Oracle 10g SQL分页查询语句和效率分析

在使用Oracle数据库进行分页查询算法设计时，分页查询的SQL语句基本上可以按照下面给出的模板来进行套用：

在使用Oracle数据库进行分页查询算法设计时，分页查询的SQL语句基本上可以按照下面给出的模板来进行套用：

分页查询格式：
SELECT * FROM
(
SELECT A.*, ROWNUM RN
FROM (SELECT * FROM TABLE_NAME) A
WHERE ROWNUM )
WHERE RN >= 11

其中最内层的查询SELECT * FROM TABLE_NAME表示不进行翻页的原始查询语句。ROWNUM = 11控制分页查询的每页的范围。
上面给出的这个分页查询语句，在大多数情况拥有较高的效率。分页的目的就是控制输出结果集大小，将结果尽快的返回。在上面的分页查询语句中，这种考虑主要体现在WHERE ROWNUM 选择第11到20条记录存在两种方法，一种是上面例子中展示的在查询的第二层通过ROWNUM SELECT * FROM
(
SELECT A.*, ROWNUM RN
FROM (SELECT * FROM TABLE_NAME) A
)
WHERE RN BETWEEN 11 AND 20

对比这两种写法，绝大多数的情况下，第一个查询的效率比第二个高得多。

这是由于CBO优化模式下，Oracle可以将外层的查询条件推到内层查询中，以提高内层查询的执行效率。对于第一个查询语句，第二层的查询条件

WHERE ROWNUM
而第二个查询语句，由于查询条件BETWEEN 11 AND 20是存在于查询的第三层，而Oracle无法将第三层的查询条件推到最内层（即使推到最内层也没有意义，，因为最内层查询不知道RN代表什么）。因此，对于第二个查询语句，Oracle最内层返回给中间层的是所有满足条件的数据，而中间层返回给最外层的也是所有数据。数据的过滤在最外层完成，显然这个效率要比第一个查询低得多。

上面分析的查询不仅仅是针对单表的简单查询，对于最内层查询是复杂的多表联合查询或最内层查询包含排序的情况一样有效。

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下面简单讨论一下多表联合的情况。对于最常见的等值表连接查询，

CBO一般可能会采用两种连接方式NESTED LOOP和HASH JOIN（MERGE JOIN效率比HASH JOIN效率低，一般CBO不会考虑）。在这里，由于使用了分页

，因此指定了一个返回的最大记录数，NESTED LOOP在返回记录数超过最大值时可以马上停止并将结果返回给中间层，而HASH JOIN必须处理完所有结果集（MERGE JOIN也是）。那么在大部分的情况下，对于分页查询选择NESTED LOOP作为查询的连接方法具有较高的效率（分页查询的时候绝大部分的情况是查询前几页的数据，越靠后面的页数访问几率越小）。
因此，如果不介意在系统中使用HINT的话，可以将分页的查询语句改写为：

SELECT /*+ FIRST_ROWS */ * FROM
(
SELECT A.*, ROWNUM RN
FROM (SELECT * FROM TABLE_NAME) A
WHERE ROWNUM )
WHERE RN >= 11

下面用几个例子来说明分页查询的效率。首先选择一个比较大的表作为测试表：
可以采用Oracle 10g R2数据库自带的示例方案中的OE用户的数据表 customers表做测试：

下面是CUSTOMERS表的DDL定义语句，主要是为了让大家理解表结构
CREATE TABLE "SH"."CUSTOMERS"
( "CUST_ID" NUMBER NOT NULL ENABLE,
"CUST_FIRST_NAME" VARCHAR2(20) NOT NULL ENABLE,
"CUST_LAST_NAME" VARCHAR2(40) NOT NULL ENABLE,
"CUST_GENDER" CHAR(1) NOT NULL ENABLE,
"CUST_YEAR_OF_BIRTH" NUMBER(4,0) NOT NULL ENABLE,
"CUST_MARITAL_STATUS" VARCHAR2(20),
"CUST_STREET_ADDRESS" VARCHAR2(40) NOT NULL ENABLE,
"CUST_POSTAL_CODE" VARCHAR2(10) NOT NULL ENABLE,
"CUST_CITY" VARCHAR2(30) NOT NULL ENABLE,
"CUST_CITY_ID" NUMBER NOT NULL ENABLE,
"CUST_STATE_PROVINCE" VARCHAR2(40) NOT NULL ENABLE,
"CUST_STATE_PROVINCE_ID" NUMBER NOT NULL ENABLE,
"COUNTRY_ID" NUMBER NOT NULL ENABLE,
"CUST_MAIN_PHONE_NUMBER" VARCHAR2(25) NOT NULL ENABLE,
"CUST_INCOME_LEVEL" VARCHAR2(30),
"CUST_CREDIT_LIMIT" NUMBER,
"CUST_EMAIL" VARCHAR2(30),
"CUST_TOTAL" VARCHAR2(14) NOT NULL ENABLE,
"CUST_TOTAL_ID" NUMBER NOT NULL ENABLE,
"CUST_SRC_ID" NUMBER,
"CUST_EFF_FROM" DATE,
"CUST_EFF_TO" DATE,
"CUST_VALID" VARCHAR2(1),
CONSTRAINT "CUSTOMERS_PK" PRIMARY KEY ("CUST_ID")
USING INDEX PCTFREE 10 INITRANS 2 MAXTRANS 255 NOLOGGING COMPUTE STATISTICS
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "EXAMPLE" ENABLE NOVALIDATE,
CONSTRAINT "CUSTOMERS_COUNTRY_FK" FOREIGN KEY ("COUNTRY_ID")
REFERENCES "SH"."COUNTRIES" ("COUNTRY_ID") ENABLE NOVALIDATE
) PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS NOLOGGING
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "EXAMPLE" ;

CREATE BITMAP INDEX "SH"."CUSTOMERS_GENDER_BIX" ON "SH"."CUSTOMERS" ("CUST_GENDER")
PCTFREE 10 INITRANS 2 MAXTRANS 255 NOLOGGING COMPUTE STATISTICS
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "EXAMPLE" ;

CREATE BITMAP INDEX "SH"."CUSTOMERS_MARITAL_BIX" ON "SH"."CUSTOMERS" ("CUST_MARITAL_STATUS")
PCTFREE 10 INITRANS 2 MAXTRANS 255 NOLOGGING COMPUTE STATISTICS
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "EXAMPLE" ;

CREATE UNIQUE INDEX "SH"."CUSTOMERS_PK" ON "SH"."CUSTOMERS" ("CUST_ID")
PCTFREE 10 INITRANS 2 MAXTRANS 255 NOLOGGING COMPUTE STATISTICS
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "EXAMPLE" ;

CREATE BITMAP INDEX "SH"."CUSTOMERS_YOB_BIX" ON "SH"."CUSTOMERS" ("CUST_YEAR_OF_BIRTH")
PCTFREE 10 INITRANS 2 MAXTRANS 255 NOLOGGING COMPUTE STATISTICS
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "EXAMPLE" ;

ALTER TABLE "SH"."CUSTOMERS" ADD CONSTRAINT "CUSTOMERS_PK" PRIMARY KEY ("CUST_ID")
USING INDEX PCTFREE 10 INITRANS 2 MAXTRANS 255 NOLOGGING COMPUTE STATISTICS
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
TABLESPACE "EXAMPLE" ENABLE NOVALIDATE;

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_ID" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_FIRST_NAME" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_LAST_NAME" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_GENDER" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_YEAR_OF_BIRTH" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_STREET_ADDRESS" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_POSTAL_CODE" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_CITY" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_CITY_ID" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_STATE_PROVINCE" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_STATE_PROVINCE_ID" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("COUNTRY_ID" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_MAIN_PHONE_NUMBER" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_TOTAL" NOT NULL ENABLE);

ALTER TABLE "SH"."CUSTOMERS" MODIFY ("CUST_TOTAL_ID" NOT NULL ENABLE);

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_ID" IS 'primary key';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_FIRST_NAME" IS 'first name of the customer';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_LAST_NAME" IS 'last name of the customer';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_GENDER" IS 'gender; low cardinality attribute';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_YEAR_OF_BIRTH" IS 'customer year of birth';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_MARITAL_STATUS" IS 'customer marital status; low cardinality attribute';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_STREET_ADDRESS" IS 'customer street address';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_POSTAL_CODE" IS 'postal code of the customer';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_CITY" IS 'city where the customer lives';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_STATE_PROVINCE" IS 'customer geography: state or province';

COMMENT ON COLUMN "SH"."CUSTOMERS"."COUNTRY_ID" IS 'foreign key to the countries table (snowflake)';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_MAIN_PHONE_NUMBER" IS 'customer main phone number';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_INCOME_LEVEL" IS 'customer income level';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_CREDIT_LIMIT" IS 'customer credit limit';

COMMENT ON COLUMN "SH"."CUSTOMERS"."CUST_EMAIL" IS 'customer email id';

COMMENT ON TABLE "SH"."CUSTOMERS" IS 'dimension table';

GRANT SELECT ON "SH"."CUSTOMERS" TO "BI";

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