Why parallel query did not kick in
A big aggregate runs single-threaded while your cores sit idle. Understand the gates that block parallelism and a 297 ms serial scan becomes an 89 ms parallel one.
Problem
What you're actually looking at
The symptom as it shows up on a real server.
Parallel query has to clear several gates, table size, max_parallel_workers_per_gather, cost thresholds, and worker availability. Miss one and PostgreSQL silently runs the plan on a single worker while the box has cores to spare.
A Meridian aggregate over a large table runs as a single-threaded HashAggregate. With parallelism allowed, the same query fans out across workers and finishes in a third of the time.
Simple terms
PostgreSQL can split a big scan across several worker processes, but only if a whole checklist passes: the table is large enough, the setting max_parallel_workers_per_gather allows it, the estimated cost clears a threshold, and workers are actually free at that moment. Miss any one item and the query quietly runs on a single core while the rest of the machine sits idle. Knowing the checklist tells you exactly which knob to turn to get the spare cores working.
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