Tag History Partitioning

I have a system that does not have tag history partitioning enabled. I was just wondering what would happen if this was turned on after say a years worth of data was stored. Would it be a seamless transition to using the partitioning where trends that cross the threshold would not break?

Yes. The part of the historian that actually performs queries is independent of the mechanism that actually stores into partitions - by design, the only “communication” the two pieces have is the sqlth_partitions table, which you’ll notice has a start and end timestamp. Once you enable partitioning, each new partition created will just make a new record in that table (along with its actual data partition table). Querying automatically bridges the gaps in those start and end timestamps.