使用Plan Hint进行调优
Plan Hint为用户提供了直接影响执行计划生成的手段，用户可以通过指定join顺序，join、scan方法，指定结果行数，等多个手段来进行执行计划的调优，以提升查询的性能。
功能描述
Plan Hint仅支持在SELECT关键字后通过如下形式指定：
/*+ <plan hint>*/
可以同时指定多个hint，之间使用空格分隔。hint只能hint当前层的计划，对于子查询计划的hint，需要在子查询的select关键字后指定hint。
例如：
select /*+ <plan_hint1> <plan_hint2> */ * from t1, (select /*+ <plan_hint3> */ from t2) where 1=1;
其中
如果在视图定义（CREATE VIEW）时指定hint，则在该视图每次被应用时会使用该hint。
当使用random plan功能（参数plan_mode_seed不为0）时，查询指定的plan hint不会被使用。
支持范围
当前版本Plan Hint支持的范围如下，后续版本会进行增强。
指定Join顺序的Hint - leading hint
指定Join方式的Hint，仅支持除semi/anti join，unique plan之外的常用hint。
指定结果集行数的Hint
指定Scan方式的Hint，仅支持常用的tablescan，indexscan和indexonlyscan的hint。
指定子链接块名的Hint
注意事项
不支持Agg、Sort、Setop和Subplan的hint。
示例
本章节使用同一个语句进行示例，便于Plan Hint支持的各方法作对比，示例语句及不带hint的原计划如下所示：
create table store
(
s_store_sk integer not null,
s_store_id char(16) not null,
s_rec_start_date date ,
s_rec_end_date date ,
s_closed_date_sk integer ,
s_store_name varchar(50) ,
s_number_employees integer ,
s_floor_space integer ,
s_hours char(20) ,
s_manager varchar(40) ,
s_market_id integer ,
s_geography_class varchar(100) ,
s_market_desc varchar(100) ,
s_market_manager varchar(40) ,
s_division_id integer ,
s_division_name varchar(50) ,
s_company_id integer ,
s_company_name varchar(50) ,
s_street_number varchar(10) ,
s_street_name varchar(60) ,
s_street_type char(15) ,
s_suite_number char(10) ,
s_city varchar(60) ,
s_county varchar(30) ,
s_state char(2) ,
s_zip char(10) ,
s_country varchar(20) ,
s_gmt_offset decimal(5,2) ,
s_tax_precentage decimal(5,2) ,
primary key (s_store_sk)
);
create table store_sales
(
ss_sold_date_sk integer ,
ss_sold_time_sk integer ,
ss_item_sk integer not null,
ss_customer_sk integer ,
ss_cdemo_sk integer ,
ss_hdemo_sk integer ,
ss_addr_sk integer ,
ss_store_sk integer ,
ss_promo_sk integer ,
ss_ticket_number integer not null,
ss_quantity integer ,
ss_wholesale_cost decimal(7,2) ,
ss_list_price decimal(7,2) ,
ss_sales_price decimal(7,2) ,
ss_ext_discount_amt decimal(7,2) ,
ss_ext_sales_price decimal(7,2) ,
ss_ext_wholesale_cost decimal(7,2) ,
ss_ext_list_price decimal(7,2) ,
ss_ext_tax decimal(7,2) ,
ss_coupon_amt decimal(7,2) ,
ss_net_paid decimal(7,2) ,
ss_net_paid_inc_tax decimal(7,2) ,
ss_net_profit decimal(7,2) ,
primary key (ss_item_sk, ss_ticket_number)
);
create table store_returns
(
sr_returned_date_sk integer ,
sr_return_time_sk integer ,
sr_item_sk integer not null,
sr_customer_sk integer ,
sr_cdemo_sk integer ,
sr_hdemo_sk integer ,
sr_addr_sk integer ,
sr_store_sk integer ,
sr_reason_sk integer ,
sr_ticket_number integer not null,
sr_return_quantity integer ,
sr_return_amt decimal(7,2) ,
sr_return_tax decimal(7,2) ,
sr_return_amt_inc_tax decimal(7,2) ,
sr_fee decimal(7,2) ,
sr_return_ship_cost decimal(7,2) ,
sr_refunded_cash decimal(7,2) ,
sr_reversed_charge decimal(7,2) ,
sr_store_credit decimal(7,2) ,
sr_net_loss decimal(7,2) ,
primary key (sr_item_sk, sr_ticket_number)
);
create table customer
(
c_customer_sk integer not null,
c_customer_id char(16) not null,
c_current_cdemo_sk integer ,
c_current_hdemo_sk integer ,
c_current_addr_sk integer ,
c_first_shipto_date_sk integer ,
c_first_sales_date_sk integer ,
c_salutation char(10) ,
c_first_name char(20) ,
c_last_name char(30) ,
c_preferred_cust_flag char(1) ,
c_birth_day integer ,
c_birth_month integer ,
c_birth_year integer ,
c_birth_country varchar(20) ,
c_login char(13) ,
c_email_address char(50) ,
c_last_review_date char(10) ,
primary key (c_customer_sk)
);
create table promotion
(
p_promo_sk integer not null,
p_promo_id char(16) not null,
p_start_date_sk integer ,
p_end_date_sk integer ,
p_item_sk integer ,
p_cost decimal(15,2) ,
p_response_target integer ,
p_promo_name char(50) ,
p_channel_dmail char(1) ,
p_channel_email char(1) ,
p_channel_catalog char(1) ,
p_channel_tv char(1) ,
p_channel_radio char(1) ,
p_channel_press char(1) ,
p_channel_event char(1) ,
p_channel_demo char(1) ,
p_channel_details varchar(100) ,
p_purpose char(15) ,
p_discount_active char(1) ,
primary key (p_promo_sk)
);
create table customer_address
(
ca_address_sk integer not null,
ca_address_id char(16) not null,
ca_street_number char(10) ,
ca_street_name varchar(60) ,
ca_street_type char(15) ,
ca_suite_number char(10) ,
ca_city varchar(60) ,
ca_county varchar(30) ,
ca_state char(2) ,
ca_zip char(10) ,
ca_country varchar(20) ,
ca_gmt_offset decimal(5,2) ,
ca_location_type char(20) ,
primary key (ca_address_sk)
);
create table item
(
i_item_sk integer not null,
i_item_id char(16) not null,
i_rec_start_date date ,
i_rec_end_date date ,
i_item_desc varchar(200) ,
i_current_price decimal(7,2) ,
i_wholesale_cost decimal(7,2) ,
i_brand_id integer ,
i_brand char(50) ,
i_class_id integer ,
i_class char(50) ,
i_category_id integer ,
i_category char(50) ,
i_manufact_id integer ,
i_manufact char(50) ,
i_size char(20) ,
i_formulation char(20) ,
i_color char(20) ,
i_units char(10) ,
i_container char(10) ,
i_manager_id integer ,
i_product_name char(50) ,
primary key (i_item_sk)
);
explain
select i_product_name product_name
,i_item_sk item_sk
,s_store_name store_name
,s_zip store_zip
,ad2.ca_street_number c_street_number
,ad2.ca_street_name c_street_name
,ad2.ca_city c_city
,ad2.ca_zip c_zip
,count(*) cnt
,sum(ss_wholesale_cost) s1
,sum(ss_list_price) s2
,sum(ss_coupon_amt) s3
FROM store_sales
,store_returns
,store
,customer
,promotion
,customer_address ad2
,item
WHERE ss_store_sk = s_store_sk AND
ss_customer_sk = c_customer_sk AND
ss_item_sk = i_item_sk and
ss_item_sk = sr_item_sk and
ss_ticket_number = sr_ticket_number and
c_current_addr_sk = ad2.ca_address_sk and
ss_promo_sk = p_promo_sk and
i_color in ('maroon','burnished','dim','steel','navajo','chocolate') and
i_current_price between 35 and 35 + 10 and
i_current_price between 35 + 1 and 35 + 15
group by i_product_name
,i_item_sk
,s_store_name
,s_zip
,ad2.ca_street_number
,ad2.ca_street_name
,ad2.ca_city
,ad2.ca_zip
;
