grafana 时间函数

简单记录一下 grafana 的时间函数说明：

- $__time(column) -> UNIX_TIMESTAMP(column) as time_sec
- $__timeEpoch(column) -> UNIX_TIMESTAMP(column) as time_sec
- $__timeFilter(column) -> column BETWEEN FROM_UNIXTIME(1492750877) AND FROM_UNIXTIME(1492750877)
- $__unixEpochFilter(column) ->  time_unix_epoch > 1492750877 AND time_unix_epoch < 1492750877
- $__unixEpochNanoFilter(column) ->  column >= 1494410783152415214 AND column <= 1494497183142514872

- $__timeGroup(column,'5m'[, fillvalue]) -> cast(cast(UNIX_TIMESTAMP(column)/(300) as signed)*300 as signed)
     by setting fillvalue grafana will fill in missing values according to the interval
     fillvalue can be either a literal value, NULL or previous; previous will fill in the previous seen value or NULL if none has been seen yet
- $__timeGroupAlias(column,'5m') -> cast(cast(UNIX_TIMESTAMP(column)/(300) as signed)*300 as signed) AS "time"
- $__unixEpochGroup(column,'5m') -> column DIV 300 * 300
- $__unixEpochGroupAlias(column,'5m') -> column DIV 300 * 300 AS "time"