The Ultimate Guide to High Availability with MariaDB

Configuring ColumnStore Local PM Query Mode

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MariaDB ColumnStore has the ability to query data from just a single PM instead of the whole database through the UM. In order to accomplish this, the infinidb_local_query variable in the my.cnf configuration file is used and maybe set as a default at system wide or set at the session level.

Enable local PM query systemwide

To enable the use of the local PM Query at the instance level, specify infinidb_local_query =1 (enabled) in the my.cnf configuration file at /usr/local/mariadb/columnstore/mysql. The default is 0 (disabled).

Enable/disable local PM query at the session level

To enable/disable the use of the local PM Query at the session level, the following command is used. Once the session has ended, any subsequent session will return to the default for the instance.

set infinidb_local_query = n
where n is:
* 0 (disabled)
* 1 (enabled)

At the session level, this variable applies only to executing a query on an individual PM and will error if executed on the UM. The PM must be set up with the local query option during installation.

Local PM Query Examples

Example 1 - SELECT from a single table on local PM to import back on local PM:

With the infinidb_local_query variable set to 1 (default with local PM Query):

mcsmysql -e 'select * from source_schema.source_table;' –N | /usr/local/Calpont/bin/cpimport target_schema target_table -s '\t' –n1

Example 2 - SELECT involving a join between a fact table on the PM node and dimension table across all the nodes to import back on local PM:

With the infinidb_local_query variable set to 0 (default with local PM Query):

Create a script (i.e., extract_query_script.sql in our example) similar to the following:

set infinidb_local_query=0;
select fact.column1, dim.column2 
from fact join dim using (key) 
where idbPm(fact.key) = idbLocalPm();

The infinidb_local_query is set to 0 to allow query across all PMs.

The query is structured so that the UM process on the PM node gets the fact table data locally from the PM node (as indicated by the use of the idbLocalPm() function), while the dimension table data is extracted from all the PM nodes.

Then you can execute the script to pipe it directly into cpimport:

mcsmysql source_schema -N < extract_query_script.sql | /usr/local/mariadb/columnstore/bin/cpimport target_schema target_table -s '\t' –n1

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