Sorted inside Union

A feature of sorting inside union.
Thank you funca I ’ll leave a post - maybe someone will find the main meaning useful - " You need to sort the result, not the intermediate data ."

1. CREATE TABLE `t1` (
2. `id1` int( 10 ) unsigned NOT NULL default '0',
3. `id2` int( 10 ) unsigned NOT NULL ,
4. `k1` decimal( 6, 5 ) unsigned NOT NULL default '0.00000',
5. `k2` int( 10 ) unsigned NOT NULL default '0',
6. PRIMARY KEY ( `id1` , `id2` ) ,
7. KEY `k2` ( `id1` , `k2` )
8. ) ENGINE = MEMORY;

.

engine memory is not a key point - I checked the same thing on MyISAM

and fill it with data similar to the one below.

1. for($i=0;$i<100;$i++){
2.   $i->query('REPLACE INTO `t1` (`id1`, `id2`, `k1`, `k2`)VALUES (?d, ?d, ?f, ?d)',
3.     rand(0,99999), rand(0,5), rand(2,99999)/50000, rand(0,20)
4.   );
5. }

* This source code was highlighted with Source Code Highlighter.

and look at the result of such a query:

1. (
2.   SELECT `id1`, `k1`, `k2`
3.   FROM `t1`
4.   WHERE `id2`=3 AND `k2`=13
5.   ORDER BY `k1` DESC
6. )
7. UNION (
8.   SELECT `id1`, `k1`, `k2`
9.   FROM `k1`
10.   WHERE `id2`=3 AND `k2`!=13
11.   ORDER BY `k1` DESC
12. )

here it is, for the above example:

id1 k1 k2
2726 1.50194 13
88207 0.25084 13
37274 0.96550 11
11059 0.42600 6
11139 1.90196 4
63593 1.42970 5
65273 1.44950 18
28721 0.79328 15
70946 0.87576 4
96673 1.71290 14
49207 1.92928 17
40697 1.82320 18

The bug in our opinion is that in the sample generated by the second query of the union there is no sorting by the middle column.
Can any of the habrasociety shed light on such behavior?

By the way, of course, we also want to share a solution to this problem:

1. SELECT `id1` , `k1` , `k2`
2. FROM `t1`
3. WHERE `id2` =3
4. ORDER BY if( `k2` =13, 0, 1 ) , `k1` DESC

2726 1.50194 13
88207 0.25084 13
49207 1.92928 17
11139 1.90196 4
40697 1.82320 18
96673 1.71290 14
65273 1.44950 18
63593 1.42970 5
37274 0.96550 11
70946 0.87576 4
28721 0.79328 15
11059 0.42600 6