key is not used

[Corin Langosch]

Hello,

i'm having a problem with mysql. when i put an index on a
datetime column, it's never used.

for example the
sql: 'EXPLAIN SELECT * FROM actions ORDER BY datum'

shows:
table  type  possible_keys  key  key_len  ref  rows  Extra
actions ALL NULL NULL NULL NULL 75853 Using filesort

even if i use the
sql: 'EXPLAIN SELECT * FROM actions USE INDEX (datum) ORDER BY datum'

mysql doesn' use the index :-((

tahnks for any help,
corin