>>This query uses natural for both tables, when I expected it to use an
>>index for the join.
>>
>>select *
>>from table1 t1
>>join table2 t2 on t2.indexed_char14 starting with t1.indexed_char10
>>where t1.unindex_varchar containing 'foo'
>>
>>This uses an index, so the optimizer knows about it.
>>
>>select *
>>from table2 t2
>>where t2.indexed_char14 starting with 'bar'
>>
>>How can I get the optimizer to use the index for the join?
>
>Anyone? Without any indexes, I won't live to see this complete.

Not a good suggestion (my simplified understanding of how Firebird JOINs tables, may not be appropriate when STARTING WITH is part of the JOIN, so I've no clue how to force that index), but I can think of two possible solutions:

a)
An expression index and
join table2 t2 on substring(t2.indexed_char14 from 1 for 10) = t1.indexed_char10

b)
Add a calculated field
t2.indexed_char10
that you index and then change your JOIN to:
join table2 t2 on t2.indexed_char10 = t1.indexed_char10

Sorry for not having any better suggestions,
Set