[bbk3153] 第62集 - Chapter 15-Application Tuning(02)

Index-Organized Tables

IOT表要求必须有主键.heap表可以没有主键.没有主键的表，不能称之为IOT表.

IOT and Heap Tables

Compared to heap tables,IOTs have:

Faster key-based access to table data

Reduced storage requirements

Secondary indexes and logical rowids

IOTs have the following restrictions:

-Must have a primary key

-Cannot be clustered

Creating IOT

CREATE TABLE country
(
country_id             CHAR(2) CONSTRAINT contry_id_nn NOT NULL,
country_name        VARCHAR2(40),
currency_nbame     VARCHAR2(25),
currency_symbol    VARCAHR2(3),
map BLOB,
flag BLOB,
CONSTRAINT country_c_id_pk PRIMARY KEY(country_id))
ORGANIZATION INDEX
TABLESPACE indx
PCTTHRESHOLD 20
OVERFLOW TABLESPACE users;

Keyword ORGANIZATION INDEX指明IOT表.

PCTTRHESHOLD 20,OVERFLOW TABLESPACE users，两个参数结合使用：当索引内容占到一个索引块大小的20%的时候，就会overflow到user tablespace里面.节省空间，使一个索引块里面存放的索引entry更多；一个索引块里面存放的索引entry更多，那么hight就会越低.height越低，I/O的次数就会越少，性能就会提高.

IOT Row Overflow

pctthreshold和including这两个关键字，是用来控制那些列放在叶子block，那些列存放在overflow指定的segment.当二者发生冲突的时候， 以关键字pctthreshold优先.

create table iot1
(
x       int,
y       date,
z       varchar2(2000),
constraint iot_pk1      primary key(x)
)
organization index
pctthreshold 10
overflow
/

上图图解:通过指定关键字pctthreshold 10，说明每行所有字段累加超过叶子节点块大小的10%时，就会将多余出的数据，存放于overflow指定的表空间中.

CREATE TABLE iot2
(
x       int,
y       date,
z       varchar2(2000),
constraint iot_pk2 primary key(x)
)
organization index
including y
overflow
/

图解：因为在脚本中指定了including y，也就是说列y(包含y)左边的所有列都将包含在叶子节点所在的块中.列y右边的字段将直接放在overflow默认的表空间里面.上图清晰可见.