綁定變量窺視是否開啟和直方圖關系

如果有直方圖信息，而且綁定變量窺視也開啟了，這里我們很好理解，在oracle 9i和oracle 10g時，sql第一次執行會將綁定變量的值帶入到sql中，然后根據直方圖等統計信息來評估合理的執行計劃，在同樣的sql下次執行時，會直接使用之前的執行計劃，這個也就是我

如果有直方圖信息，而且綁定變量窺視也開啟了，這里我們很好理解，在oracle 9i和oracle 10g時，sql第一次執行會將綁定變量的值帶入到sql中，然后根據直方圖等統計信息來評估合理的執行計劃，在同樣的sql下次執行時，會直接使用之前的執行計劃，這個也就是我們經常所接觸的綁定變量窺視。

1） 開啟了綁定變量窺視，收集該列的直方圖：
SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE 10.2.0.4.0 Production
TNS for 64-bit Windows: Version 10.2.0.4.0 - Production
NLSRTL Version 10.2.0.4.0 - Production

SQL> alter system set "_optim_peek_user_binds"=true;

System altered.

SQL> create table t002 as select * from dba_objects;

Table created.

SQL> update t002 set object_id=100 where rownum<50000;

49999 rows updated.

SQL> commit;

Commit complete.

SQL> execute dbms_stats.gather_table_stats(ownname=>'XIAOYU',tabname=>'T002',met
hod_opt=>'for all columns size 254');

PL/SQL procedure successfully completed.

SQL> select num_rows,blocks from user_tables where table_name='T002';

NUM_ROWS BLOCKS
---------- ----------
50328 712

SQL> select num_distinct,density,num_nulls from user_tab_columns where table_nam
e='T002' and column_name='OBJECT_ID';

NUM_DISTINCT DENSITY NUM_NULLS
------------ ---------- ----------
29 9.8243E-06 0

SQL> variable x number;
SQL> exec : x:=100;

PL/SQL procedure successfully completed.

SQL> select count(*) from t002 where object_id=:x;

COUNT(*)
----------
49999
SQL> select * from table(dbms_xplan.display_cursor(null,null,'all'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
------------------------------------------------------------
SQL_ID 4yqsqnawx85ty, child number 0
-------------------------------------
select count(*) from t002 where object_id=:x

Plan hash value: 3014849763

---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 158 (100)| |
| 1 | SORT AGGREGATE | | 1 | 4 | | |
|* 2 | TABLE ACCESS FULL| T002 | 50065 | 195K| 158 (1)| 00:00:02 |
---------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

1 - SEL$1
2 - SEL$1 / T002@SEL$1

Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_ID"=:X)

Column Projection Information (identified by operation id):
-----------------------------------------------------------

1 - (#keys=0) COUNT(*)[22]

30 rows selected.

SQL> exec : x:=1;

PL/SQL procedure successfully completed.

SQL> select count(*) from t002 where object_id=:x;

COUNT(*)
----------
0

SQL> select * from table(dbms_xplan.display_cursor(null,null,'all'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
------------------------------------------------------------
SQL_ID 4yqsqnawx85ty, child number 0
-------------------------------------
select count(*) from t002 where object_id=:x

Plan hash value: 3014849763

---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 158 (100)| |
| 1 | SORT AGGREGATE | | 1 | 4 | | |
|* 2 | TABLE ACCESS FULL| T002 | 50065 | 195K| 158 (1)| 00:00:02 |
---------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

1 - SEL$1
2 - SEL$1 / T002@SEL$1

Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_ID"=:X)

Column Projection Information (identified by operation id):
-----------------------------------------------------------

1 - (#keys=0) COUNT(*)[22]

30 rows selected.

這里比較好理解，由于開啟了綁定變量窺視，然后該列又有直方圖信息，所以第一次執行時會把具體值帶入，然后根據具體值的直方圖信息來評估rows，這里的通過謂詞過濾后估算返回的rows是50065，關于有直方圖的情況下估算rows的方式，非常復雜，小魚自己也沒有過多的深究，后面有機會整理相應的文章來分享。

2）如果開啟了綁定變量窺視，但是沒有收集直方圖：
SQL> alter system set "_optim_peek_user_binds"=true;

System altered.

SQL> execute dbms_stats.gather_table_stats(ownname=>'XIAOYU',tabname=>'T002',met
hod_opt=>'for all columns size 1');

PL/SQL procedure successfully completed.

SQL>variable y number;
SQL>exec : y:=100

PL/SQL procedure successfully completed

SQL> select count(*) from t002 where object_id=:y;

COUNT(*)
----------
49999

SQL> select * from table(dbms_xplan.display_cursor(null,null,'all'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------

SQL_ID 86ngbvm962n14, child number 0
-------------------------------------
select count(*) from t002 where object_id=:y

Plan hash value: 3014849763

--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 158 (100)|
| 1 | SORT AGGREGATE | | 1 | 4 | |
|* 2 | TABLE ACCESS FULL| T002 | 1290 | 5160 | 158 (1)| 00:00:02
--------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

1 - SEL$1
2 - SEL$1 / T002@SEL$1

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter("OBJECT_ID"=:Y)

Column Projection Information (identified by operation id):
-----------------------------------------------------------

1 - (#keys=0) COUNT(*)[22]

30 rows selected.

這里我們來算算這個rows 1290是如何估算出來的

這里介紹最簡單的如何計算rows，selectivity、density的公式：（下列計算公式在該列沒有直方圖前提下）

小魚之前介紹cbo優化器的基本知識里面提過一個selectivity可選擇率這個概念

可選擇率selectivity=釋放指定謂詞條件返回結果集的記錄數/未施加任何謂詞條件的原始結果集的記錄數，可選擇率越大，那么cbo估算返回的rows也越大。

那么集的勢rows=selectivity*未施加任何謂詞條件的原始結果集的記錄數

那么這個可選擇率selectivity如何計算了，在列的統計信息中num_nulls為0時，selectivity=1/num_distinct

SQL> select num_distinct,num_nulls,density,num_buckets from dba_tab_columns wher
e table_name='T002' and column_name='OBJECT_ID';

NUM_DISTINCT NUM_NULLS DENSITY NUM_BUCKETS
------------ ---------- ---------- -----------
39 0 .025641026 1

SQL> select num_rows from user_tables where table_name='T002';

NUM_ROWS
----------
50328

SQL> select 1/39 from dual;

1/39
----------
.025641026

SQL> select 50328*1/39 from dual;

50328*1/39
----------
1290.46154

這里我們通過統計信息發現計算而來的1290跟執行計劃的rows 1290完全一致

列沒有直方圖，而且num_nulls為0時:
Selectivity_without_null=(1/num_distinct),也就是列的統計信息中num_nulls為0時，列的選擇率是1/num_distinct，此時density也是等于1/num_distinct

列沒有直方圖，但是num_nulls又不為0時:
selectivity_with_null=(1/num_distinct)*(num_rows-num_nulls)/(num_rows)，而density還是等于1/num_distinct

對于第一點num_nulls為0,列沒有直方圖，selectivity選擇率和density上面已經進行了驗證，下面稍微擴展點，來驗證下num_nulls不為0，沒有直方圖時，可選擇率selectivity、rows和density關系

SQL> update t002 set object_id=null where rownum<1000;
SQL> commit;
SQL> execute dbms_stats.gather_table_stats(ownname=>'XIAOYU',tabname=>'T002',method_opt=>'for all columns size 1');

SQL> select NUM_ROWS, --表中的記錄數
2 BLOCKS, --表中數據所占的數據塊數
3 EMPTY_BLOCKS, --表中的空塊數
4 AVG_SPACE, --數據塊中平均的使用空間
5 CHAIN_CNT, --表中行連接和行遷移的數量
6 AVG_ROW_LEN --每條記錄的平均長度
7 from dba_tables
8 where owner=&owner and table_name=&tabname;
Enter value for owner: 'XIAOYU'
Enter value for tabname: 'T002'
old 8: where owner=&owner and table_name=&tabname
new 8: where owner='XIAOYU' and table_name='T002'

NUM_ROWS BLOCKS EMPTY_BLOCKS AVG_SPACE CHAIN_CNT AVG_ROW_LEN
---------- ---------- ------------ ---------- ---------- -----------
50328 712 0 0 0 91

SQL> SELECT column_name,
2 num_distinct,
3 num_nulls,
4 density,
5 num_buckets,
6 low_value,
7 high_value
8 FROM dba_tab_col_statistics
9 WHERE owner = &owner
10 AND table_name = &tabname
11 AND column_name IN (&col1);
Enter value for owner: 'XIAOYU'
old 9: WHERE owner = &owner
new 9: WHERE owner = 'XIAOYU'
Enter value for tabname: 'T002'
old 10: AND table_name = &tabname
new 10: AND table_name = 'T002'
Enter value for col1: 'OBJECT_ID'
old 11: AND column_name IN (&col1)
new 11: AND column_name IN ('OBJECT_ID')

COLUMN_NAME NUM_DISTINCT NUM_NULLS DENSITY NUM_BUCKETS LOW_VALUE HIGH_VALUE
------------------------------ ------------ ---------- ---------- ----------- --
---------------------------- ------------------------------
OBJECT_ID 44 943 .022727273 1 C2
02 C3064A3F

SQL> select count(*) from t002 where object_Id=100;

COUNT(*)
----------
49000

SQL> select * from table(dbms_xplan.display_cursor(null,null));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
------------------------------------------------------------
SQL_ID 3yaw02xfsf7c8, child number 0
-------------------------------------
select count(*) from t002 where object_Id=100

Plan hash value: 3014849763

---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 158 (100)| |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | TABLE ACCESS FULL| T002 | 1122 | 3366 | 158 (1)| 00:00:02 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter("OBJECT_ID"=100)


19 rows selected.

來算算選擇率selectivity_with_null=(1/num_distinct)*(num_rows-num_nulls)/(num_rows)

SQL> select (50328-943)/50328*1/44 from dual;

(50328-943)/50328*1/44
----------------------
.02230143

算算density=1/num_distinct也跟dba_tab_columns中值一樣

SQL> select 1/44 from dual;

1/44
----------
.022727273

根據選擇率selectivity_with_null跟執行計劃的預估的rows也是相符的，這個地方要注意rows是嚴格根據num_rows*selectivity的，而不是num_rows*density，因為在沒直方圖時density計算方式始終是1/num_distinct

SQL> select 0.02230143* 50328 from dual;

0.02230143*50328
----------------
1122.38637

細心點我們發覺上面計算selectivity的方式可以直接簡化為：
（1/num_distinct）*(num_rows-num_nulls)/num_rows,如果num_null為0， 此時（1/num_distinct）*(num_rows-num_nulls)/num_rows就直接等于1/num_distinct

3）關閉綁定變量窺視，也不收集直方圖：
SQL> create table t003 as select * from dba_objects;

Table created.

SQL> alter system set "_optim_peek_user_binds"=false;

System altered.

SQL> update t003 set object_id=10000 where object_id<50000;

48524 rows updated.

SQL> commit;

Commit complete.

SQL> execute dbms_stats.gather_table_stats(ownname=>'SYS',tabname=>'T003',method
_opt=>'for all columns size 1');

PL/SQL procedure successfully completed.

SQL> select num_rows from user_tables where table_name='T003';

NUM_ROWS
----------
50325

SQL> select num_distinct,density,num_nulls from user_tab_columns where table_nam
e='T003' and column_name='OBJECT_ID';

NUM_DISTINCT DENSITY NUM_NULLS
------------ ---------- ----------
184 .005434783 0

SQL> variable a number;
SQL> exec : a:=10000;

PL/SQL procedure successfully completed.

SQL> select count(object_name) from t003 where object_id=:a;

COUNT(OBJECT_NAME)
------------------
48524

SQL> select * from table(dbms_xplan.display_cursor(null,null,'all'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
------------------------------------------------------------
SQL_ID dq92pjhyfrg1n, child number 0
-------------------------------------
select count(object_name) from t003 where object_id=:a

Plan hash value: 3872854764

---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 154 (100)| |
| 1 | SORT AGGREGATE | | 1 | 29 | | |
|* 2 | TABLE ACCESS FULL| T003 | 274 | 7946 | 154 (1)| 00:00:02 |
---------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

1 - SEL$1
2 - SEL$1 / T003@SEL$1

Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_ID"=:A)

Column Projection Information (identified by operation id):
-----------------------------------------------------------

1 - (#keys=0) COUNT("OBJECT_NAME")[22]
2 - "OBJECT_NAME"[VARCHAR2,128]

31 rows selected.

SQL> exec : a:=10;

PL/SQL procedure successfully completed.

SQL> select count(object_name) from t003 where object_id=:a;

COUNT(OBJECT_NAME)
------------------
0

SQL> select * from table(dbms_xplan.display_cursor(null,null,'all'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
------------------------------------------------------------
SQL_ID dq92pjhyfrg1n, child number 0
-------------------------------------
select count(object_name) from t003 where object_id=:a

Plan hash value: 3872854764

---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 154 (100)| |
| 1 | SORT AGGREGATE | | 1 | 29 | | |
|* 2 | TABLE ACCESS FULL| T003 | 274 | 7946 | 154 (1)| 00:00:02 |
---------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

1 - SEL$1
2 - SEL$1 / T003@SEL$1

Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_ID"=:A)

Column Projection Information (identified by operation id):
-----------------------------------------------------------

1 - (#keys=0) COUNT("OBJECT_NAME")[22]
2 - "OBJECT_NAME"[VARCHAR2,128]

31 rows selected.

關閉綁定變量，沒有直方圖時:
可選擇率selectivity=1/num_distinct*(num_rows-num_nulls)/num_rows

SQL> select 50325*1/184 from dual;

50325*1/184
-----------
273.505435

SQL> select 50325*0.005434783 from dual;

50325*0.005434783
-----------------
273.505454

特別注意點：
而且我們查看v$sql時發現，關閉綁定變量窺視并不是不共享sql，而是說sql第一次執行時不帶入具體值，這個如果大家沒有測試過可能會想當然的以為是關閉綁定變量窺視是不共享綁定的sql語句，其實綁定變量窺視真正含義是sql第一次執行時帶入綁定具體值，而如果關閉了綁定變量窺視，則不會帶入具體值，那么由于不帶入具體值，直方圖也不會影響selectivity計算

SQL> select child_number,sql_id,sql_text from v$sql where sql_text like 'select
count(object_name) from t003 where object_id=:a%';

CHILD_NUMBER SQL_ID
------------ -------------
SQL_TEXT
--------------------------------------------------------------------------------
------------------------------------------------------------
0 dq92pjhyfrg1n
select count(object_name) from t003 where object_id=:a

4）關閉綁定變量窺視，收集直方圖：
SQL> alter system set "_optim_peek_user_binds"=false;

System altered.

SQL> execute dbms_stats.gather_table_stats(ownname=>'SYS',tabname=>'T003',method
_opt=>'for all columns size 254');

PL/SQL procedure successfully completed.

SQL> select num_rows from user_tables where table_name='T003';

NUM_ROWS
----------
50325

SQL> select num_distinct,density,num_nulls from user_tab_columns where table_nam
e='T003' and column_name='OBJECT_ID';

NUM_DISTINCT DENSITY NUM_NULLS
------------ ---------- ----------
197 .000010034 0

SQL> variable b number;
SQL> exec : b:=10000;

PL/SQL procedure successfully completed.

SQL> select count(object_name) from t003 where object_id=:b;

COUNT(OBJECT_NAME)
------------------
48524

SQL> select * from table(dbms_xplan.display_cursor(null,null,'all'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
------------------------------------------------------------
SQL_ID 9qunh3ms4kjzw, child number 0
-------------------------------------
select count(object_name) from t003 where object_id=:b

Plan hash value: 3872854764

---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 154 (100)| |
| 1 | SORT AGGREGATE | | 1 | 29 | | |
|* 2 | TABLE ACCESS FULL| T003 | 255 | 7395 | 154 (1)| 00:00:02 |
---------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

1 - SEL$1
2 - SEL$1 / T003@SEL$1

Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("OBJECT_ID"=:B)

Column Projection Information (identified by operation id):
-----------------------------------------------------------

1 - (#keys=0) COUNT("OBJECT_NAME")[22]
2 - "OBJECT_NAME"[VARCHAR2,128]

31 rows selected.

SQL> exec : b:=10;

PL/SQL procedure successfully completed.

SQL> select count(object_name) from t003 where object_id=:b;

COUNT(OBJECT_NAME)
------------------
0

SQL> select * from table(dbms_xplan.display_cursor(null,null,'all'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
------------------------------------------------------------
SQL_ID 9qunh3ms4kjzw, child number 0
-------------------------------------
select count(object_name) from t003 where object_id=:b

Plan hash value: 3872854764

---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 154 (100)| |
| 1 | SORT AGGREGATE | | 1 | 29 | | |
|* 2 | TABLE ACCESS FULL| T003 | 255 | 7395 | 154 (1)| 00:00:02 |
---------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

1 - SEL$1
2 - SEL$1 / T003@SEL$1

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter("OBJECT_ID"=:B)

Column Projection Information (identified by operation id):
-----------------------------------------------------------

1 - (#keys=0) COUNT("OBJECT_NAME")[22]
2 - "OBJECT_NAME"[VARCHAR2,128]

31 rows selected.

如果有直方圖，但是關閉綁定變量窺視，由于無法把綁定變量的值帶入sql語句中，此時selectivity計算方式還是1/num_distinct*(num_rows-num_nulls)/num_rows
SQL> select 50325*1/197 from dual;

50325*1/197
-----------
255.456853

在關閉綁定變量窺視后，sql語句還是會軟解析，只是綁定變量的sql語句第一次執行時無法帶入到sql語句中，selectivity計算無法應用到直方圖信息，所以此時有無直方圖對于這類綁定變量的sql語句沒有影響。

關于有直方圖時，字段的可選擇、density等如何計算，小魚后續會整理文檔來分享。

原文地址：綁定變量窺視是否開啟和直方圖關系, 感謝原作者分享。

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