> Running SET STATISTICS on this particular index
> Query RDB$INDICES.RDB$STATISTICS for this index

No Change: 0.0000049277592

>Also you can generate a histogram of the used values/permutation with:
>select
> sourcetype
> , transactiontype
> , refno
> , count(*)
>from
> stk_stocktransaction
>group by
> sourcetype
> , transactiontype
> , refno

>This gives you the value distribution across the entire index.

Neat trick, but not sure what I should be looking at exactly. Returns about 200 000 records.

The SQL I am testing with, returns no records.

Interestingly though, this histogram query DOES use my index ;-)

Which gave me an idea:

I changed my original query to:
select t.SalesOrderNo from stk_stocktransaction t
where (t.SourceType = 34)
and (t.TransactionType = 'EXTCST')
and (t.RefNo = 'I00001')
order by t.SourceType, t.TransactionType, t.RefNo

This DOES use the index.

BUT, it's on average 50% SLOWER than the NATURAL plan the Firebird chose to use.

<scratching head>