[SOLVED] MySQL UNION ALL is too slow

Issue

I have a query in MySQL from 4 tables. When I run these queries individually they run quick within 1 second.

But when I combine them using UNION ALL, machine comes under heavy load and take at least 10 seconds for the same query in UNION ALL. And I have digged whole the internet including YouTube, but unfortunately I did not find any solution yet.

Please advise.
Thanks.

code

SELECT *
FROM (
    SELECT id, sessionId FROM promotion
    UNION ALL
    SELECT id, sessionId FROM transport_details
    UNION ALL
    SELECT id, sessionId FROM fee_details
) a;

Solution

Thank you all the developers here who participated in my question.

Although I did not find any satisfactory resolution of MySQL UNION ALL slow query. But I have managed it up to 80% by optimizing MySQL individual queries.

Also there is a provision to increase the memory and CPU limits.

Answered By – kk4You

Answer Checked By – Cary Denson (BugsFixing Admin)

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