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

Function

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

Syntax

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[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, HDFS tables, HDFS foreign tables, OBS tables, and subquery tables. HDFS tables include primary tables and delta tables. The delta tables are invisible to users. Therefore, scan operation hints are used only for primary tables.

If indexscan is specified, indexscan or indexonlyscan takes effect. indexscan and indexonlyscan can also take effect at the same time. When indexscan and indexonlyscan hints appear at the same time, indexonlyscan takes effect first.

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.

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create index i on item(i_item_sk);

Hint the query plan in Examples as follows:

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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