Help Center/ GaussDB/ Developer Guide(Centralized_8.x)/ SQL Optimization/ Hint-based Optimization/ Plan Hint Optimization
Updated on 2024-08-20 GMT+08:00
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Plan Hint Optimization

In plan hints, you can specify a join order, join and scan operations, and the number of rows in a result to tune an execution plan, improving query performance.

GaussDB also provides the SQL patch function. You can create an SQL patch to make hints take effect without modifying service statements.

Description

Plan hints are specified in the following format after keywords such as SELECT, INSERT, UPDATE, DELETE, and MERGE:

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/*+ <plan hint>*/

You can specify multiple hints for a query plan and separate them with spaces. A hint specified for a query plan does not apply to its subquery plans. To specify a hint for a subquery, add the hint following the SELECT of this subquery.

Example:

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select /*+ <plan_hint1> <plan_hint2> */ * from t1, (select /*+ <plan_hint3> */ * from t2) where 1=1;

In the preceding command, <plan_hint1> and <plan_hint2> are the hints of a query, and <plan_hint3> is the hint of its subquery.

You can use the EXPLAIN syntax to analyze the plan hint optimization effect. You can use EXPLAIN to view the plan of the target SQL statement after the plan hint is used and check whether the plan meets the requirements to verify the plan hint effect. EXPLAIN has multiple plan display modes, which are controlled by explain_perf_mode. In some examples in this section, explain_perf_mode is set to pretty to display complete plan information. In some examples, explain_perf_mode is set to normal to simplify the output information.

If a hint is specified in the CREATE VIEW statement, the hint will be applied each time this view is used.

If the random plan function is enabled (plan_mode_seed is set to a value other than 0), the specified hint will not be used.

Scope

Currently, the following hints are supported:

  • Join order hints (leading).
  • Join operation hints, excluding the semi join, anti join, and unique plan hints.
  • Rows hints.
  • Stream operation hints.
  • Scan operation hints, supporting only the tablescan, indexscan, and indexonlyscan hints.
  • Sublink name hints.
  • Hints specifying not to expand subqueries.
  • Hints for internal table materialization.
  • Bitmap scan hints.
  • Hints of the Agg method

Precautions

Hints do not support Sort, Setop, or Subplan.

Examples

The following are the statements (used in most examples in this section) and the original plan without hints for comparing the methods supported by Plan Hint:

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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)
Parent topic: Hint-based Optimization