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All two minute intervals or all two minute "whole" intervals - e.g. are you bucketing the data by interval, or do you want to see all possible intervals (e.g. any span of time that is 2 minutes (to the second?) or just a bucketed set of minutes?
Perhaps there's an index on my_column that the optimizer decided not to use in the former case but did use in the latter? When was the last time you gathered stats on the tables? Any number of possibilities... but we need the real query plans...