Ceil(), floor() and round() throws error

Queries throw error while using ceil(), floor() and round() function.
For Example:

the query:
select ceil(fieldKey) from rt.meas group by tag order by time desc limit 1
throws error:
ERR: undefined function ceil()

Please help

Not sure about the error, but you cannot use whose functions directly with a “group by” since those functions do not aggregate data.

A valid syntax should be ie: (Docs here)

SELECT CEIL( MEAN(fieldKey) ) from rt.meas GROUP BY tag ORDER BY time DESC LIMIT 1