min_parallel_table_scan_size
No restart — settable per sessionsize (MB)
The smallest table scan size for which the planner considers going parallel.
What it does
Parallelism has startup overhead, so the planner only parallelizes a scan once the table is at least this big. min_parallel_table_scan_size (default 8 MB) sets that floor; the number of workers it considers scales with how much larger the table is.
How to tune it
Lower it to make the planner parallelize smaller scans (useful in analytics where even mid-size scans benefit); raise it to keep small scans serial and avoid worker overhead on a busy OLTP box. There's an index counterpart, min_parallel_index_scan_size.