Fast query, in theory using index, still in the slow-query log

Question

I'm having a hard time interpreting the EXPLAIN results of these queries. They both end up in the slow-query log, but the execution time is ~0.0050ms and the final result set is always under 100 rows. What's wrong here? Is my second "improved" version any better?
Any suggestion?

  mysql> # Original
  mysql> EXPLAIN SELECT SQL_NO_CACHE relSrc, relDst, 1 as relType, relTypeDesc, 0 as fracQty, '24794' as source FROM productsRelationships1
      -> LEFT JOIN productsRelationshipsDesc on 1=relTypeID
      -> WHERE relDst='24794' OR relSrc='24794'
      -> UNION ALL
      -> SELECT relSrc, relDst, 2 as relType, relTypeDesc, 0 as fracQty, '24794' as source FROM productsRelationships2
      -> LEFT JOIN productsRelationshipsDesc on 2=relTypeID
      -> WHERE relDst='24794' OR relSrc='24794'
      -> UNION ALL
      -> SELECT relSrc, relDst, 3 as relType, relTypeDesc, 0 as fracQty, '24794' as source FROM productsRelationships3
      -> LEFT JOIN productsRelationshipsDesc on 3=relTypeID
      -> WHERE relDst='24794' OR relSrc='24794'
      -> UNION ALL
      -> SELECT relSrc, relDst, 5 as relType, relTypeDesc, 0 as fracQty, '24794' as source FROM productsRelationships5
      -> LEFT JOIN productsRelationshipsDesc on 5=relTypeID
      -> WHERE relDst='24794' OR relSrc='24794'
      -> UNION ALL
      -> SELECT relSrc, relDst, 6 as relType, relTypeDesc, fracQty, '24794' as source      FROM productsRelationships6
      -> LEFT JOIN productsRelationshipsDesc on 6=relTypeID
      -> WHERE relDst='24794' OR relSrc='24794'
      -> UNION ALL
      -> SELECT relSrc, relDst, 7 as relType, relTypeDesc, 0 as fracQty, '24794' as source FROM productsRelationships7
      -> LEFT JOIN productsRelationshipsDesc on 7=relTypeID
      -> WHERE relDst='24794' OR relSrc='24794'
      -> ORDER BY relType, relSrc, RelDst;
  +----+--------------+---------------------------+-------------+---------------------+---------------+---------+-------+-------+-----------------------------------------+
  | id | select_type  | table                     | type        | possible_keys       | key           | key_len | ref   | rows  | Extra                                   |
  +----+--------------+---------------------------+-------------+---------------------+---------------+---------+-------+-------+-----------------------------------------+
  |  1 | PRIMARY      | productsRelationships1    | index       | PRIMARY,src-1       | src-1         | 2       | NULL  |   663 | Using where; Using index                |
  |  1 | PRIMARY      | productsRelationshipsDesc | ref         | relTypeID           | relTypeID     | 1       | const |     1 | Using index                             |
  |  2 | UNION        | productsRelationships2    | index       | src-dst-2           | src-dst-2     | 4       | NULL  | 13126 | Using where; Using index                |
  |  2 | UNION        | productsRelationshipsDesc | ref         | relTypeID           | relTypeID     | 1       | const |     1 | Using index                             |
  |  3 | UNION        | productsRelationships3    | index       | PRIMARY             | PRIMARY       | 4       | NULL  | 11459 | Using where; Using index                |
  |  3 | UNION        | productsRelationshipsDesc | ref         | relTypeID           | relTypeID     | 1       | const |     1 | Using index                             |
  |  4 | UNION        | productsRelationships5    | index       | PRIMARY,src-5       | src-5         | 2       | NULL  |   369 | Using where; Using index                |
  |  4 | UNION        | productsRelationshipsDesc | ref         | relTypeID           | relTypeID     | 1       | const |     1 | Using index                             |
  |  5 | UNION        | productsRelationships6    | index_merge | PRIMARY,src-6,dst-6 | dst-6,PRIMARY | 2,2     | NULL  |     2 | Using union(dst-6,PRIMARY); Using where |
  |  5 | UNION        | productsRelationshipsDesc | ref         | relTypeID           | relTypeID     | 1       | const |     1 | Using index                             |
  |  6 | UNION        | productsRelationships7    | index       | PRIMARY,src-7       | src-7         | 2       | NULL  |     1 | Using where; Using index                |
  |  6 | UNION        | productsRelationshipsDesc | ref         | relTypeID           | relTypeID     | 1       | const |     1 | Using index                             |
  | NULL | UNION RESULT | <union1,2,3,4,5,6>        | ALL         | NULL                | NULL          | NULL    | NULL  |  NULL | Using filesort                          |
  +----+--------------+---------------------------+-------------+---------------------+---------------+---------+-------+-------+-----------------------------------------+
  13 rows in set (0.00 sec)

  mysql>
  mysql>
  mysql> # Improved?
  mysql> EXPLAIN SELECT SQL_NO_CACHE relSrc, relDst, relType, fracQty, source, relTypeDesc FROM (
      -> SELECT relSrc, relDst, 1 as relType, 0 as fracQty, '24794' as source FROM productsRelationships1
      -> WHERE relDst='24794' OR relSrc='24794'
      -> UNION ALL
      -> SELECT relSrc, relDst, 2 as relType, 0 as fracQty, '24794' as source FROM productsRelationships2
      -> WHERE relDst='24794' OR relSrc='24794'
      -> UNION ALL
      -> SELECT relSrc, relDst, 3 as relType, 0 as fracQty, '24794' as source FROM productsRelationships3
      -> WHERE relDst='24794' OR relSrc='24794'
      -> UNION ALL
      -> SELECT relSrc, relDst, 5 as relType, 0 as fracQty, '24794' as source FROM productsRelationships5
      -> WHERE relDst='24794' OR relSrc='24794'
      -> UNION ALL
      -> SELECT relSrc, relDst, 6 as relType,      fracQty, '24794' as source FROM productsRelationships6
      -> WHERE relDst='24794' OR relSrc='24794'
      -> UNION ALL
      -> SELECT relSrc, relDst, 7 as relType, 0 as fracQty, '24794' as source FROM productsRelationships7
      -> WHERE relDst='24794' OR relSrc='24794'
      -> ) AS rels
      -> LEFT JOIN productsRelationshipsDesc ON relType=relTypeID
      -> ORDER BY relType, relSrc, RelDst;
  +----+--------------+---------------------------+-------------+---------------------+---------------+---------+--------------+-------+------------------------------------------------------+
  | id | select_type  | table                     | type        | possible_keys       | key           | key_len | ref          | rows  | Extra                                                |
  +----+--------------+---------------------------+-------------+---------------------+---------------+---------+--------------+-------+------------------------------------------------------+
  |  1 | PRIMARY      | <derived2>                | ALL         | NULL                | NULL          | NULL    | NULL         |    38 | Using filesort                                       |
  |  1 | PRIMARY      | productsRelationshipsDesc | ref         | relTypeID           | relTypeID     | 1       | rels.relType |     1 | Using index                                          |
  |  2 | DERIVED      | productsRelationships1    | index       | PRIMARY,src-1       | src-1         | 2       | NULL         |   663 | Using where; Using index                             |
  |  3 | UNION        | productsRelationships2    | index       | src-dst-2           | src-dst-2     | 4       | NULL         | 13126 | Using where; Using index                             |
  |  4 | UNION        | productsRelationships3    | index       | PRIMARY             | PRIMARY       | 4       | NULL         | 11459 | Using where; Using index                             |
  |  5 | UNION        | productsRelationships5    | index       | PRIMARY,src-5       | src-5         | 2       | NULL         |   369 | Using where; Using index                             |
  |  6 | UNION        | productsRelationships6    | index_merge | PRIMARY,src-6,dst-6 | dst-6,PRIMARY | 2,2     | NULL         |     2 | Using union(dst-6,PRIMARY); Using where; Using index |
  |  7 | UNION        | productsRelationships7    | index       | PRIMARY,src-7       | src-7         | 2       | NULL         |     1 | Using where; Using index                             |
  | NULL | UNION RESULT | <union2,3,4,5,6,7>        | ALL         | NULL                | NULL          | NULL    | NULL         |  NULL |                                                      |
  +----+--------------+---------------------------+-------------+---------------------+---------------+---------+--------------+-------+------------------------------------------------------+
  9 rows in set (0.00 sec)

Those are the tables definitions

  mysql> SHOW CREATE TABLE productsRelationships1\G
  *************************** 1. row ***************************
         Table: productsRelationships1
  Create Table: CREATE TABLE `productsRelationships1` (
    `relSrc` smallint(5) unsigned NOT NULL,
    `relDst` smallint(5) unsigned NOT NULL,
    PRIMARY KEY (`relSrc`,`relDst`),
    UNIQUE KEY `src-1` (`relSrc`)
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8
  1 row in set (0.00 sec)

  mysql> SHOW CREATE TABLE productsRelationships2\G
  *************************** 1. row ***************************
         Table: productsRelationships2
  Create Table: CREATE TABLE `productsRelationships2` (
    `relSrc` smallint(5) unsigned NOT NULL,
    `relDst` smallint(5) unsigned NOT NULL,
    KEY `src-dst-2` (`relSrc`,`relDst`)
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8
  1 row in set (0.00 sec)

  mysql> SHOW CREATE TABLE productsRelationships3\G
  *************************** 1. row ***************************
         Table: productsRelationships3
  Create Table: CREATE TABLE `productsRelationships3` (
    `relSrc` smallint(5) unsigned NOT NULL,
    `relDst` smallint(5) unsigned NOT NULL,
    PRIMARY KEY (`relSrc`,`relDst`)
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8
  1 row in set (0.00 sec)

  mysql> SHOW CREATE TABLE productsRelationships5\G
  *************************** 1. row ***************************
         Table: productsRelationships5
  Create Table: CREATE TABLE `productsRelationships5` (
    `relSrc` smallint(5) unsigned NOT NULL,
    `relDst` smallint(5) unsigned NOT NULL,
    PRIMARY KEY (`relSrc`,`relDst`),
    UNIQUE KEY `src-5` (`relSrc`)
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8
  1 row in set (0.00 sec)

  mysql> SHOW CREATE TABLE productsRelationships6\G
  *************************** 1. row ***************************
         Table: productsRelationships6
  Create Table: CREATE TABLE `productsRelationships6` (
    `relSrc` smallint(5) unsigned NOT NULL,
    `relType` tinyint(2) unsigned NOT NULL DEFAULT '6',
    `fracQty` int(2) unsigned NOT NULL,
    `relDst` smallint(5) unsigned NOT NULL,
    PRIMARY KEY (`relSrc`,`relDst`),
    UNIQUE KEY `src-6` (`relSrc`),
    UNIQUE KEY `dst-6` (`relDst`)
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8
  1 row in set (0.00 sec)

  mysql> SHOW CREATE TABLE productsRelationships7\G
  *************************** 1. row ***************************
         Table: productsRelationships7
  Create Table: CREATE TABLE `productsRelationships7` (
    `relSrc` smallint(5) unsigned NOT NULL,
    `relDst` smallint(5) unsigned NOT NULL,
    PRIMARY KEY (`relSrc`,`relDst`),
    UNIQUE KEY `src-7` (`relSrc`)
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8
  1 row in set (0.00 sec)

  mysql> SHOW CREATE TABLE productsRelationshipsDesc\G
  *************************** 1. row ***************************
         Table: productsRelationshipsDesc
  Create Table: CREATE TABLE `productsRelationshipsDesc` (
    `relTypeID` tinyint(2) unsigned NOT NULL AUTO_INCREMENT,
    `relTypeDesc` varchar(100) NOT NULL,
    UNIQUE KEY `relTypeID` (`relTypeID`,`relTypeDesc`)
  ) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8
  1 row in set (0.00 sec)

And this is the amount of data in every table.

  mysql> SELECT COUNT(1) FROM productsRelationships1\G
  *************************** 1. row ***************************
  COUNT(1): 663
  1 row in set (0.00 sec)

  mysql> SELECT COUNT(1) FROM productsRelationships2\G
  *************************** 1. row ***************************
  COUNT(1): 263
  1 row in set (0.00 sec)

  mysql> SELECT COUNT(1) FROM productsRelationships3\G
  *************************** 1. row ***************************
  COUNT(1): 8551
  1 row in set (0.01 sec)

  mysql> SELECT COUNT(1) FROM productsRelationships5\G
  *************************** 1. row ***************************
  COUNT(1): 369
  1 row in set (0.00 sec)

  mysql> SELECT COUNT(1) FROM productsRelationships6\G
  *************************** 1. row ***************************
  COUNT(1): 80
  1 row in set (0.00 sec)

  mysql> SELECT COUNT(1) FROM productsRelationships7\G
  *************************** 1. row ***************************
  COUNT(1): 0
  1 row in set (0.00 sec)

  mysql> SELECT COUNT(1) FROM productsRelationshipsDesc\G
  *************************** 1. row ***************************
  COUNT(1): 7
  1 row in set (0.00 sec)

---

UPDATE: Acording to the EXPLAIN It APPEARS to be using index but the slow query log says this # Query_time: 0.005458 Lock_time: 0.000340 Rows_sent: 38 Rows_examined: 50579 and this is when I take out the ORDER clause
It may be a fast query, but it's called nearly in every single operation of the system.

---

UPDATE2: Ok, I'm going crazy here

# Query_time: 0.003527  Lock_time: 0.000164 Rows_sent: 38  Rows_examined: 8554
SET timestamp=1370017780;
SELECT SQL_NO_CACHE relSrc, relDst, 3 as relType, 0 as fracQty, '24794' as source
FROM   productsRelationships3
WHERE relSrc='24794' OR relDst='24794';

How is this possible? productsRelationships3 has a composed index PRIMARY KEY (relSrc,relDst) why it 's examining every single row?