在分析表的是否有一個參數no_invalidate:缺省值是DBMS_STATS.AUTO_INVALIDATE.
10g中默認是AUTO_INVALIDATE,就是說分析表後,遊標不會馬上invalidate,已經存在的SQL的執行計劃不會受新的統計信息影響。可以手工DDL
invalidate遊標。又或者等待隱藏參數_optimizer_invalidation_period(time window for invalidation of cursors of analyzed objects)秒後,
Oracle自動invalidate遊標並使SQL能夠讀取新的統計信息產生新的執行計劃。
如果想要dbms_stats分析立馬見效,需要使用no_invalidate=false option或者DBA自己手工invalidate遊標。
--說明一下,我個人感覺這個參數理解起來很煩,validate表示有效,no_invalidate反了2次,也是表示有效的意思。
dbms_stats收集統計信息時候no_invalidate參數
用於是否與收集相關object的cursor失效,defalut(9i false, 10g dbms_stats.auto_invalidate(既null))
true:當收集完統計信息後,收集對象的cursor不會失效(不會產生新的執行計劃,子游標)
false:當收集完統計信息後,收集對象的cursor會立即失效(新的執行計劃,新的子游標)
no_invalidate=>DBMS_STATS.AUTO_INVALIDATE,分析表後,遊標不會馬上invalidate,已經存在的SQL的執行計劃不會受新的統計信息影響。可以手工
DDL invalidate遊標。又或者等待隱藏參數_optimizer_invalidation_period(time window for invalidation of cursors of analyzed objects)秒後,
Oracle自動invalidate遊標並使SQL能夠讀取新的統計信息產生新的執行計劃。
缺省隱藏參數_optimizer_invalidation_period設置的時間太長={{18000:0}}。
再建立一個例子來說明問題:
1.建立測試環境:
SQL> select * from v$version ;
BANNER--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - ProductionCORE 11.2.0.1.0 Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production
create table t as select rownum id ,'test' name from dual connect by level <={{10000:0}};
create index i_t_id on t(id);
SQL> select dbms_stats.get_param('no_invalidate') from dual;
DBMS_STATS.GET_PARAM('NO_INVALIDATE')
-------------------------------------------------------------
DBMS_STATS.AUTO_INVALIDATE
--可以發現缺省no_invalidate=DBMS_STATS.AUTO_INVALIDATE.
--如果查看dbms_stats文檔,可以發現實際上就是等於NULL。
-- oracle decides when to invalidate dependend cursors
AUTO_INVALIDATE CONSTANT BOOLEAN := null;
SQL> exec dbms_stats.gather_table_stats(user,'t',cascade=>true,method_opt=>'for all columns size 254',no_invalidate=>DBMS_STATS.AUTO_INVALIDATE);
PL/SQL procedure successfully completed.
--分析表T,並且在表T上建立直方圖。
SQL> select column_name,data_type,histogram from dba_tab_cols where wner=user and table_name='T';
COLUMN_NAME DATA_TYPE HISTOGRAM
------------ ---------- ---------------
ID NUMBER HEIGHT BALANCED
NAME CHAR FREQUENCY
2.測試:
SQL> select * from t where id={{10001:0}};
no rows selected
SQL> @dpc
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------
SQL_ID 0yzrd8s6vbfcc, child number 0
-------------------------------------
select * from t where id={{10001:0}}
Plan hash value: {{4153437776:0}}
--------------------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
--------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | 2 (100)|| 1
| TABLE ACCESS BY INDEX ROWID| T | 1 | 2 (0)|
|* 2 | INDEX RANGE SCAN | I_T_ID | 1 | 1 (0)|
--------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"={{10001:0}})
--測試發現可以選擇索引。
但是如果我改變id的分佈,加入大量id={{10001:0}},在分析後是什麼情況呢?
SQL> insert into t select {{10001:0}} id ,'book' name from dual connect by level<={{10000:0}};
{{10000:0}} rows created.
SQL> commit ;
Commit complete.
3.分析表後在測試:
SQL> exec dbms_stats.gather_table_stats(user,'t',cascade=>true,method_opt=>'for all columns size 254',no_invalidate=>DBMS_STATS.AUTO_INVALIDATE);
PL/SQL procedure successfully completed.
--正常按照許多人的理解如果查詢:
select * from t where id={{10001:0}};
應該不會使用索引,實際情況呢?
select * from t where id={{10001:0}};
SQL> @dpc
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------
SQL_ID 0yzrd8s6vbfcc, child number 0
-------------------------------------
select * from t where id={{10001:0}}
Plan hash value: {{4153437776:0}}
--------------------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
--------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | 2 (100)|| 1
| TABLE ACCESS BY INDEX ROWID| T | 1 | 2 (0)|
|* 2 | INDEX RANGE SCAN | I_T_ID | 1 | 1 (0)|
--------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"={{10001:0}})
--可以發現執行計劃並沒有因爲我們分析表而改變執行計劃。
修改語句,select換成大寫,再測試可以很好的說明問題:
SQL> @dpc
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------
SQL_ID 0sf1updb7x3xf, child number 0
-------------------------------------
SELECT * from t where id={{10001:0}}
Plan hash value: {{1601196873:0}}
--------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
--------------------------------------------------------
| 0 | SELECT STATEMENT | | | 14 (100)|
|* 1 | TABLE ACCESS FULL| T | {{10039:0}} | 14 (0)|
--------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("ID"={{10001:0}})
--可以發現執行計劃選擇全表掃描。
4.而隱藏參數_optimizer_invalidation_period缺省測試={{18000:0}}.
$ P _optimizer_invalidation_period
NAME DESCRIPTION DEFAULT_VA SESSION_VALUE SYSTEM_VALUE
------------------------------------ ---------------------------------------------------------------------------- ---------- -------------------- --------------------
_optimizer_invalidation_period time window for invalidation of cursors of analyzed objects TRUE {{18000:0}} {{18000:0}}
我在.bashrc中定義了一個shell函數P。
P()
{
echo ' '
if [ -z "$1" ];
then sqlplus -S "/ as sysdba" <<EOF
set echo off lin 9999 trimsp on feedb off head on pages 0 newp 0 tab off
col name for a36
col description format a76
col default_value format a10
col session_value format a20
col system_value format a20
select a.ksppinm name, a.ksppdesc DESCRIPTION, b.ksppstdf DEFAULT_VALUE, b.ksppstvl SESSION_VALUE, c.ksppstvl SYSTEM_VALUE
from sys.x\$ksppi a, sys.x\$ksppcv b, sys.x\$ksppsv c
EOF
else
sqlplus -S "/ as sysdba" <<EOF | grep -i $1 -B 2 -A 2
set echo off lin 9999 trimsp on feedb off head on pages 0 newp 0 tab off
col name for a36
col description format a76
col default_value format a10
col session_value format a20
col system_value format a20
select a.ksppinm name, a.ksppdesc DESCRIPTION, b.ksppstdf DEFAULT_VALUE, b.ksppstvl SESSION_VALUE, c.ksppstvl SYSTEM_VALUE
from sys.x\$ksppi a, sys.x\$ksppcv b, sys.x\$ksppsv c
EOF
fi
echo ' '}
{{18000:0}}/3600=5 ,要5個小時後,select * from t where id={{10001:0}};執行計劃纔會變成full 掃描,顯然測試不能等這麼長時間。
SQL> select sql_id,child_number,executions,parse_calls,loads,invalidations from v$sql
where sql_id ='0yzrd8s6vbfcc';
SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS LOADS INVALIDATIONS
------------- ------------ ---------- ----------- ---------- -------------
0yzrd8s6vbfcc 0 2 2 1 0
SQL> alter system set "_optimizer_invalidation_period" = 10 scope=memory;
System altered.
--等待10秒............
SQL> @dpc
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------
SQL_ID 0yzrd8s6vbfcc, child number 1
-------------------------------------
select * from t where id={{10001:0}}
Plan hash value: {{1601196873:0}}
--------------------------------------------------------
| Id | Operation | Name | E-Rows | Cost (%CPU)|
--------------------------------------------------------
| 0 | SELECT STATEMENT | | | 14 (100)|
|* 1 | TABLE ACCESS FULL| T | {{10039:0}} | 14 (0)|
--------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("ID"={{10001:0}})
--可以發現執行計劃變成了全表掃描。
SQL> select sql_id,child_number,executions,parse_calls,loads,invalidations from v$sql where sql_id ='0yzrd8s6vbfcc';
SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS LOADS INVALIDATIONS
------------- ------------ ---------- ----------- ---------- -------------
0yzrd8s6vbfcc 0 2 2 1 0
0yzrd8s6vbfcc 1 1 1 1 0
--可以發現執行計劃生成了新的子光標。
總結:
oracle系統缺省存在自動分析一般在晚上10點分析,而no_invalidate的缺省值正好是DBMS_STATS.AUTO_INVALIDATE.這樣缺省分析
DBMS_STATS.AUTO_INVALIDATE,如果處理不好,一些性能問題會延遲出現,在優化SQL應該引起注意。