How to speed up datetime MySQL queries or improve db scheme for time series data?
You may need to add proper indexes to your data... My guess is that you need a multi-column index on (data_node_id, timestamp).

Check the mySQL documentation on Multiple-Column Indexes

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Side note: 

I suspect that a data_node cannot have two readings at the same time. That would make the (data_node_id, timestamp) pair always unique (and not null). If this were the case, I would use this as the primary key, and take out of the table the id column (and the corresponding index). Taking out the surrogate id key will save some data that doesn't need to be read... if you're looking for the last bit of performance, it also helps.

As mySQL + innoDB keeps your data clustered by primary index, having (data_node_id, timestamp) as your PK would clearly be an advantage for retrieving information. Like always, it comes with a price: INSERTS will cause fragmentation (and thus, be more expensive), because your PK will not be an increasing number.