DB uses wrong index

Hi,

I have a custom table, with 2 indexes:

1) Z01 with the ff. fields: client, plant, doc1, movmt_typ

2) Z02 with only one field: doc1

my select statement is below:

select single * from zcustom where doc1 = p_doc.

when i check on st05, the system uses the Z01 index instead of Z02 which has the field in my where condition.

i need to know why the system uses Z01 and not Z02.

Thansks!