Question

Let’s say I want to perform this query:

(SELECT a FROM t1 WHERE a=10 AND B=1) 
UNION ALL 
(SELECT a FROM t2 WHERE a=11 AND B=2) 
UNION ALL 
(SELECT a FROM t3 WHERE a=12 AND B=3) 
ORDER BY a LIMIT 1000;

Is MySQL smart enough to skip “t3” if 550 results are available in “t1” and 450 in “t2”?

I’m looking at MySQL docs (http://dev.mysql.com/doc/refman/5.1/en/union.html) but can’t seem to find the answer.

Answer 1

As specified in UNION Syntax description (http://dev.mysql.com/doc/refman/5.1/en/union.html):

The default behavior for UNION is that
duplicate rows are removed from the
result. The optional DISTINCT keyword
has no effect other than the default
because it also specifies
duplicate-row removal. With the
optional ALL keyword, duplicate-row
removal does not occur and the result
includes all matching rows from all
the SELECT statements.

I suppose, that’s the answer to your question.