For all the dnodes in a TDengine cluster, the below parameters must be configured exactly the same, any node whose configuration is different from dnodes already in the cluster can't join the cluster.
**Description**: The linear regression function of the specified column and the timestamp column (primary key), `start_val` is the initial value and `step_val` is the step value.
**Description**: The linear regression function of a specified column, `start_val` is the initial value and `step_val` is the step value.
**Return value type**: A string in the format of "(slope, intercept)"