Hints for Specifying the Degree of Parallelism for Scans

Description

The degree of parallelism (DOP) is specified for table scans in a parallel execution plan.

Syntax

scandop([@queryblock] table dop_num)

Parameters

• For details about @queryblock, see Hint Specifying the Query Block Where the Hint Is Located. @queryblock can be omitted, indicating that the hint takes effect in the current query block.
• table specifies the table to be scanned. You can specify only one table. Use a table alias (if any) instead of a table name.
• dop_num specifies the DOP for table scans.
• scandop specifies a hint for specifying the DOP for scans.

Examples

-- Preparation
CREATE TABLE cst1(a int, b int, c int, d bigint);
set explain_perf_mode = pretty;  -- Open the explain pretty option to view a detailed plan.
set enable_fast_query_shipping = off;  -- Disable FQS optimization.
-- Usage
gaussdb=# EXPLAIN (costs off) SELECT /*+ Set(query_dop 2) scandop(cst1 2)*/ * FROM cst1;
 id |                   operation
----+-----------------------------------------------
  1 | ->  Streaming (type: GATHER)
  2 |    ->  Streaming(type: LOCAL GATHER dop: 1/2)
  3 |       ->  Seq Scan on cst1
(3 rows)

In a parallel plan, you can use scandop hints to specify the degree of parallelism.

The hint takes effect only when dop_num is the same as the current degree of parallelism (query_dop) or is set to 1.

gaussdb=# EXPLAIN (costs off) SELECT /*+ Set(query_dop 2) scandop(cst1 4)*/ * FROM cst1;
 id |      operation       
----+----------------------
  1 | ->  Seq Scan on cst1
(1 row)