我有两张表，一张有 200 万条记录（员工），另一张有大约一千条记录（城市）。我正在加入基于 id (City_Id) 的表。City_Id 是 Cities 表的主键，我在Employees 表的 City_Id 列上有一个索引。
我在这些表上运行以下查询
1)SELECT * FROM EMPLOYEES, CITIES WHERE EMPLOYEES.CITY_ID=CITIES.CITY_ID and EMPLOYEES.EMPLOYEE_NAME='XYZ';
2)SELECT * FROM EMPLOYEES, CITIES WHERE EMPLOYEES.CITY_ID=CITIES.CITY_ID and CITIES.CITY_CLASS='ABC';
我还在 EMPLOYEES 表的 EMPLOYEE_NAME 和 CITIES 表的 CITY_CLASS 上有索引。
第一个查询执行得非常快，但第二个查询执行得非常慢。你能告诉我需要做什么才能让第二个跑得更快吗？
寻找正确的索引
让我在这里建立你的架构
CREATE TABLE EMPLOYEES ( EMPLOYEE_NAME VARCHAR2 ( 50 ),
CITY_ID NUMBER ( 2 ),
DUMMYCOL VARCHAR2 ( 100 ) );
CREATE TABLE CITIES ( CITY_CLASS VARCHAR2 ( 10 ),
CITY_ID NUMBER ( 2 ),
DUMMYCOL VARCHAR2 ( 100 ) );
你提到的索引
CREATE INDEX IDX_T1
ON EMPLOYEES ( EMPLOYEE_NAME );
CREATE INDEX IDX_T2
ON CITIES ( CITY_CLASS );
模拟行数
BEGIN
DBMS_STATS.SET_TABLE_STATS ( OWNNAME => 'REALSPIRITUALS',
TABNAME => 'EMPLOYEES',
NUMROWS => 2000000 );
END;
/
BEGIN
DBMS_STATS.SET_TABLE_STATS ( OWNNAME => 'REALSPIRITUALS',
TABNAME => 'CITIES',
NUMROWS => 1000 );
END;
/
尝试您的第一个查询
SET AUTOTRACE ON
SELECT
*
FROM
EMPLOYEES,
CITIES
WHERE
EMPLOYEES.CITY_ID = CITIES.CITY_ID
AND EMPLOYEES.EMPLOYEE_NAME = 'XYZ';
注意：查询使用 IDX_T1 进行更快的检索，并通过索引访问员工表，因此速度更快
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer Mode=ALL_ROWS (Cost=27 Card=20 K Bytes=3 M)
1 0 HASH JOIN (Cost=27 Card=20 K Bytes=3 M)
2 1 TABLE ACCESS FULL REALSPIRITUALS.CITIES (Cost=21 Card=1 K Bytes=97 K)
3 1 TABLE ACCESS BY INDEX ROWID REALSPIRITUALS.EMPLOYEES (Cost=5 Card=20 K Bytes=1 M)
4 3 INDEX RANGE SCAN REALSPIRITUALS.IDX_T1 (Cost=1 Card=8 K)
Statistics
----------------------------------------------------------
203 recursive calls
0 spare statistic 3
0 gcs messages sent
29 db block gets from cache
0 physical reads direct (lob)
0 queue position update
0 queue single row
0 queue ocp pages
0 HSC OLTP Compressed Blocks
0 HSC IDL Compressed Blocks
0 rows processed
您现在的第二个查询
SET AUTOTRACE ON
SELECT
*
FROM
EMPLOYEES,
CITIES
WHERE
EMPLOYEES.CITY_ID = CITIES.CITY_ID
AND CITIES.CITY_CLASS = 'ABC';
注意：查询使用 IDX_T2 以加快检索速度，但员工表正在进行全表扫描，因此速度较慢
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer Mode=ALL_ROWS (Cost=52 Card=20 K Bytes=3 M)
1 0 HASH JOIN (Cost=52 Card=20 K Bytes=3 M)
2 1 TABLE ACCESS BY INDEX ROWID REALSPIRITUALS.CITIES (Cost=5 Card=10 Bytes=1000)
3 2 INDEX RANGE SCAN REALSPIRITUALS.IDX_T2 (Cost=1 Card=4)
4 1 TABLE ACCESS FULL REALSPIRITUALS.EMPLOYEES (Cost=38 Card=2 M Bytes=190 M)
Statistics
----------------------------------------------------------
0 recursive calls
0 spare statistic 3
0 gcs messages sent
0 db block gets from cache
0 physical reads direct (lob)
0 queue position update
0 queue single row
0 queue ocp pages
0 HSC OLTP Compressed Blocks
0 HSC IDL Compressed Blocks
0 rows processed
现在我正在添加一个索引
CREATE INDEX IDX_T3
ON EMPLOYEES ( CITY_ID );
重试第二个查询
SET AUTOTRACE ON
SELECT
*
FROM
EMPLOYEES,
CITIES
WHERE
EMPLOYEES.CITY_ID = CITIES.CITY_ID
AND CITIES.CITY_CLASS = 'ABC';
注意：避免使用 FTS，查询使用索引进行检索
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer Mode=ALL_ROWS (Cost=5 Card=20 K Bytes=3 M)
1 0 NESTED LOOPS
2 1 NESTED LOOPS (Cost=5 Card=20 K Bytes=3 M)
3 2 TABLE ACCESS BY INDEX ROWID REALSPIRITUALS.CITIES (Cost=5 Card=10 Bytes=1000)
4 3 INDEX RANGE SCAN REALSPIRITUALS.IDX_T2 (Cost=1 Card=4)
5 2 INDEX RANGE SCAN REALSPIRITUALS.IDX_T3 (Cost=0 Card=1)
6 1 TABLE ACCESS BY INDEX ROWID REALSPIRITUALS.EMPLOYEES (Cost=0 Card=2 K Bytes=195 K)
Statistics
----------------------------------------------------------
1 recursive calls
0 spare statistic 3
0 gcs messages sent
0 db block gets from cache
0 physical reads direct (lob)
0 queue position update
0 queue single row
0 queue ocp pages
0 HSC OLTP Compressed Blocks
0 HSC IDL Compressed Blocks
0 rows processed
这应该更快 :) 通常，为了加快查询速度，始终只对 where 子句中使用的列进行索引