ORATEA ORACLE的统计信息在执行SQL的过程中扮演着非常重要的作用，而且ORACLE在表的各个层次都会有不同的统计信息，通过这些统计信息来描述表的，列的各种各样的统计信息。下面通过一个复合分区表来说明一些常见的和常见的统计信息。
SQL>
create table test
partition by range(object_id)
subpartition by hash(object_type) subpartitions 4
(partition p1 values less than(10000),
partition p2 values less than(20000),
partition p3 values less than(30000),
partition p4 values less than(maxvalue))
as
select * from dba_objects;
表已创建。
sql>
BEGIN
dbms_stats.gather_table_stats(ownname => 'SCOTT',
tabname => 'TEST',
estimate_percent => 100,
block_sample => FALSE,
method_opt => 'FOR ALL COLUMNS SIZE 10',
granularity => 'ALL',
cascade => TRUE);
END;
1,表级的统计信息
SQL> select table_name,num_rows,blocks,empty_blocks,avg_space from user_tables where table_name = 'TEST';
TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS AVG_SPACE
------------------------------ ---------- ---------- ------------ ----------
TEST 50705 788 0 0
2,表上列的统计信息
SQL> select table_name,column_name,num_distinct,density from user_tab_columns where table_name = 'TEST';
TABLE_NAME COLUMN_NAME NUM_DISTINCT DENSITY
------------------------------ ------------------------------ ------------ ----------
TEST OWNER 25 .365014295
TEST OBJECT_NAME 30275 .000039205
TEST SUBOBJECT_NAME 191 .015657993
TEST OBJECT_ID 50705 .000019722
TEST DATA_OBJECT_ID 4334 .000248075
TEST OBJECT_TYPE 42 .271207855
TEST CREATED 2305 .001608457
TEST LAST_DDL_TIME 2369 .001566737
TEST TIMESTAMP 2412 .001610251
TEST STATUS 2 .000009861
TEST TEMPORARY 2 .000009861
TEST GENERATED 2 .000009861
TEST SECONDARY 2 .000009861
13 rows selected.
3,表上列的直方图信息
SQL>
select table_name,column_name,endpoint_number,endpoint_value
from user_tab_histograms
where table_name = 'TEST'
and column_name = 'OBJECT_ID';
TABLE_NAME COLUMN_NAM ENDPOINT_NUMBER ENDPOINT_VALUE
---------- ---------- --------------- --------------
TEST OBJECT_ID 0 2
TEST OBJECT_ID 1 5160
TEST OBJECT_ID 2 10587
TEST OBJECT_ID 3 15658
TEST OBJECT_ID 4 20729
TEST OBJECT_ID 5 25800
TEST OBJECT_ID 6 30870
TEST OBJECT_ID 7 35940
TEST OBJECT_ID 8 41089
TEST OBJECT_ID 9 46821
TEST OBJECT_ID 10 53497
4,分区的统计信息
SQL>
select partition_name,num_rows,blocks,empty_blocks,avg_space
from user_tab_partitions
where table_name = 'TEST';
PARTITION_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS AVG_SPACE
--------------- ---------- ---------- ------------ ----------
P1 9581 140 0 0
P2 9973 164 0 0
P3 10000 158 0 0
P4 21151 326 0 0
5,分区上列的统计信息
SQL> select column_name,num_distinct,density,num_nulls
from user_part_col_statistics
where table_name = 'TEST'
and partition_name = 'P1';
COLUMN_NAME NUM_DISTINCT DENSITY NUM_NULLS
--------------- ------------ ---------- ----------
OWNER 7 .000052187 0
OBJECT_NAME 7412 .000156925 0
SUBOBJECT_NAME 26 .47017301 9496
OBJECT_ID 9581 .000104373 0
DATA_OBJECT_ID 1765 .000664385 7780
OBJECT_TYPE 34 .18494854 0
CREATED 913 .001977449 0
LAST_DDL_TIME 994 .001882695 0
TIMESTAMP 982 .001928775 0
STATUS 2 .000052187 0
TEMPORARY 2 .000052187 0
GENERATED 2 .000052187 0
SECONDARY 1 .000052187 0
6,分区上列的直方图信息
SQL> select column_name,bucket_number,endpoint_value
from user_part_histograms
where table_name = 'TEST'
and partition_name = 'P1'
and column_name = 'OBJECT_ID';
COLUMN_NAME BUCKET_NUMBER ENDPOINT_VALUE
--------------- ------------- --------------
OBJECT_ID 0 2
OBJECT_ID 1 1005
OBJECT_ID 2 1963
OBJECT_ID 3 2921
OBJECT_ID 4 3888
OBJECT_ID 5 4859
OBJECT_ID 6 5941
OBJECT_ID 7 6899
OBJECT_ID 8 7885
OBJECT_ID 9 8864
OBJECT_ID 10 9999
7,子分区的统计信息
SQL> select subpartition_name,num_rows,blocks,empty_blocks
from user_tab_subpartitions
where table_name = 'TEST'
and partition_name = 'P1';
SUBPARTITION_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS
------------------------------ ---------- ---------- ------------
SYS_SUBP21 3597 50 0
SYS_SUBP22 3566 52 0
SYS_SUBP23 637 11 0
SYS_SUBP24 1781 27 0
8,子分区上的列的统计信息
SQL> select column_name,num_distinct,density
from user_subpart_col_statistics
where table_name = 'TEST'
and subpartition_name = 'SYS_SUBP21';
COLUMN_NAME NUM_DISTINCT DENSITY
--------------- ------------ ----------
OWNER 6 .000139005
OBJECT_NAME 3595 .000278319
SUBOBJECT_NAME 4 .014285714
OBJECT_ID 3597 .000278009
DATA_OBJECT_ID 155 .006451613
OBJECT_TYPE 8 .000139005
CREATED 751 .002392334
LAST_DDL_TIME 784 .002302524
TIMESTAMP 768 .00235539
STATUS 1 .000139005
TEMPORARY 2 .000139005
GENERATED 2 .000139005
SECONDARY 1 .000139005
9,子分区上的列的直方图信息
SQL> select column_name,bucket_number,endpoint_value
from user_subpart_histograms
where table_name = 'TEST'
and subpartition_name = 'SYS_SUBP21'
and column_name = 'OBJECT_ID';
COLUMN_NAME BUCKET_NUMBER ENDPOINT_VALUE
--------------- ------------- --------------
OBJECT_ID 0 208
OBJECT_ID 1 1525
OBJECT_ID 2 2244
OBJECT_ID 3 2892
OBJECT_ID 4 3252
OBJECT_ID 5 4047
OBJECT_ID 6 5238
OBJECT_ID 7 6531
OBJECT_ID 8 7661
OBJECT_ID 9 8474
OBJECT_ID 10 9998
我们对这个复合分区分析之后产生了上面这九种不同层次的统计信息。CBO想要得要一个高效的执行计划需要如此多的统计信息.