Help Center > > Developer Guide> Setting GUC Parameters> Cluster Transaction Parameters

Cluster Transaction Parameters

Updated at: Jul 15, 2020 GMT+08:00

This section describes the settings and value ranges of cluster transaction parameters.

transaction_isolation

Parameter description: Specifies the isolation level of the current transaction.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1.

Value range:

  • READ COMMITTED: Only committed data is read. This is the default.
  • READ UNCOMMITTED: DWS does not support READ UNCOMMITTED. If READ UNCOMMITTED is set, READ COMMITTED is executed instead.
  • REPEATABLE READ: Only the data committed before transaction start is read. Uncommitted data or data committed in other concurrent transactions cannot be read.
  • SERIALIZABLE: DWS does not support SERIALIZABLE. If SERIALIZABLE is set, REPEATABLE READ is executed instead.

Default value: READ COMMITTED

transaction_read_only

Parameter description: Specifies that the current transaction is a read-only transaction.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1.

Value range: Boolean

  • on indicates that the current transaction is a read-only transaction.
  • off indicates that the current transaction can be a read/write transaction.

Default value: off

xc_maintenance_mode

Parameter description: Specifies whether the system is in maintenance mode.

This parameter is a SUSET parameter. Set it based on method 3 in Table 1.

Value range: Boolean

  • on indicates that maintenance mode is enabled.
  • off indicates that the maintenance mode is disabled.

Enable the maintenance mode with caution to avoid cluster data inconsistencies.

Default value: off

allow_concurrent_tuple_update

Parameter description: Specifies whether to allow concurrent update.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1.

Value range: Boolean

  • on indicates that concurrent update is allowed.
  • off indicates that concurrent update is disallowed.

Default value: on

gtm_backup_barrier

Parameter description: Specifies whether to create a restoration point for the GTM starting point.

This parameter is a SUSET parameter. Set it based on instructions provided in Table 1.

Value range: Boolean

  • on indicates that a restoration point will be created for the GTM starting point.
  • off indicates that a restoration point will not be created for the GTM starting point.

Default value: off

transaction_deferrable

Parameter description: Specifies whether to delay the execution of a read-only serial transaction without incurring an execution failure. Assume this parameter is set to on. When the server detects that the tuples read by a read-only transaction are being modified by other transactions, it delays the execution of the read-only transaction until the other transactions finish modifying the tuples. Currently, this parameter is not used in DWS. Similar to this parameter, the default_transaction_deferrable parameter is used to specify whether to allow delayed execution of a transaction.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1.

Value range: Boolean

  • on indicates that the execution of a read-only serial transaction can be delayed.
  • off indicates that the execution of a read-only serial transaction cannot be delayed.

Default value: off

enforce_two_phase_commit

Parameter description: This parameter is reserved for compatibility with earlier versions. This parameter is invalid in the current version.

enable_show_any_tuples

Parameter description: This parameter is available only in a read-only transaction and is used for analysis. When this parameter is set to on/true, all versions of tuples in the table are displayed.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1.

Value range: Boolean

  • on/true indicates that all versions of tuples in the table are displayed.
  • off/false indicates that no versions of tuples in the table are displayed.

Default value: off

enable_redistribute

Parameter description: Specifies whether unmatched nodes are redistributed.

This parameter is a SUSET parameter. Set it based on instructions provided in Table 1.

Value range: Boolean

  • on indicates that unmatched nodes are redistributed.
  • off indicates that unmatched nodes are not redistributed.

Default value: off

replication_type

Parameter description: Specifies what nodes are deployed for the HA mode. The nodes can consist of a primary, a standby, and a secondary node; or consist of a primary and multiple standby nodes.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1.

This parameter is an internal parameter used for CM deployment. Do not set it.

Value range: an integer

  • 1 indicates that the HA nodes consist of a primary and multiple standby nodes.
  • 0: indicates that the HA nodes consist of a primary, a standby, and a secondary node.

Default value: 0

enable_gtm_free

Parameter description: Specifies whether the GTM-FREE mode is enabled. In large concurrency scenarios, the snapshots delivered by the GTM increase in number and size. The network between the GTM and the CN becomes the performance bottleneck. The GTM-FREE mode is used to eliminate the bottleneck. In this mode, the CN communicates with DNs instead of the GTM. The CN sends queries to each DN, which locally generates snapshots and xids, ensuring external write consistency but not external read consistency.

You are not advised to set this parameter to on in OLTP or OLAP scenarios where strong read consistency is required. This parameter is invalid for DWS.

This parameter is a SUSET parameter. Set it based on instructions provided in Table 1.

Value range: Boolean

  • on indicates that the GTM-FREE mode is enabled and the cluster ensures eventual read consistency.
  • off indicates that the GTM-FREE mode is disabled.

Default value: off

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