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This is basically how they work:

• Everytime SQL DM for MySQL sends a statement like SHOW VARIABLES/STATUS of some kind (or fetching a OS metric from Linux/proc folder) one row is INSERTED into [timestamp_master] table with information about current time and the metrics retrieved for each such statement is INSERTED into [snapshot_master]. The [snapshot_master] table contains the metrics details. The [timestamp_id] column identifies when metric details were like that. Also, that timestamps in SQL DM for MySQL databases are unix_timestamps.
• And actually, we do not always INSERT into [snapshot_master]. Only if the particular metric was changed since last time something was INSERTED for that metric we will INSERT again. So if you want to find the value of a metric at some particular time you need to find the most recent value stored before that particular time for that particular metric.
• Finally, the [snapshot_master] does not have the names of the metrics. It is not possible to know in advance what metrics the server returns as it depends on server details (version and configuration). And actually a server may be upgraded.
  Saving each textual description only once which would save disk space. So [snapshot_master] only contains a number in the [metric_id] column referring to the textual description the [metric_master] table. So, if the query returns the name of the metric or the metric name should be used in a WHERE-clause also [metric_master] table must be referenced in the query. If you are familiar with the SHOW statements and what information they return you identify the meaning of each row in [metric_master] table.

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