PolarDB:Transactional consistency of read-only column store nodes

The transactional consistency described in this topic is applicable to the following versions of clusters and PolarProxy:

• Your PolarDB cluster must use one of the following versions:

  • PolarDB for MySQL 8.0.1.1.31 or later

  • PolarDB for MySQL 8.0.2.2.12 or later

• The PolarProxy version is 2.8.11 or later.

For more information about how to check the cluster version and PolarProxy version, see Query the engine version.

The replication log sequence number (LSN) of a read-only column store node is the minimum value between the column store replication LSN and the row store replication LSN. The node handles complex queries at different consistency levels provided by PolarDB based on the following principles. For more information, see Consistency levels.

• After you enable automatic request distribution among row store and column store nodes, PolarProxy handles complex queries in which the estimated execution cost of SQL statements exceeds a specified threshold in a different manner from common queries. For complex queries, PolarProxy sends requests only to the read-only column store node. Even if the consistency LSN of the node times out, PolarProxy does not forward requests to the primary node. Instead, PolarProxy sends an error to the application side. For more information, see Automatic request distribution among row store and column store nodes.

  Note

  To avoid a large number of errors due to request timeouts on the node, the complex queries on the node have a longer timeout than the common queries.

• If you do not enable automatic request distribution among row store and column store nodes, PolarProxy regards the node as a common read-only node. For more information, see Automatic request distribution among row store and column store nodes.