Help Center> GaussDB> Distributed_2.x> Performance Tuning> SQL Optimization> Hint-based Tuning> Scan Operation Hints
Updated on 2023-10-23 GMT+08:00
View PDF

Scan Operation Hints

Function

These hints specify a scan operation, which can be tablescan, indexscan, or indexonlyscan.

Syntax

1
 
[no] tablescan|indexscan|indexonlyscan(table [index])

Parameter Description

  • no indicates that the specified hint will not be used for a join.
  • table specifies the table to be scanned. You can specify only one table. Use a table alias (if any) instead of a table name.
  • index indicates the index for indexscan or indexonlyscan. You can specify only one index.

indexscan and indexonlyscan hints can be used only when the specified index belongs to the table.

Scan operation hints can be used for row-store tables, column-store tables, OBS tables (The current feature is a lab feature. Contact Huawei engineers for technical support before using it.), and subquery tables.

Example

To specify an index-based hint for a scan, create an index named i on the i_item_sk column of the item table.

1
 
create index i on item(i_item_sk);

Hint the query plan in Example as follows:

1
2
 
explain
select /*+ indexscan(item i) */ i_product_name product_name ...

item is scanned based on an index. The optimized plan is as follows:

Parent topic: Hint-based Tuning