SHOW EXPLAIN: Plan produced by SHOW EXPLAIN is different from standard EXPLAIN: type ALL vs 'index_merge', keys is different, 'Using sort_union(a,b);' is missing

SELECT a+SLEEP(0.01) FROM t1

WHERE a IN ( 255, 0 ) OR b BETWEEN 6 AND 129                                     

ORDER BY b;

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

1	SIMPLE	t1	index_merge	a,b	a,b	5,5	NULL	8	Using sort_union(a,b); Using where; Using filesort

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

1	SIMPLE	t1	ALL	a,b	NULL	NULL	NULL	30	Using where; Using filesort

revision-id: psergey@askmonty.org-20120607081906-rkc1r1wyc9e0n1hj

date: 2012-06-07 12:19:06 +0400

revno: 3409

CREATE TABLE t1 (a INT, b INT, KEY(a), KEY(b)) ENGINE=MyISAM;

INSERT INTO t1 VALUES

(8,0),(128,5050),(5372,8),(234,7596),(2,0),(2907,8930),(1,0),

(0,5224),(8,7638),(960,5),(9872,1534),(0,2295),(3408,9809),

(7,0),(1168,0),(2089,5570),(0,205),(88,1018),(0,26528),

(0,0),(4,5567),(1444,145),(6,0),(1,7535),(7793,534),(70,9),

(178,1),(44,5),(189,0),(3,0);

SELECT a+SLEEP(0.01) FROM t1

WHERE a IN ( 255, 0 ) OR b BETWEEN 6 AND 129

ORDER BY b;

--connect (con1,localhost,root,,)

let $con_id = `SELECT CONNECTION_ID()`;

EXPLAIN

SELECT a+SLEEP(0.01) FROM t1

WHERE a IN ( 255, 0 ) OR b BETWEEN 6 AND 129

ORDER BY b;

--send

SELECT a+SLEEP(0.01) FROM t1

WHERE a IN ( 255, 0 ) OR b BETWEEN 6 AND 129

ORDER BY b;

--connection default

let $fail = 100;

while ($fail)

{

  --error 0,ER_ERROR_WHEN_EXECUTING_COMMAND

  --eval SHOW EXPLAIN FOR $con_id

  --dec $fail

  if (!$mysql_errno)

  {

    let $fail = 0;

  }

}

let $pass = 1;

while ($pass)

{

  --error 0,ER_ERROR_WHEN_EXECUTING_COMMAND

  --eval SHOW EXPLAIN FOR $con_id

  if ($mysql_errno)

  {

    let $pass = 0;

  }

}

--connection con1

--disable_result_log

--reap