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.（帶*為準確值）

oracle收集如下的索引統計信息。對于常規索引，可以在視圖USER_INDEXES, ALL_INDEXES, and DBA_INDEXES中查看到如下的統計信息。

Where are the statistics stored?

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.

統計信息存儲在數據字典中，在sys用戶下的表內。通過視圖我們可以非常方便的從這些表中獲取信息。視圖通常以DBA_ USER_ ALL_開始。為了簡便，我們以DBA_開頭的視圖為例。

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:

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:

Columns to look at are:

 BLEVEL : B*Tree level : depth of the index from its root 
 block to its leaf blocks (從0開始)
 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（可以通過alter index index_name update block references來更新）

Column level statistics can be retrieved from:

The last three views extract statistics data from DBA_TAB_COLUMNS.（后三個視圖是從DBA_TAB_COLUMNS獲取數據）

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 vs. Estimate statistics

Both computed and estimated statistics are used by the Oracle optimizer to choose the execution plan for SQL statements that access analyzed objects. These statistics may also be useful to application developers who write such statements.

無論是采用compute還是采用estimat的方式計算統計信息，優化器都會根據這些信息來選擇執行計劃。程序員也可以根據這些統計信息來編寫sql語句。

COMPUTE STATISTICS

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.

當COMPUTE STATISTICS時，oracle會精確計算被分析對象的統計信息，并將其存儲在數據字典中。oracle會掃描整個對象來獲取數據，并根據這些數據計算統計信息。對于這種方式，基本是輕微的變化也會被計算在內。因為整個對象都會被掃描，因此對象越大就會需要越多的工作量來完成統計。

為了完成精確統計，oracle需要足夠的空間來執行掃描和排序作業。如果在內存中不存在足夠的空間，就會占用磁盤的臨時空間。對于estimation方式，oracle僅僅需要掃描和排序所采樣的內容。如果我們統計的對象是索引，computation方式不會占用太多的時間和空間，因此對于索引我們最好采用compute方式。

某些統計信息總是精確計算的，例如表所占用的數據塊數量和索引的深度。

對于表和聚簇，我們建議使用estimation的方式，除非真的需要精確的統計信息。因此estatimation方式通常不會發生排序，速度更快，尤其在分析大表時。

ESTIMATE STATISTICS

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.

estimate statistics 使得oracle評估待分析對象的統計信息并將它們存儲在數據字典中。當評估統計信息時，oracle在待分析對象的部分區間內收集信息。這部分信息為分析對象提供了足夠的內容。estimate方式的準確程度主要依賴于oracle是如何采樣的。由于只有部分內容被掃描，因此速度更快。我們可以指定oracle采樣的百分比。

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.

對于estimate方式，oracle會隨機采樣數據。我們可以指定采樣的百分比，也可以指定是根據記錄還是根據塊來采樣。

Notes on estimating statistics

DBMS_STATS functions and variable definitions

Most of the DBMS_STATS procedures include the three parameters statown, stattab, and statid. These parameters allow you to store statistics in your own tables (outside of the dictionary), which does not affect the optimizer. Therefore, you can maintain and experiment with sets of statistics.

大部分DBMS_STAT過程包含三個參數STATOWN,STATTAB和statid。這些參數允許我們將統計信息存放到自己的表中，這些統計信息不回影響優化器。因此，我們可以維護和測試統計信息。

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 stattabidentifiers to hold separate sets of statistics.stattab參數規定了保存統計信息的表明，通常情況下，如果沒有指定statown參數，oracle以被統計對象所在的模式用戶為stattab的擁有者。我們可以使用不同的stattab來分別存儲不同的統計信息。

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.

靈位，我們也可以指定statid參數，從而在相同的stattab中存儲不同的統計信息，這樣可以使用戶模式顯得井井有條。

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.

對于所有的set和get過程，如果我們沒有指定stattab，oracle會將統計信息寫入數據字典，如果指定了stattab，orcle只會將統計信息寫入用戶自定義表，而不會更新數據字典。

Create Stats Table

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

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);

stattab : User stat table identifier.

Gather Schema Stats (本人在測試過程中，即便指定了stattab，該過程依然更新了數據字典)

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',（size 1 指在該列上不創建histogram，如果該值大于1，則創建histogram）
 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);

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(從數據字典導出到用戶表）

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

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（從用戶表導入到數據字典）

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

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

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);

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);

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);

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);

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.

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).

Automated table monitoring and stale statistics gathering example

在oracle10g中 statistics_level 初始化參數作為一個全局設置影響對表的監控操作，本文下面涉及的alter_schema_tab_monitoring已經不再被使用，但是到我們調用這些過程時，不會報錯，只是沒有任何事情發生。

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 : 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.（通過*_tab_modifications視圖）

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.

通過alter table 語句我們可以設置oracle數據庫自動監控某張表的變化，從9i開始，我們還可以在schema或者數據庫級別設置是否監控數據變化，監控結構會存儲在*_tab_modifications視圖中。

Monitor only the EMP table.

alter table emp monitoring;

Table altered.

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

PL/SQL procedure successfully completed.

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.

可以通過*_tables視圖的monitoring字段來判斷某張表是否開啟了自動監控

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.

exec dbms_stats.flush_database_monitoring_info;

PL/SQL procedure successfully completed.

Step 7 : Check for what it has 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

How to determine if dictionary statistics are RDBMS-generated or user-defined

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:（我們可以通過如下方式生成統計信息）

The column USER_STATS from DBA_TABLES, ALL_TABLES, USER_TABLES displays:

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