min_parallel_index_scan_size
No restart — settable per sessionsize (kB)
Smallest index portion to scan that justifies considering a parallel index scan.
What it does
Mirrors min_parallel_table_scan_size for indexes: a parallel index scan only kicks in once the indexed range to scan is at least this large, so small lookups aren't burdened with worker startup.
How to tune it
Default 512 kB suits most cases. Lower it to let smaller index scans go parallel (analytical workloads); raise it to keep small scans serial (OLTP).