Union All

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Mar 3, 2020 · Databases

Optimizing a Slow MySQL Query with Derived Tables and UNION ALL

This article analyzes a slow MySQL aggregation query that joins a large table with a derived table built from many UNION ALL subqueries, explains the execution plan and derived‑table materialization, and presents two optimization approaches—adding taskname indexes and rewriting the query or using a temporary indexed table—to dramatically reduce execution time while preserving result semantics.