Help Center/ GaussDB(DWS)/ Performance Tuning/ SQL Optimization Guide/ Hint-based Tuning/ Plan Hint Optimization
Updated on 2024-04-28 GMT+08:00
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Plan Hint Optimization

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

Function

Plan hints can be specified using the keywords such as SELECT, INSERT, UPDATE, MERGE, and DELETE, in the following format:

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/*+ <plan hint> */

You can specify multiple hints for a query plan and separate them by 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 keyword of this subquery.

For 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.

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.

Supported Hints

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 tablescan, indexscan, and indexonlyscan
  • Sublink name hints
  • Skew hints, supporting only the skew in the redistribution involving Join or HashAgg
  • Hint used for Agg distribution columns Only clusters of 8.1.3.100 and later versions support this function.
  • Hint that disables subquery pull-up. Only clusters of 8.2.0 and later versions support this function.
  • Configuration parameter hints. For details about supported parameters, see Configuration Parameter Hints.

Precautions

  • Sort, Setop, and Subplan hints are not supported.
  • Hints do not support SMP or Node Groups.
  • Hints cannot be used for the target table of the INSERT statement.

Examples

The following is the original plan and is used for comparing with the optimized ones:

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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
;

Parent topic: Hint-based Tuning