对于analyze table使用

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Oracle

首先创建四个临时表t1,t2,t3,t4,和他们相对应的索引
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代码:
create table t1 as select * from user_objects;
create table t2 as select * from user_objects;
create table t3 as select * from user_objects;
create table t4 as select * from user_objects;
create unique index pk_t1_idx on t1(object_id);
create unique index pk_t2_idx on t2(object_id);
create unique index pk_t3_idx on t3(object_id);
create unique index pk_t4_idx on t4(object_id);
查看这个时候各个表对应的数据库统计信息(表,字段,索引)
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代码:
--查看表的统计信息
select table_name,num_rows,blocks,empty_blocks from user_table where table_names in ('T1','T2','T3','T4');
TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS
T1
T2
T3
T4

--查看字段的统计信息
select table_name,column_name,num_distinct,low_value,high_value,density from user_tab_columns where table_name in ('T1','T2','T3','T4');
TABLE_NAME COLUMN_NAME NUM_DISTINCT LOW_VALUE HIGH_VALUE DENSITY
T1 OBJECT_NAME
T1 SUBOBJECT_NAME
T1 OBJECT_ID
T1 DATA_OBJECT_ID
T1 OBJECT_TYPE
T1 CREATED
T1 LAST_DDL_TIME
T1 TIMESTAMP
T1 STATUS
T1 TEMPORARY
T1 GENERATED
T1 SECONDARY
T2 OBJECT_NAME
T2 SUBOBJECT_NAME
T2 OBJECT_ID
T2 DATA_OBJECT_ID
T2 OBJECT_TYPE
T2 CREATED
T2 LAST_DDL_TIME
T2 TIMESTAMP
T2 STATUS
T2 TEMPORARY
T2 GENERATED
T2 SECONDARY
T3 OBJECT_NAME
T3 SUBOBJECT_NAME
T3 OBJECT_ID
T3 DATA_OBJECT_ID
T3 OBJECT_TYPE
T3 CREATED
T3 LAST_DDL_TIME
T3 TIMESTAMP
T3 STATUS
T3 TEMPORARY
T3 GENERATED
T3 SECONDARY
T4 OBJECT_NAME
T4 SUBOBJECT_NAME
T4 OBJECT_ID
T4 DATA_OBJECT_ID
T4 OBJECT_TYPE
T4 CREATED
T4 LAST_DDL_TIME
T4 TIMESTAMP
T4 STATUS
T4 TEMPORARY
T4 GENERATED
T4 SECONDARY

--查看索引的统计信息
select table_name,index_name,blevel,leaf_blocks,distinct_keys,
avg_leaf_blocks_per_key avg_leaf_blocks,avg_data_blocks_per_key avg_data_blocks,clustering_factor,num_rows
from user_indexes where table_name in ('T1','T2','T3','T4');
TABLE_NAME INDEX_NAME BLEVEL LEAF_BLOCKS DISTINCT_KEYS AVG_LEAF_BLOCKS AVG_DATA_BLOCKS CLUSTERING_FACTOR NUM_ROWS
T1 PK_T1_IDX
T2 PK_T2_IDX
T3 PK_T3_IDX
T4 PK_T4_IDX
现在我们分别对这个表做不同形式的analyze table处理
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代码:
analyze table t1 compute statistics for table;
analyze table t2 compute statistics for all columns;
analyze table t3 compute statistics for all indexed columns;
analyze table t4 compute statistics;
我们再回头看看这是的oracle数据库对于各种统计信息
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代码:
--这是对于表的统计信息
select table_name,num_rows,blocks,empty_blocks from user_tables where table_name in ('T1','T2','T3','T4');
TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS
T1 3930 55 1
T2
T3
T4 3933 55 1
--我们可以据此得出结论,只有我们在analyze table命令中指定了for table或者不指定任何参数的时候,oracle数据库才会给我们统计基于表的统计信息

--这是对于表中字段的统计信息
select table_name,column_name,num_distinct,low_value,high_value,density from user_tab_columns where table_name in ('T1','T2','T3','T4');
TABLE_NAME COLUMN_NAME NUM_DISTINCT LOW_VALUE HIGH_VALUE DENSITY
T1 OBJECT_NAME
T1 SUBOBJECT_NAME
T1 OBJECT_ID
T1 DATA_OBJECT_ID
T1 OBJECT_TYPE
T1 CREATED
T1 LAST_DDL_TIME
T1 TIMESTAMP
T1 STATUS
T1 TEMPORARY
T1 GENERATED
T1 SECONDARY
T2 OBJECT_NAME 3823 41423030 D3F1BBB736D4C2B7DDCFFABBA7C7E5B5A5 .000270447891062615
T2 SUBOBJECT_NAME 77 503031 52455354 .012987012987013
T2 OBJECT_ID 3930 C304062D C30F4619 .000254452926208651
T2 DATA_OBJECT_ID 3662 C304062D C30F4619 .000273074822501365
T2 OBJECT_TYPE 15 4441544142415345204C494E4B 56494557 .000127194098193844
T2 CREATED 3684 7867081E111F33 7868071211152F .000547559423988464
T2 LAST_DDL_TIME 3574 7867081E11251B 7868071211152F .000565522924083892
T2 TIMESTAMP 3649 323030332D30382D33303A31363A33303A3530 323030342D30372D31383A31363A32303A3436 .000559822349362313
T2 STATUS 2 494E56414C4944 56414C4944 .000127194098193844
T2 TEMPORARY 2 4E 59 .000127194098193844
T2 GENERATED 2 4E 59 .000127194098193844
T2 SECONDARY 2 4E 59 .000127194098193844
T3 OBJECT_NAME
T3 SUBOBJECT_NAME
T3 OBJECT_ID 3931 C304062D C30F461A .000254388196387688
T3 DATA_OBJECT_ID
T3 OBJECT_TYPE
T3 CREATED
T3 LAST_DDL_TIME
T3 TIMESTAMP
T3 STATUS
T3 TEMPORARY
T3 GENERATED
T3 SECONDARY
T4 OBJECT_NAME 3825 41423030 D3F1BBB736D4C2B7DDCFFABBA7C7E5B5A5 .000261437908496732
T4 SUBOBJECT_NAME 77 503031 52455354 .012987012987013
T4 OBJECT_ID 3932 C304062D C30F461B .000254323499491353
T4 DATA_OBJECT_ID 3664 C304062D C30F461B .00027292576419214
T4 OBJECT_TYPE 15 4441544142415345204C494E4B 56494557 .0666666666666667
T4 CREATED 3685 7867081E111F33 78680712111530 .000271370420624152
T4 LAST_DDL_TIME 3575 7867081E11251B 78680712111530 .00027972027972028
T4 TIMESTAMP 3650 323030332D30382D33303A31363A33303A3530 323030342D30372D31383A31363A32303A3437 .000273972602739726
T4 STATUS 2 494E56414C4944 56414C4944 .5
T4 TEMPORARY 2 4E 59 .5
T4 GENERATED 2 4E 59 .5
T4 SECONDARY 2 4E 59 .5
/*
在这个结果中我们可以看到,oracle数据库给t2,t4的所有字段都做了统计信息.
对表t3的object_id(索引字段)做了统计信息.
由此得出结论,
在指定for all columns 和不指定任何参数的时候oracle会给所有字段做统计信息,在指定for indexed columns时,oracle只给[b]有索引的字段进行字段信息统计[/b],如果我们别有必要给所有字段统计信息时,这个属性就很有用了.
*/

--这里是对于索引的统计信息
select table_name,index_name,blevel,leaf_blocks,distinct_keys,
avg_leaf_blocks_per_key avg_leaf_blocks,avg_data_blocks_per_key avg_data_blocks,clustering_factor,num_rows
from user_indexes where table_name in ('T1','T2','T3','T4');
TABLE_NAME INDEX_NAME BLEVEL LEAF_BLOCKS DISTINCT_KEYS AVG_LEAF_BLOCKS AVG_DATA_BLOCKS CLUSTERING_FACTOR NUM_ROWS
T1 PK_T1_IDX
T2 PK_T2_IDX
T3 PK_T3_IDX
T4 PK_T4_IDX 1 9 3932 1 1 2143 3932

--从这里我们可以看出,只有表t4有索引统计信息.
--再综合前面的我们就会发现,如果在运行analyze table是我们不指定参数,oracle将收集对于特定表的所有统计信息(表,索引,表字段的统计信息)
补充,truncate命令不修改以上统计信息
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代码:
truncate table t1;
truncate table t2;
truncate table t3;
truncate table t4;
--我们在查看表和索引的统计信息
select table_name,num_rows,blocks,empty_blocks from user_tables where table_name in ('T1','T2','T3','T4');
TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS
T1 3930 55 1
T2
T3
T4 3933 55 1

--我们再对以上各表做一次分析
analyze table t1 compute statistics for table;
analyze table t2 compute statistics for all columns;
analyze table t3 compute statistics for all indexed columns;
analyze table t4 compute statistics;

--现在再来查看表和索引的统计信息
select table_name,num_rows,blocks,empty_blocks,initial_extent,'8192' block_size from user_tables where table_name in ('T1','T2','T3','T4');
TABLE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS INITIAL_EXTENT BLOCK_SIZE
T1 0 0 8 65536 8192
T2 65536 8192
T3 65536 8192
T4 0 0 8 65536 8192

--索引的统计信息
select table_name,index_name,blevel,leaf_blocks,distinct_keys,
avg_leaf_blocks_per_key avg_leaf_blocks,avg_data_blocks_per_key avg_data_blocks,clustering_factor,num_rows
from user_indexes where table_name in ('T1','T2','T3','T4');
TABLE_NAME INDEX_NAME BLEVEL LEAF_BLOCKS DISTINCT_KEYS AVG_LEAF_BLOCKS AVG_DATA_BLOCKS CLUSTERING_FACTOR NUM_ROWS
T1 PK_T1_IDX
T2 PK_T2_IDX
T3 PK_T3_IDX
T4 PK_T4_IDX 0 0 0 0 0 0 0
--由此得出结论,truncate命令不会修改数据的统计信息,
--也就是如果我们想让CBO利用合理利用数据的统计信息的时候,需要我们及时的使用analyze命令或者dbms_stats重新统计数据的统计信息

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