drop table test;
select count(*) from test;
--創建測試表
create table test
(
id number(9),
nick varchar2(30)
);
--插入測試數據
begin
for i in 1..100000 loop
insert into test(id) values(i);
end loop;
commit;
end;
select * from test;
--更新nick字段,使數據發生嚴重傾斜
update test set nick='abc' where rownum<99999;
--創建索引
create index idx_test_nick on test(nick);
update test set nick='def' where nick is null;
--只對索引進行分析
analyze index idx_test_nick compute statistics;
select * from user_indexes;
--查看索引名,對應存儲的數據塊,不同的key數量,記錄數(行數)的分析信息
select index_name, LEAF_BLOCKS, DISTINCT_KEYS, NUM_ROWS
from user_indexes
where index_name = 'IDX_TEST_NICK';
--dba_tab_col_statistics
--查看錶的統計信息
select COLUMN_NAME, NUM_BUCKETS, num_distinct
from USER_tab_columns
where table_name = 'TEST';
select * from test where nick ='abc';
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST'
2 1 INDEX (RANGE SCAN) OF 'IDX_TEST_NICK' (NON-UNIQUE)
select * from test where nick ='def';
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST'
2 1 INDEX (RANGE SCAN) OF 'IDX_TEST_NICK' (NON-UNIQUE)
--由上可以看到,對索引分析之後,sql的執行路徑都是基於規則的,索引的字段的偏移
--先根據索引找到rowid,然後再根據rowid讀取記錄,這個過程肯定比全表掃描讀取記錄要慢
--user_part_col_statistics 分區分析信息
--分析表的第二列nick
analyze table test compute statistics for columns size 2 nick;
select * from test where nick ='abc';
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST'
2 1 INDEX (RANGE SCAN) OF 'IDX_TEST_NICK' (NON-UNIQUE)
--根據上面的執行計劃,還是按照規則來執行的
--分析表
analyze table test compute statistics for table;
select * from test where nick ='abc';
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=49 Card=99998 Bytes=
1499970)
1 0 TABLE ACCESS (FULL) OF 'TEST' (Cost=49 Card=99998 Bytes=14
99970)
--分析表之後,完全按照成本來執行
--刪除所有的統計數據,並只對表與列進行分析,不分析索引,
--ORACLE使用CBO的優化器,併產生了正確的執行計劃
analyze table test delete statistics;
--分析列nick
analyze table test compute statistics for table for columns size 2 nick;
select * from test where nick ='abc';
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=49 Card=99998 Bytes=
1499970)
1 0 TABLE ACCESS (FULL) OF 'TEST' (Cost=49 Card=99998 Bytes=14
99970)
--
select * from test where nick ='def';
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=2 Bytes=30)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST' (Cost=2 Card=2 Byt
es=30)
2 1 INDEX (RANGE SCAN) OF 'IDX_TEST_NICK' (NON-UNIQUE) (Cost
=1 Card=2)
--創建TEST表ID列上的索引,但不對索引進行分析
create index idx_test_id on test(id);
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1000 Bytes=15
000)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST' (Cost=2 Card=1000
Bytes=15000)
2 1 INDEX (RANGE SCAN) OF 'IDX_TEST_ID' (NON-UNIQUE) (Cost=1
Card=400)
--當條件中即有id,又有nick時,因爲nick上有直方圖,ORACLE知道nick='abc'的值特別的多,所以不走IDX_TEST_NICK索引,走IDX_TEST_ID上的索引
select * from test where id=5 and nick='abc';
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1000 Bytes=15
000)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST' (Cost=2 Card=1000
Bytes=15000)
2 1 INDEX (RANGE SCAN) OF 'IDX_TEST_ID' (NON-UNIQUE) (Cost=1
Card=400)
--當條件中即有id,又有nick時,因爲nick上有直方圖,ORACLE知道nick='def'的值特別的少,所以走IDX_TEST_NICK上的索引,不走IDX_TEST_ID索引
select * from test where id=5 and nick='def';
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=15)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST' (Cost=2 Card=1 Byt
es=15)
2 1 INDEX (RANGE SCAN) OF 'IDX_TEST_NICK' (NON-UNIQUE) (Cost
=1 Card=2)
select * from test where nick='def' and id=5;
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=15)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST' (Cost=2 Card=1 Byt
es=15)
2 1 INDEX (RANGE SCAN) OF 'IDX_TEST_NICK' (NON-UNIQUE) (Cost
=1 Card=2)
--在分析ID列後,ORACLE發現ID列的選擇度更高,所以不再選擇IDX_TEST_NICK索引,而是選擇IDX_TEST_ID
analyze table test compute statistics for columns size 1 id;
select * from test where id=5 and nick='def';
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=7)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST' (Cost=2 Card=1 Byt
es=7)
2 1 INDEX (RANGE SCAN) OF 'IDX_TEST_ID' (NON-UNIQUE) (Cost=1
Card=1)
/*
下面來看另外一種情況,我們刪除所有的統計數據,然後在ID列上創建唯一索引,在此條件下,
只分析表與分析列nick,我們看到ORACLE走了正確的執行計劃,
走了UK_TEST_ID,其實從這裏也給我們帶來很多的啓示:
在主鍵與唯一鍵約束的列上是否需要直方圖的問題?
如果在這些列上有像這樣的查詢where id > 100 and id < 1000,
我們還是需要有直方圖的,但除此之外,好像真的沒有直方圖的必要了!
*/
analyze table test delete statistics;
drop index idx_test_id;
create unique index uk_test_id on test(id);
--分析表的第二列nick
analyze table test compute statistics for table for columns size 2 nick;
select * from test where id=5 and nick='def';
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=15)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST' (Cost=2 Card=1 Byt
es=15)
2 1 INDEX (UNIQUE SCAN) OF 'UK_TEST_ID' (UNIQUE) (Cost=1 Car
d=100000)
從以上一系列的實驗可以看出,對ORACLE的優化器CBO來說,表的分析與列的分析纔是最重要的,索引的分析次之。還有我們可以考慮我們的哪些列上需要直方圖,對於bucket的個數問題,oracle的默認值是75個,所以根據你的應用規則,選擇合適的桶數對性能也是有幫助的。因爲不必要的桶的個數的大量增加,必然會帶來SQL語句硬解析時產生執行計劃的複雜度問題。
完全計算法: analyze table abc compute statistics;
抽樣估算法(抽樣20%): analyze table abc estimate statistics sample 20 percent
我們可以採用以下方法,對數據庫的表和索引及簇表定期分析生成統計信息,保證應用的正常性能
生成索引分析,表分析的sql語句: