Oracle like statement not using correct index - sql

Question

Oracle database.
I've got the following segment of SQL that's performing a full table scan on PROVIDER P1 table. I believe this is because it's dynamically building a like clause as you can see on line XXX.
I've got an index on PROVIDER.TERMINAL_NUMBER and the following SQL snippet does use the correct index.
select * from providers where terminal_number like '1234%'
so why does the following not hit that index?
SELECT P1.PROVIDER_NUMBER, P1.TERMINAL_NUMBER, PC."ORDER" FROM PROVIDERS P1
INNER JOIN PROVIDER_CONFIG PC
ON PC.PROVIDER_NUMBER = P1.PROVIDER_NUMBER
WHERE EXISTS (
SELECT E2.* FROM EQUIPMENT E1
INNER JOIN EQUIPMENT E2
ON E1.MERCHANT_NUMBER = E2.MERCHANT_NUMBER
WHERE E1.TERMINAL_NUMBER = 'SA323F'
AND E1.STATUS IN (0, 9)
AND E2.STATUS IN (0, 9)
XXX
AND P1.TERMINAL_NUMBER LIKE SUBSTR(E2.TERMINAL_NUMBER, 0, length(E2.TERMINAL_NUMBER) - 1) || '%'
)
ORDER BY PC."ORDER" DESC

Answer 1

Here ...
select * from providers where terminal_number like '1234%'
... the Optimiser knows all the fitting numbers start with a fixed prefix and so will be co-located in the index. Hence reading the index is likely to be very efficient.
But here there is no such knowledge ...
P1.TERMINAL_NUMBER LIKE SUBSTR(E2.TERMINAL_NUMBER, 0, length(E2.TERMINAL_NUMBER) - 1) || '%'
There can be any number of different prefixes from E2.TERMINAL_NUMBER and the query will be returning records from all over the PROVIDERS table. So indexed reads will be highly inefficient, and a blunt approach of full scans is the right option.
It may be possible to rewrite the query so it works more efficiently - for instance you would want a Fast Full Index Scan rather than a Full Table Scan. But without knowing your data and business rules we're not really in a position to help, especially when dynamic query generation is involved.
One thing which might improve performance would be to replace the WHERE EXISTS with a WHERE IN...
SELECT P1.PROVIDER_NUMBER, P1.TERMINAL_NUMBER, PC."ORDER" FROM PROVIDERS P1
INNER JOIN PROVIDER_CONFIG PC
ON PC.PROVIDER_NUMBER = P1.PROVIDER_NUMBER
WHERE substr(P1.TERMINAL_NUMBER, 1, 5) IN (
SELECT SUBSTR(E2.TERMINAL_NUMBER, 1, 5)
FROM EQUIPMENT E1
INNER JOIN EQUIPMENT E2
ON E1.MERCHANT_NUMBER = E2.MERCHANT_NUMBER
WHERE E1.TERMINAL_NUMBER = 'SA323F'
AND E1.STATUS IN (0, 9)
AND E2.STATUS IN (0, 9)
)
ORDER BY PC."ORDER" DESC
This would work if the length of the terminal number is constant. Only you know your data, so only you can tell whether it will fly.

Answer 2

If this query does not use an index:
select *
from providers
where terminal_number like '1234%'
Then presumably terminal_number is numeric and not a string. The type conversion prevents the use of the index.
If you want to use an index, then convert the value to a string and use a string index:
create index idx_providers_terminal_number_str on providers(cast(terminal_number as varchar2(255)));
Then write the query as:
select *
from providers
where cast(terminal_number as varchar2(255)) like '1234%'