更新时间：2024-07-03 GMT+08:00
Plan Hint调优概述
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。
GaussDB还提供了SQL PATCH功能，在不修改业务语句的前提下通过创建SQL PATCH的方式使得Hint生效。
功能描述
Plan Hint在SELECT、INSERT、UPDATE、DELETE、MERGE等关键字后通过如下形式指定：
1
|
/*+ <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：
1
|
select /*+ <plan_hint1> <plan_hint2> */ * from t1, (select /*+ <plan_hint3> */ * from t2) where 1=1; |
其中<plan_hint1>，<plan_hint2>为外层查询的hint，<plan_hint3>为内层子查询的hint。
检查Plan Hint调优的效果可以借助explain语法进行分析。通过explain可以查看使用Plan Hint后目标SQL的计划，对比计划是否符合要求以验证Plan Hint的效果。explain有多种计划展示的模式，通过explain_perf_mode进行控制。本节的示例一般通过设置explain_perf_mode为pretty模式来展示计划，展示较全的计划相关信息。部分示例设置explain_perf_mode为normal模式以精简输出信息。
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Stream方式的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。
- 指定子查询不展开的hint。
- 指定内表物化的hint。
- 指定Bitmapscan的hint。
- 指定Agg方法的hint。
注意事项
不支持Sort、Setop和Subplan的hint。
示例
本章节大部分示例使用下述语句，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：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 |
create table t1(c1 int, c2 int, c3 int); create table t2(c1 int, c2 int, c3 int); create table t3(c1 int, c2 int, c3 int); create index it1 on t1(c1,c2); create index it2 on t2(c1,c2); create index it3 on t1(c3,c2); 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 ; |
HashAggregate (cost=18.09..18.10 rows=1 width=776)
Group By Key: item.i_product_name, item.i_item_sk, store.s_store_name, store.s_zip, ad2.ca_street_number, ad2.ca_street_name, ad2.ca_city, ad2.ca_zip
-> Nested Loop (cost=0.00..18.06 rows=1 width=776)
-> Nested Loop (cost=0.00..17.37 rows=1 width=416)
-> Nested Loop (cost=0.00..17.08 rows=1 width=420)
-> Nested Loop (cost=0.00..16.67 rows=1 width=420)
-> Nested Loop (cost=0.00..16.26 rows=1 width=262)
Join Filter: (item.i_item_sk = store_sales.ss_item_sk)
-> Nested Loop (cost=0.00..15.46 rows=2 width=216)
-> Seq Scan on item (cost=0.00..11.16 rows=1 width=208)
Filter: ((i_current_price >= 35::numeric) AND (i_current_price <= 45::numeric) AND (i_current_price >= 36::numeric) AND (i_current_price <= 50::numeric) AN
D (i_color = ANY ('{maroon,burnished,dim,steel,navajo,chocolate}'::bpchar[])))
-> Index Only Scan using store_returns_pkey on store_returns (cost=0.00..4.29 rows=2 width=8)
Index Cond: (sr_item_sk = item.i_item_sk)
-> Index Scan using store_sales_pkey on store_sales (cost=0.00..0.38 rows=1 width=62)
Index Cond: ((ss_item_sk = store_returns.sr_item_sk) AND (ss_ticket_number = store_returns.sr_ticket_number))
-> Index Scan using store_pkey on store (cost=0.00..0.40 rows=1 width=166)
Index Cond: (s_store_sk = store_sales.ss_store_sk)
-> Index Scan using customer_pkey on customer (cost=0.00..0.40 rows=1 width=8)
Index Cond: (c_customer_sk = store_sales.ss_customer_sk)
-> Index Only Scan using promotion_pkey on promotion (cost=0.00..0.28 rows=1 width=4)
Index Cond: (p_promo_sk = store_sales.ss_promo_sk)
-> Index Scan using customer_address_pkey on customer_address ad2 (cost=0.00..0.68 rows=1 width=368)
Index Cond: (ca_address_sk = customer.c_current_addr_sk)
(23 rows)
父主题： 使用Plan Hint进行调优
