最近忙着把公司的数据库从mysql迁移到Oracle,期间作了很多工作来优化oracle平台的性能，不过这里面最大的性能调整还是来自sql。下面举一个web翻页sql调整的例子。
环境：
Linux version 2.4.20-8custom (root@web2) (gcc version 3.2.2 20030222 (Red Hat Linux 3.2.2-5)) #3 SMP Thu Jun 5 22:03:36 CST 2003
Mem:2113466368
Swap: 4194881536
CPU:两个超线程的Intel(R) Xeon(TM) CPU 2.40GHz
优化前语句在mysql里面查询15秒左右出来，转移到oracle后进行在不调整索引和语句的情况下执行时间大概是4-5秒，调整后执行时间小于0.5秒。
翻页语句：
SELECT * FROM(SELECT T1.*, rownum as linenum FROM(
SELECT /*+ index(a ind_old)*/
a.category FROM aUCtion_auctions a WHERE a.category =' 170101 ' AND a.closed='0' AND ends sysdate AND (a.approve_status=0)ORDER BY a.ends) T1WHERE rownum < 18681) WHERE linenum = 18641
被查询的表：auction_auctions（产品表）
表结构：
Code: [Copy to clipboard]
SQL desc auction_auctions;
NameNull?Type
----------------------------------------- -------- ----------------------------
IDNOT NULL VARCHAR2(32)
USERNAME VARCHAR2(32)
TITLECLOB
GMT_MODIFIEDNOT NULL DATE
STARTSNOT NULL DATE
DESCRIPTIONCLOB
PICT_URL CLOB
CATEGORYNOT NULL VARCHAR2(11)
MINIMUM_BIDNUMBER
RESERVE_PRICENUMBER
BUY_NOWNUMBER
AUCTION_TYPE CHAR(1)
DURATION VARCHAR2(7)
INCREMENTNUMNOT NULL NUMBER
CITY VARCHAR2(30)
PROV VARCHAR2(20)
LOCATION VARCHAR2(40)
LOCATION_ZIP VARCHAR2(6)
SHIPPING CHAR(1)
PAYMENTCLOB
INTERNATIONALCHAR(1)
ENDSNOT NULL DATE
CURRENT_BIDNUMBER
CLOSED CHAR(2)
PHOTO_UPLOADED CHAR(1)
QUANTITY NUMBER(11)
STORYCLOB
HAVE_INVOICENOT NULL NUMBER(1)
HAVE_GUARANTEENOT NULL NUMBER(1)
STUFF_STATUSNOT NULL NUMBER(1)
APPROVE_STATUSNOT NULL NUMBER(1)
OLD_STARTSNOT NULL DATE
ZOOVARCHAR2(10)
PROMOTED_STATUS NOT NULL NUMBER(1)
REPOST_TYPECHAR(1)
REPOST_TIMESNOT NULL NUMBER(4)
SECURE_TRADE_AGREENOT NULL NUMBER(1)
SECURE_TRADE_TRANSACTION_FEE VARCHAR2(16)
SECURE_TRADE_ORDINARY_POST_FEE NUMBER
SECURE_TRADE_FAST_POST_FEE NUMBER
表记录数及大小
SQL select count(*) from auction_auctions;
COUNT(*)
----------
537351
SQL select segment_name,bytes,blocks from user_segments where segment_name ='AUCTION_AUCTIONS';
SEGMENT_NAMEBYTES BLOCKS
AUCTION_AUCTIONS1059061760 129280
表上原有的索引
create index ind_old on auction_auctions(closed,approve_status,category,ends) tablespace tbsindex compress 2;
SQL select segment_name,bytes,blocks from user_segments where segment_name = 'IND_OLD';
SEGMENT_NAME BYTES BLOCKS
IND_OLD 20971520 2560
表和索引都已经分析过，我们来看一下sql执行的费用
SQL set autotrace trace;
SQL SELECT * FROM(SELECT T1.*, rownum as linenum FROM(SELECT a.* FROM auction_auctions a WHERE a.category like '18%' AND a.closed='0' AND ends sysdate AND (a.approve_status=0)ORDER BY a.ends) T1WHERE rownum <18681) WHERE linenum = 18641;
40 rows selected.
Execution Plan
----------------------------------------------------------
0SELECT STATEMENT Optimizer=CHOOSE (Cost=19152 Card=18347 Byt
es=190698718)
10 VIEW (Cost=19152 Card=18347 Bytes=190698718)
21 COUNT (STOPKEY)
32 VIEW (Cost=19152 Card=18347 Bytes=190460207)
43 TABLE Access (BY INDEX ROWID) OF 'AUCTION_AUCTIONS'
(Cost=19152 Card=18347 Bytes=20860539)
54 INDEX (RANGE SCAN) OF 'IND_OLD' (NON-UNIQUE) (Cost
=810 Card=186003)
Statistics
----------------------------------------------------------
0recursive calls
0db block gets
19437consistent gets
18262physical reads
0redo size
114300bytes sent via SQL*Net to client
56356bytes received via SQL*Net from client
435SQL*Net roundtrips to/from client
0sorts (memory)
0sorts (disk)
40rows processed
我们可以看到这条sql语句通过索引范围扫描找到最里面的结果集，然后通过两个view操作最后得出数据。
其中18502consistent gets，17901physical reads
我们来看一下这个索引建的到底合不合理，先看下各个查寻列的distinct值
select count(distinct ends) from auction_auctions;
COUNT(DISTINCTENDS)
-------------------
338965
SQL select count(distinct category) from auction_auctions;
COUNT(DISTINCTCATEGORY)
-----------------------
1148
SQL select count(distinct closed) from auction_auctions;
COUNT(DISTINCTCLOSED)
---------------------
2
SQL select count(distinct approve_status) from auction_auctions;
COUNT(DISTINCTAPPROVE_STATUS)
-----------------------------
5
页索引里列平均存储长度
SQL select avg(vsize(ends)) from auction_auctions;
AVG(VSIZE(ENDS))
----------------
7
SQL select avg(vsize(closed)) from auction_auctions;
AVG(VSIZE(CLOSED))
------------------
2
SQL select avg(vsize(category)) from auction_auctions;
AVG(VSIZE(CATEGORY))
--------------------
5.52313106
SQL select avg(vsize(approve_status)) from auction_auctions;
AVG(VSIZE(APPROVE_STATUS))
--------------------------
1.67639401
我们来估算一下各种组合索引的大小，可以看到closed,approve_status，category都是相对较低集势的列（重复值较多），下面我们来大概计算下各种页索引需要的空间
column distinct numcolumn len
ends3389657
category 11485.5
closed 2 2
approve_status5 1.7
index1: (ends,closed,category,approve_status) compress 2
en
