Session-level Transaction Pushdown and Related Parameters

session_type

Parameter description: Specifies the type of the current session. The value is case-insensitive. An execution plan is generated based on the actual value.

Parameter type: enumerated type

Unit: none

Value range:

single_node: single-node session. Read and write operations can be pushed down. Nodes to be pushed down are determined and an execution plan is generated based on the GUC parameter exec_target_node.
global_read_only: cross-node read-only session. A distributed plan is generated normally, but distributed write operations are forbidden.
normal: default session, which is the same as the original behavior. The plan is executed based on the original logic of the database.

Default value: normal

Setting method: This is a USERSET parameter. Set it based on instructions provided in Table 2. Note that this parameter cannot be set globally using gs_guc.

Setting suggestion: Retain the default value.

Risks and impacts of improper settings: When data is inserted in single-node session mode (session_type is set to single_node), correct data distribution is ensured by the application layer. If an incorrect node is specified, data may be inconsistent.

exec_target_node

Parameter description: Specifies the name of the DN where DML statements and some DDL statements (TRUNCATE and ALTER TABLE...TRUNCATE PARTITION...) of the current session are executed. The name is case-sensitive. If the DN name is invalid, an error is reported during the setting.

This parameter is valid only when the GUC parameter session_type is set to single_node.

Parameter type: string.

Unit: none

Value range: names of all DNs in the current cluster. You can obtain the names of all DNs by querying the PGXC_NODE system catalog.

Default value: ""

Setting method: This is a USERSET parameter. Set it based on instructions provided in Table 2. Note that this parameter cannot be set globally using gs_guc.

Setting suggestion: Retain the default value.

Risks and impacts of improper settings: In single-node session mode, if the parameters are not used according to the restrictions, data inconsistency may occur.

The single-node session mode has the following restrictions:

The router function does not take effect in this scenario.
The DN specified by node_name hint does not take effect and is delivered by exec_target_node.
Statements involving row-level security are not supported.
Reading on the distributed standby node is not supported.
Advanced package statements are not supported.
GPC is not supported. When the enable_global_plancache parameter is enabled, if PBE is executed, an error is reported.
In single-node session mode, when the PBE statement is used, the plan type is directly set to GPlan. If the single-node session mode is specified after a plan is generated, the original plan type remains unchanged.

Examples

gaussdb=# SET log_min_messages = debug2;
gaussdb=# SET logging_module = 'on(opt_choice)';
gaussdb=# CREATE TABLE t1(a int ,b int);
gaussdb=# PREPARE x(int) AS SELECT * FROM t1;
gaussdb=# EXPLAIN EXECUTE x(10);
NOTICE:  [Choosing C/G/A Plan]: GPlan, reason: Using pbe optimization.
NOTICE:  [Choosing C/G/A Plan]: CPlan, reason: First 5 times using CPlan.
                    QUERY PLAN                    
--------------------------------------------------
 Data Node Scan  (cost=0.00..0.00 rows=0 width=0)
   Node/s: All datanodes
(2 rows)
gaussdb=# DEALLOCATE x;

gaussdb=# PREPARE x(int) AS SELECT * FROM t1;
gaussdb=# SET session_type=single_node;
gaussdb=# SET exec_target_node=datanode1;
gaussdb=# EXPLAIN EXECUTE x(10);
NOTICE:  [Choosing C/G/A Plan]: GPlan, reason: Using pbe optimization.
NOTICE:  [Choosing C/G/A Plan]: GPlan, reason: Using pbe optimization.
                    QUERY PLAN                    
--------------------------------------------------
 Data Node Scan  (cost=0.00..0.00 rows=0 width=0)
   Node/s: datanode1
(2 rows)
gaussdb=# DEALLOCATE x;
gaussdb=# DROP TABLE t1;