If I run this query:

SELECT 1 UNION SELECT 2 INTO OUTFILE 'test.txt';

It correctly creates a file 'test.txt' with two lines:

1

2

Now if I do "rm test.txt" and execute another query (with parentheses):

(SELECT 1) UNION (SELECT 2 INTO OUTFILE 'test.txt');

it still puts the same two lines from both UNION parts.

This looks confusing, as parentheses a kind of assume that only the right UNION part is to be exported.

We could introduce a less confusing syntax to export UNION queries, for example:

((SELECT 1) UNION (SELECT 2)) INTO OUTFILE 'test.txt';

But "MDEV-10028 Syntax error on ((SELECT ...) UNION (SELECT ...))" should be fixed first.

Note, one can write something like this:

SELECT * INTO OUTFILE 'test.txt'

FROM ((SELECT ...) UNION (SELECT ...)) AS t1;

But it looks longer.

Sergei thinks that even if we add a less confusing syntax, we should not disallow the confusing syntax not to break existing applications.
https://lists.launchpad.net/maria-developers/msg09599.html

But at least just sending a warning suggesting a better syntax would probably be fine.