query mystery: union de-optimizes component queries

Why would using UNION cause the subqueries to be de-optimized?

explain
(SELECT count(gamename) as gname ,variation from zertz_gamerecord
where (gmtdate > date_sub(current_timestamp(),interval 90 day)) and (player1='13213' or player2='13213' ) group by variation limit 3)

shows using index on gmtdate

explain
(SELECT count(gamename) as gname ,variation from mp_gamerecord
where (gmtdate > date_sub(current_timestamp(),interval 90 day))
and (player1='13213' or player2='13213' or player3='13213' or player4='13213' or player5='13213' or player6='13213')
group by variation limit 3)

shows using index gmtdate

explain
(SELECT count(gamename) as gname ,variation from zertz_gamerecord
where (gmtdate > date_sub(current_timestamp(),interval 90 day)) and (player1='13213' or player2='13213' ) group by variation limit 3)
UNION
(SELECT count(gamename) as gname ,variation from mp_gamerecord
where (gmtdate > date_sub(current_timestamp(),interval 90 day))
and (player1='13213' or player2='13213' or player3='13213' or player4='13213' or player5='13213' or player6='13213')
group by variation limit 3)

ie: the same two queries shows using no indexes on the first half
of the query.


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