Hello,

We have 2 similar thanos query (observee / observer) architectures. (query->queriers->sidecars):
One with the prometheus promql engine and one with the thanos promql engine and the following query gave different results with the thanos promql engines compared to thanos with prometheus engine and directly executed in prometheus also:

avg by (hrStorageDescr) (hrStorageUsed{nodename =~"xxxxx", } * on (instance, hrStorageIndex ) group_left(hrStorageDescr) (sum by (instance, hrStorageIndex, hrStorageDescr) (hrStorageDescr{nodename =~"xxxxx", hrStorageDescr!~".*Data.*"} ) ))

results with thanos engine (wrong one):

results with prometheus engine (ok one):

results in prometheus ui directly:

Sorry I had no time yet to try to reproduce with a "generic" exporter/query. I may try in the coming days.
I suspect group_left queries (maybe only with negative matchers?). Are those supported by the engine yet?

Thanks