Apache Druid

is it jdbc or http query? ive seen some evidence that heavy jdbc workloads can seem to clog the broker parallel merge pool, i suspect there might be some not great behavior, especially when some jdbc clients are not fetching all of their results or fetching them very slowly (since jdbc shifts the query completing onto the client, and on the broker the query is pulling stuff out of a blocking queue of the parallel merge pool)

in the flame graphs i saw the merge pool itself appeared to be blocked trying to put results into these blocking queues, indicating that it was waiting for downstream stuff to finish consuming the sequence and close everything out

it could be something simple like waiting for a merge buffer: this time isn't counted in `query/cpu/time` since we're just waiting

if that's in, increasing the # of merge buffers would be useful