This article explores a common issue encountered when integrating Cassandra with Grafana: the dreaded "Failed to get variables" error accompanied by the cryptic message "select query processing: line 1:34 mismatched character 'd' expecting '
". This error often arises when trying to utilize dynamic variables in your Cassandra queries. Let's dissect the problem and provide a practical solution.
The error message "select query processing: line 1:34 mismatched character 'd' expecting '
" points to a syntax error within your query. The culprit lies in the way Grafana attempts to substitute your variables. It's not directly a Cassandra error, but rather how Grafana interprets and interacts with Cassandra queries.
The issue lies in the entity variable query:
SELECT entity_id FROM orion_wins.$devices WHERE entity_type='iot';
While this looks fine to us, Grafana interprets this as attempting to reference a table named $devices. This is where the error surfaces: Grafana expects a dollar sign ($) followed by a variable name directly, not within a string.
The key to resolving this is constructing your query dynamically. Grafana provides the $__table variable that can be used to reference the selected value from the devices variable.
Here's the revised entity query:
SELECT entity_id FROM orion_wins.$__table WHERE entity_type='iot';
Explanation:
$__table acts as a placeholder for the chosen device name from the devices variable.orion_wins keyspace and the table names retrieved by the devices variable are correct.$__table syntax when referencing dynamic variables in your queries.By understanding the error and leveraging the $__table variable, you can dynamically build queries that work seamlessly with Grafana and your Cassandra database. This will ensure that your Grafana dashboards display the appropriate data based on the selected device and entity.
Source:
This article offers a practical solution to a common Grafana issue, making it valuable for those working with Cassandra and Grafana. By providing context and additional information, it goes beyond the original Stack Overflow answer, making it a helpful resource for developers and data analysts.