Why does MySQL stop using an index for a join when I select non-indexed fields in the field list

I have the following two tables:

CREATE TABLE `temporal_expressions` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`dated_obj_type` varchar(255) DEFAULT NULL,
`dated_obj_id` int(11) DEFAULT NULL,
`start_date` datetime DEFAULT NULL,
`end_date` datetime DEFAULT NULL,
`start_time` int(11) DEFAULT NULL,
`end_time` int(11) DEFAULT NULL,
`created_at` datetime DEFAULT NULL,
`updated_at` datetime DEFAULT NULL,
`lock_version` int(11) NOT NULL DEFAULT '0',
`wday` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `te_search` (`dated_obj_type`,`dated_obj_id`,`start_date`,`end_date`),
KEY `te_calendar`   (`dated_obj_type`,`dated_obj_id`,`start_date`,`end_date`,`start_time`,`end_time`),
KEY `te_search_wday` (`dated_obj_type`,`dated_obj_id`,`start_date`,`end_date`,`wday`),
KEY `te_calendar_wday` (`dated_obj_type`,`dated_obj_id`,`start_date`,`end_date`,`start_time`,`end_time`,`wday开发者_如何学编程`),
KEY `te_index` (`wday`,`dated_obj_type`,`start_date`,`end_date`,`start_time`,`end_time`,`dated_obj_id`)
) ENGINE=InnoDB AUTO_INCREMENT=8162445 DEFAULT CHARSET=latin1

CREATE TABLE `asset_blocks` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`block_type` int(11) DEFAULT '0',
`spaces_left` int(11) DEFAULT NULL,
`provider_note` varchar(255) DEFAULT NULL,
`extra_data` text,
`lock_version` int(11) DEFAULT '0',
`created_at` datetime DEFAULT NULL,
`updated_at` datetime DEFAULT NULL,
`type` varchar(255) DEFAULT NULL,
`service_provider_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `type` (`type`,`id`),
KEY `service_provider_id` (`service_provider_id`,`type`,`id`),
) ENGINE=InnoDB AUTO_INCREMENT=516867 DEFAULT CHARSET=latin1

If I run explain on this query (note that I am only selecting fields in the te_calendar_wday index from temporal_expressions) it uses the index for the join as expected

EXPLAIN SELECT asset_blocks.*, temporal_expressions.id,
temporal_expressions.dated_obj_type, temporal_expressions.dated_obj_id,
temporal_expressions.start_date, temporal_expressions.end_date,
temporal_expressions.start_time
FROM `asset_blocks`
LEFT OUTER JOIN `temporal_expressions`
ON `temporal_expressions`.dated_obj_id = `asset_blocks`.id 
  AND `temporal_expressions`.dated_obj_type = 'AssetBlock'
WHERE ( temporal_expressions.start_date <= '2010-11-25' 
AND temporal_expressions.end_date >= '2010-11-01'
AND temporal_expressions.start_time < 1000 AND temporal_expressions.end_time > 1200 
AND temporal_expressions.wday IN (1,2,3,4,5,6) 
AND asset_blocks.id IN (1,2,3,4,5,6,7,8,9) )

1 SIMPLE temporal_expressions range    te_search,te_calendar,te_search_wday,te_calendar_wday,te_index te_calendar_wday 272 NULL 9 Using where; Using index
1 SIMPLE asset_blocks eq_ref PRIMARY PRIMARY 4 lb_production.temporal_expressions.dated_obj_id 1

However, if I run this query (note that I have added a non-indexed field to the field list) it no longer uses the index (it uses a join buffer). Is this intentional or am I missing something?

EXPLAIN SELECT asset_blocks.*, temporal_expressions.id,
temporal_expressions.dated_obj_type, temporal_expressions.dated_obj_id,
temporal_expressions.start_date, temporal_expressions.end_date,
temporal_expressions.start_time, temporal_expressions.created_at
FROM `asset_blocks`
LEFT OUTER JOIN `temporal_expressions`
ON `temporal_expressions`.dated_obj_id = `asset_blocks`.id 
  AND `temporal_expressions`.dated_obj_type = 'AssetBlock'
WHERE ( temporal_expressions.start_date <= '2010-11-25' 
AND temporal_expressions.end_date >= '2010-11-01'
AND temporal_expressions.start_time < 1000 AND temporal_expressions.end_time > 1200 
AND temporal_expressions.wday IN (1,2,3,4,5,6) 
AND asset_blocks.id IN (1,2,3,4,5,6,7,8,9) )


1 SIMPLE asset_blocks range PRIMARY PRIMARY 4 NULL 9 Using where
1 SIMPLE temporal_expressions range te_search,te_calendar,te_search_wday,te_calendar_wday,new_te_index te_search 272 NULL 9 Using where; Using join buffer

I cannot be sure if this is the case here, but:

If you select only indexed fields, MySQL can answer the whole query out of the index and does not even load the table data file.

If you select a field that is not indexed, it has to load the table data.

When making its execution plan, in certain cases (see comment) MySQL decides to do a full table scan although an index is present. This is because it's much quicker to read all data blindly than to look up every entry in the index and then read the data.