SHOW EXPLAIN: ref returned by SHOW EXPLAIN is different from the normal EXPLAIN ('const' vs empty string)

EXPLAIN SELECT SLEEP(0.5) FROM t1 WHERE a=1;

id      1

select_type     SIMPLE

table   t1

type    ref

possible_keys   a

key     a

key_len 5

ref     const

rows    1

Extra   Using index

SELECT SLEEP(0.5) FROM t1 WHERE a=1;

SHOW EXPLAIN FOR 2;

id      1

select_type     SIMPLE

table   t1

type    ref

possible_keys   a

key     a

key_len 5

ref

rows    1

Extra   Using index

Warnings:

Level   Note

Code    1003

Message SELECT SLEEP(0.5) FROM t1 WHERE a=1

revision-id: psergey@askmonty.org-20120524182239-gq6jtnnovydnthtl

date: 2012-05-24 22:22:39 +0400

revno: 3405

CREATE TABLE t1 (a INT, KEY(a));

INSERT INTO t1 VALUES

(3),(1),(5),(1);

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

let $con_id = `SELECT CONNECTION_ID()`;

EXPLAIN SELECT SLEEP(0.5) FROM t1 WHERE a=1;

--send

SELECT SLEEP(0.5) FROM t1 WHERE a=1;

--connection default

let $run = 100;

while ($run)

{

  --error 0,ER_ERROR_WHEN_EXECUTING_COMMAND

  --eval SHOW EXPLAIN FOR $con_id

  --dec $run

  if (!$mysql_errno)

  {

    let $run = 0;

  }

}

--connection con1

--reap