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The article explains MySQL's Index Merge optimization, detailing its three types—Intersection, Union, and Sort‑Union—when it is beneficial, how to design indexes and rewrite queries for better performance, and the associated costs and limitations.
This article explains MySQL EXPLAIN type column, detailing the inefficiencies of type=ALL, how adding ORDER BY can change it to index, and demonstrates the use of INDEX_MERGE for UNION and INTERSECT queries, including practical examples and cost comparisons to improve query performance.
This article explains the MySQL Index Merge access method, details the underlying merge‑sort algorithm, enumerates the three index‑merge strategies, analyzes source‑code snippets, presents a real‑world case where index merge hurts performance, and offers practical optimization recommendations.
This article analyzes a production MySQL deadlock caused by two identical UPDATE statements that trigger an index‑merge optimization, explains the lock interactions between the idx_status and PRIMARY indexes, and provides code‑level and MySQL‑level solutions to prevent such deadlocks.
This article reviews MySQL composite index syntax, explains the mandatory condition that the leftmost column must appear in the query filter, analyzes seven representative SQL statements that cannot benefit from a composite index, and provides practical refactoring suggestions such as adding single‑column indexes or creating more appropriate composite indexes.
This article explains the concepts of cardinality and HINT in MySQL, demonstrates how inaccurate cardinality can lead to sub‑optimal execution plans, and shows how applying index_merge hints dramatically reduces query cost and row scans for multi‑column predicates.