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    • Home

    Slow Union Query - MySQL

    Published Jan 22, 2021 [  MySQL  ]

    Problem

    query1

UNION

query2

    query1 and query2 are fast queries, but the UNION one is slow

    Explanation

    A UNION (which is the same as UNION DISTINCT) or a UNION ALL causes a temporary table to be created. For UNION, the table is created with an index so that duplicates can be removed. With UNION ALL, there is no index.

    Reference