to create the stat tables. If none is specified, then they are created in the user's default tablespace.

Drop Stats Table

DBMS_STATS.drop_stat_table (
  ownname VARCHAR2, 
  stattab VARCHAR2);

• ownname : Name of the schema.

• stattab : User stat table identifier.

Gather Schema Stats

DBMS_STATS.gather_schema_stats (
  ownname          VARCHAR2,
  estimate_percent NUMBER   DEFAULT NULL, 
  block_sample     BOOLEAN  DEFAULT FALSE,
  method_opt       VARCHAR2 DEFAULT 'FOR ALL COLUMNS SIZE 1',
  degree           NUMBER   DEFAULT NULL,
  granularity      VARCHAR2 DEFAULT 'DEFAULT', 
  cascade          BOOLEAN  DEFAULT FALSE,
  stattab          VARCHAR2 DEFAULT NULL, 
  statid           VARCHAR2 DEFAULT NULL,
  options          VARCHAR2 DEFAULT 'GATHER', 
  objlist     OUT  ObjectTab,
  statown          VARCHAR2 DEFAULT NULL);

• ownname : Schema to analyze (NULL means current schema).

• estimate_percent : Percentage of rows to estimate (NULL means compute): The valid range is [0.000001,100).

• block_sample : Whether or not to use random block sampling instead of random row sampling. Random block sampling is more efficient, but if the data is not randomly distributed on disk, then the sample values may be somewhat correlated. Only pertinent when doing an estimate statistics.

• method_opt : Method options of the following format (the phrase 'SIZE 1' is required to ensure gathering statistics in parallel and for use with the phrase hidden):

  FOR ALL [INDEXED | HIDDEN] COLUMNS [SIZE integer]

  This value is passed to all of the individual tables.

• degree : Degree of parallelism (NULL means use table default value).

• granularity : Granularity of statistics to collect (only pertinent if the table is partitioned).
  • DEFAULT: Gather global- and partition-level statistics.
  • SUBPARTITION: Gather subpartition-level statistics.
  • PARTITION: Gather partition-level statistics.
  • GLOBAL: Gather global statistics.
  • ALL: Gather all (subpartition, partition, and global) statistics.

• cascade : Gather statistics on the indexes as well.

  Index statistics gathering is not parallelized. Using this option is equivalent to running the gather_index_stats procedure on each of the indexes in the schema in addition to gathering table and column statistics.

• stattab : User stat table identifier describing where to save the current statistics.

• statid : Identifier (optional) to associate with these statistics within stattab.

• options : Further specification of which objects to gather statistics for:
  • GATHER: Gather statistics on all objects in the schema.
  • GATHER STALE: Gather statistics on stale objects as determined by looking at the *_tab_modifications views. Also, return a list of objects found to be stale.
  • GATHER EMPTY: Gather statistics on objects which currently have no statistics. also, return a list of objects found to have no statistics.
  • LIST STALE: Return list of stale objects as determined by looking at the *_tab_modifications views.
  • LIST EMPTY: Return list of objects which currently have no statistics.

• objlist : List of objects found to be stale or empty.

• statown : Schema containing stattab (if different than ownname).

Export Schema Stats

DBMS_STATS.export_schema_stats (
  ownname VARCHAR2,
  stattab VARCHAR2, 
  statid  VARCHAR2 DEFAULT NULL,
  statown VARCHAR2 DEFAULT NULL);

• ownname : Name of the schema.

• stattab : User stat table identifier describing where to store the statistics.

• statid : Identifier (optional) to associate with these statistics within stattab.

• statown : Schema containing stattab (if different than ownname).

Import Schema Stats

DBMS_STATS.import_schema_stats (
  ownname VARCHAR2,
  stattab VARCHAR2, 
  statid  VARCHAR2 DEFAULT NULL,
  statown VARCHAR2 DEFAULT NULL);

• ownname : Name of the schema.

• stattab : User stat table identifier describing from where to retrieve the statistics.

• statid : Identifier (optional) to associate with these statistics within stattab.

• statown : Schema containing stattab (if different than ownname).

Delete Schema Stats

DBMS_STATS.delete_schema_stats (
  ownname VARCHAR2, 
  stattab VARCHAR2 DEFAULT NULL,
  statid  VARCHAR2 DEFAULT NULL,
  statown VARCHAR2 DEFAULT NULL);

• ownname : Name of the schema.

• stattab : User stat table identifier describing from where to delete the statistics. If stattab is NULL, then the statistics are deleted directly in the dictionary.

• statid : Identifier (optional) to associate with these statistics within stattab (Only pertinent if stattab is not NULL).

• statown : Schema containing stattab (if different than ownname).

Set Table Stats

DBMS_STATS.set_table_stats (
  ownname  VARCHAR2, 
  tabname  VARCHAR2, 
  partname VARCHAR2 DEFAULT NULL,
  stattab  VARCHAR2 DEFAULT NULL, 
  statid   VARCHAR2 DEFAULT NULL,
  numrows  NUMBER   DEFAULT NULL, 
  numblks  NUMBER   DEFAULT NULL,
  avgrlen  NUMBER   DEFAULT NULL, 
  flags    NUMBER   DEFAULT NULL,
  statown  VARCHAR2 DEFAULT NULL);

• ownname : Name of the schema.

• tabname : Name of the table.

• partname : Name of the table partition in which to store the statistics. If the table is partitioned and partname is NULL, then the statistics are stored at the global table level.

• stattab : User stat table identifier describing where to store the statistics. If stattab is NULL, then the statistics are stored directly in the dictionary.

• statid : Identifier (optional) to associate with these statistics within stattab (Only pertinent if stattab is not NULL).

• numrows : Number of rows in the table (partition).

• numblks : Number of blocks the table (partition) occupies.

• avgrlen : Average row length for the table (partition).

• flags : For internal Oracle use (should be left as NULL).

• statown : Schema containing stattab (if different than ownname).

Get Table Stats

DBMS_STATS.get_table_stats (
  ownname     VARCHAR2, 
  tabname     VARCHAR2, 
  partname    VARCHAR2 DEFAULT NULL,
  stattab     VARCHAR2 DEFAULT NULL, 
  statid      VARCHAR2 DEFAULT NULL,
  numrows OUT NUMBER, 
  numblks OUT NUMBER,
  avgrlen OUT NUMBER,
  statown     VARCHAR2 DEFAULT NULL);

• ownname : Name of the schema.

• tabname : Name of the table to which this column belongs.

• partname : Name of the table partition from which to get the statistics. If the table is partitioned and if partname is NULL, then the statistics are retrieved from the global table level.

• stattab : User stat table identifier describing from where to retrieve the statistics. If stattab is NULL, then the statistics are retrieved directly from the dictionary.

• statid : Identifier (optional) to associate with these statistics within stattab (Only pertinent if stattab is not NULL).

• numrows : Number of rows in the table (partition).

• numblks : Number of blocks the table (partition) occupies.

• avgrlen : Average row length for the table (partition).

• statown : Schema containing stattab (if different than ownname).

Get Index Stats

DBMS_STATS.GET_INDEX_STATS (
  ownname      VARCHAR2, 
  indname      VARCHAR2,
  partname     VARCHAR2 DEFAULT NULL,
  stattab      VARCHAR2 DEFAULT NULL, 
  statid       VARCHAR2 DEFAULT NULL,
  numrows  OUT NUMBER, 
  numlblks OUT NUMBER,
  numdist  OUT NUMBER, 
  avglblk  OUT NUMBER,
  avgdblk  OUT NUMBER, 
  clstfct  OUT NUMBER,
  indlevel OUT NUMBER,
  statown      VARCHAR2 DEFAULT NULL);

• ownname : Name of the schema.

• indname : Name of the index.

• partname : Name of the index partition for which to get the statistics. If the index is partitioned and if partname is NULL, then the statistics are retrieved for the global index level.

• stattab : User stat table identifier describing from where to retrieve the statistics. If stattab is NULL, then the statistics are retrieved directly from the dictionary.

• statid : Identifier (optional) to associate with these statistics within stattab (Only pertinent if stattab is not NULL).

• numrows : Number of rows in the index (partition).

• numlblks : Number of leaf blocks in the index (partition).

• numdist : Number of distinct keys in the index (partition).

• avglblk : Average integral number of leaf blocks in which each distinct key appears for this index (partition).

• avgdblk : Average integral number of data blocks in the table pointed to by a distinct key for this index (partition).

• clstfct : Clustering factor for the index (partition).

• indlevel : Height of the index (partition).

• statown : Schema containing stattab (if different than ownname).

Create Stats Table

BEGIN
  DBMS_STATS.create_stat_table (
    ownname  => 'scott',
    stattab  => 'stats_table',
    tblspace => 'users');
END;
/

BEGIN
  DBMS_STATS.drop_stat_table (
    wnname => 'scott',
    stattab => 'stats_table');
END;
/

BEGIN
  DBMS_STATS.gather_schema_stats (
    ownname          => 'scott',
    estimate_percent => null,
    block_sample     => false,
    method_opt       => 'FOR ALL COLUMNS SIZE 1',
    degree           => null,
    granularity      => 'ALL',
    cascade          => true,
    options          => 'GATHER');
END;
/

BEGIN
  DBMS_STATS.gather_schema_stats (
    ownname          => 'scott',
    estimate_percent => null,
    block_sample     => false,
    method_opt       => 'FOR ALL COLUMNS SIZE 1',
    degree           => null,
    granularity      => 'ALL',
    cascade          => true,
    stattab          => 'stats_table',
    statid           => 'TEST1',
    options          => 'GATHER',
    statown          => 'scott');
END;
/

BEGIN
  DBMS_STATS.export_schema_stats (
    ownname  => 'scott',
    stattab  => 'stats_table_backup',
    statid   => 'BACKUP_TEST1',
    statown  => 'scott');
END;
/

BEGIN
  DBMS_STATS.import_schema_stats (
    ownname  => 'scott',
    stattab  => 'stats_table',
    statid   => 'TEST1',
    statown  => 'scott');
END;
/

BEGIN
  DBMS_STATS.delete_schema_stats ('scott');
END;
/

BEGIN
  DBMS_STATS.delete_schema_stats (
    ownname  => 'scott',
    stattab  => 'stats_table_backup',
    statid   => 'BACKUP_TEST1',
    statown  => 'scott');
END;
/

BEGIN
  DBMS_STATS.set_table_stats (
    ownname  => 'scott',
    tabname  => 'emp',
    partname => null,
    numrows  => 650000000,
    numblks  => 53455443,
    avgrlen  => 212,
    flags    => null);
END;
/

BEGIN
  DBMS_STATS.set_table_stats (
    ownname  => 'scott',
    tabname  => 'emp',
    partname => null,
    stattab  => 'stats_table',
    statid   => 'TEST1',
    numrows  => 650000000,
    numblks  => 53455443,
    avgrlen  => 212,
    flags    => null,
    statown  => 'scott');
END;
/

SQL> variable NUMROWS number
SQL> variable NUMBLKS number
SQL> variable AVGRLEN number

BEGIN
  DBMS_STATS.get_table_stats (
    'scott', 
    'emp', 
    NUMROWS=>:numrows, 
    NUMBLKS=>:numblks, 
    AVGRLEN=>:avgrlen);
END;
/

PL/SQL procedure successfully completed.

SQL> print NUMROWS NUMBLKS AVGRLEN

   NUMROWS
----------
      1000

   NUMBLKS
----------
        28

   AVGRLEN
----------
        92

SQL> variable NUMROWS number
SQL> variable NUMLBLKS number
SQL> variable NUMDIST number
SQL> variable AVGLBLK number
SQL> variable AVGDBLK number
SQL> variable CLSTFCT number
SQL> variable INDLEVEL number

BEGIN
  DBMS_STATS.get_index_stats (
    'SCOTT',
    'EMP_PK',
    NUMROWS   => :NUMROWS,
    NUMLBLKS  => :NUMLBLKS,
    NUMDIST   => :NUMDIST,
    AVGLBLK   => :AVGLBLK,
    AVGDBLK   => :AVGDBLK,
    CLSTFCT   => :CLSTFCT,
    INDLEVEL  => :INDLEVEL);
 END;
/

PL/SQL procedure successfully completed.

SQL> print NUMROWS NUMLBLKS NUMDIST AVGLBLK AVGDBLK CLSTFCT INDLEVEL

   NUMROWS
----------
      1000

  NUMLBLKS
----------
         3

   NUMDIST
----------
      1000

   AVGLBLK
----------
         1

   AVGDBLK
----------
         1

   CLSTFCT
----------
        15

  INDLEVEL
----------
         1

You can automatically gather statistics or create lists of tables that have stale or no statistics.

To automatically gather statistics, run the DBMS_STATS.GATHER_SCHEMA_STATS and DBMS_STATS.GATHER_DATABASE_STATS procedures with the OPTIONS and objlist parameters. Use the following values for the options parameter:

• GATHER STALE : Gathers statistics on tables with stale statistics.

• GATHER : Gathers statistics on all tables. (default)

• GATHER EMPTY : Gathers statistics only on tables without statistics.

• LIST STALE : Creates a list of tables with stale statistics.

• LIST EMPTY : Creates a list of tables that do not have statistics.

The objlist parameter identifies an output parameter for the LIST STALE and LIST EMPTY options. The objlist parameter is of type DBMS_STATS.OBJECTTAB.

Step 1 : Perform. a quick analyze to load in base statistics

BEGIN
DBMS_STATS.GATHER_SCHEMA_STATS (
  ownname           => 'scott',
  estimate_percent  => null,              -- Small table, lets compute
  block_sample      => false,
  method_opt        => 'FOR ALL COLUMNS',
  degree            => null,              -- No parallelism used in this example
  granularity       => 'ALL',
  cascade           => true,              -- Make sure we include indexes
  options           => 'GATHER'           -- Gather mode
  );
END;
/

PL/SQL procedure successfully completed.

Step 2 : Examine the current statistics

SELECT table_name, num_rows, blocks, avg_row_len    
FROM user_tables
WHERE table_name='EMP';

TABLE_NAME                       NUM_ROWS     BLOCKS AVG_ROW_LEN
------------------------------ ---------- ---------- -----------
EMP                                  1500         28          92

Step 3 : Turn on Automatic Monitoring

Now turn on automatic monitoring for the emp table. This can be done using the alter table method. Starting with Oracle 9i, you can also perform. this at the "schema", and "entire database" level. I provide the syntax for all three methods below.

Monitor only the EMP table.

alter table emp monitoring;

Table altered.

Monitor all of the tables within Scott's schema. (Oracle 9i and higher)

BEGIN
  DBMS_STATS.alter_schema_tab_monitoring('scott', true);
END;
/

PL/SQL procedure successfully completed.

Monitor all of the tables within the database. (Oracle 9i and higher)

Note: Although the option to collect statistics for SYS tables is available via ALTER_DATABASE_TAB_MONITORING, Oracle continues to recommend against this practice until the next major release after 9i Release 2. Also note that the ALTER_DATABASE_TAB_MONITORING procedure in the DBMS_STATS package only monitors tables; there is an ALTER INDEX...MONITORING statement which can be used to monitor indexes. Thanks to Nabil Nawaz for providing this and pointing out an error I made in the previous version of this article.

BEGIN
  DBMS_STATS.alter_database_tab_monitoring (
    monitoring => true,
    sysobjs    => false);      -- Don't set to true, see note above.
END;
/

PL/SQL procedure successfully completed.

Step 4 : Verify that monitoring is turned on.

Note: The results of the following query are from running the alter table ... statement on the emp table only.

SELECT table_name, monitoring
FROM user_tables
ORDER BY monitoring;

TABLE_NAME                     MONITORING
------------------------------ ----------
DEPT                           NO
EMP                            YES

Step 5 : Delete some rows from the database.

SQL> DELETE FROM emp WHERE rownum < 501;

500 rows deleted.

SQL> commit;

Commit complete.

Step 6 : Wait until the monitered data is flushed.

Data can be flushed in several ways.

• In Oracle 8i, you can wait it out for 3 hours.
• In Oracle 9i and higher, you only need to wait 15 minutes.
• In either version, restart the database.
• For immediate results in Oracle 9i and higher, use the DBMS_STATS.flush_database_monitoring_info package.

OK, I'm impatient...

exec dbms_stats.flush_database_monitoring_info;

PL/SQL procedure successfully completed.

Step 7 : Check for what it has collected.

As user "scott", check USER_TAB_MODIFICATIONS to see what it was collected.

SELECT * FROM user_tab_modifications;

TABLE_NAME PARTITION_NAME SUBPARTITION_NAME INSERTS UPDATES DELETES TIMESTAMP TRUNCATED
---------- -------------- ----------------- ------- ------- ------- --------- ---------
EMP                                               0       0     500 18-SEP-02 NO

Step 8 : Execute DBMS_STATS to gather stats on all "stale" tables.

BEGIN
  DBMS_STATS.GATHER_SCHEMA_STATS(
    ownname           => 'scott',
    estimate_percent  => null,
    block_sample      => false,
    method_opt        => 'FOR ALL COLUMNS',
    degree            => null,
    granularity       => 'ALL',
    cascade           => true,
    options           => 'GATHER STALE');
END;
/

PL/SQL procedure successfully completed.

Step 9 : Verify that the table is no longer listed in USER_TAB_MODIFICATIONS.

SQL> SELECT * FROM user_tab_modifications;

no rows selected.

Step 10 : Examine some of new statistics collected.

SELECT table_name, num_rows, blocks, avg_row_len    
FROM user_tables where table_name='EMP';

TABLE_NAME                       NUM_ROWS     BLOCKS AVG_ROW_LEN
------------------------------ ---------- ---------- -----------
EMP                                  1000         28          92

This is crucial for development environments that are testing the performance of SQL statements with various sets of statistics. The DBA will need to know if the relying statistics are RDBMS-defined or user-defined.

RDBMS-generated statistics are generated by the following:

• ANALYZE SQL command
• DBMS_UTILITY.ANALYZE_SCHEMA procedure
• DBMS_UTILITY.ANALYZE_DATABASE procedure
• DBMS_DDL.ANALYZE_OBJECT procedure
• 8.1 DBMS_STATS.GATHER_xx_STATS procedures

The column USER_STATS from DBA_TABLES, ALL_TABLES, USER_TABLES displays:

• YES, when statistics are entered directly by a user.
• NO, when statistics are generated by RDBMS through an ANALYZE statement.