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Parallel Query

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PostgreSQL parallel scans: why minimum sizes also affect worker counts

PostgreSQL uses minimum parallel scan sizes both to admit scan candidates and to estimate worker counts. PostgreSQL 18 documentation and code show how to separate size thresholds, planner costs, and worker availability when tuning.

Database Postgresql

2026-10-03

Why PostgreSQL COUNT(DISTINCT) cannot use parallel aggregation—and how to rewrite it

PostgreSQL cannot use parallel aggregation when an aggregate call contains DISTINCT. Using the primary source’s execution plans and benchmark as the starting point, this article explains a GROUP BY rewrite, NULL semantics, and what to validate with EXPLAIN.

Database Postgresql

2026-08-06