One source of plateauing effect at 8 processes on unsaturated IO system *might* be the Linux readahead window, defaulting to 128kB. Parallel Sequential Scan starts to look like random access when each process is reading data with a step size > that size.
Replying to @MengTangmu @pwramsey and
There's also often the issue that there's a substantial portion of non-parallel work, which then means that even if the parallel part of the work scales close to linearly, the gain of that for the overall runtime of the query decreases.
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