Index question

Is there any reason an index would not be used in optimising a query?
I have an SQL which does a join and correctly picks up an index on a numeric
column
but fails to use another index that is on a character column (char I think)
Maybe its because its character column - as most joins are done on numeric
values

I just wondered if theres a simple answer to this? If not I'll post the SQL
which I don't have
with me!

I think the SQL went something like this:

Select * From Order JOIN Customer ON Customer.account_no = Order.account_no
WHERE Order.order_no = 12345

The explain function told me it was using the index on order_no but scanning the
customer
table!

Either a) I've made a stupid mistake
or b) Something else

PS Is there a way of finding out which indexes exist on a table???

Thanks in advance,

MC.

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