Parallel query: the Gather node, and planned vs launched workers
Simple terms
For a big scan Postgres can hand pieces of the work to helper processes and collect the results at a Gather node. How many helpers? It decides from the size of the table being scanned, not from how busy the machine is right now. That gap is why EXPLAIN shows two numbers, workers planned and workers launched. When they don't match, the planner asked for help that wasn't there, the server's worker slots were already taken, and your query ran with fewer hands than its cost assumed.
You might be asked
Explain how a parallel sequential scan actually runs. Where do the workers come from, who decides how many, what is the difference between 'Workers Planned' and 'Workers Launched', and what does the leader do while the workers run?
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