MySQL Performance: MySQL => Does MySQL optimize UNION with a LIMIT clause?

Does MySQL optimize UNION with a LIMIT clause? [message #1758]

Fri, 07 September 2007 06:46

Spuerhund
Messages: 4
Registered: August 2007

Junior Member

Assume i have a query like:

(SELECT a FROM t1)
UNION ALL
(SELECT a FROM t2)
ORDER BY a LIMIT 10;

Will MySQL be so clever and execute it in the same way (and time) as:

(SELECT a FROM t1 LIMIT 10)
UNION ALL
(SELECT a FROM t2 LIMIT 10)
ORDER BY a LIMIT 10;

In other words:
will the limit clause from the outer UNION statement be applied to the inner SELECT queries automatically? Otherwise the inner queries might return a lot (millions) of rows, although only the first 10 rows have a chance to appear in the final result.

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