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This article explains how to read MySQL's EXPLAIN EXTRA column, covering SEMI JOIN hints such as FirstMatch, Start/End temporary, LooseScan, and the Range‑checked‑for‑each‑record optimization, with concrete examples and command‑line output to illustrate each case.
The article analyzes a slow MySQL query that uses an IN subquery on a massive user table, explains how the optimizer’s semi‑join and materialized temporary tables cause full‑table scans, and demonstrates practical fixes such as disabling the semi‑join optimizer switch or rewriting the SQL to regain index usage.
This article explains how MySQL transforms eligible subqueries into semi‑joins, describes the four semi‑join strategies, and details the Semi‑join Materialization approach—including Materialization‑scan and Materialization‑lookup—while showing how to identify the chosen strategy via EXPLAIN output.