There is a Managed Package on which some of the custom fields are to be indexed so as to improve SOQL Query performance for one of the subscriber's org? Our initial connect with the Salesforce support rep was that they will not be able to edit Managed Package metadata which seemed a little surprising for me. Please can you let me know your thoughts.
Salesforce support can absolutely index managed fields. They did this for me via partner support on a number of occasions at past jobs, including indexing deterministic formulas and using two-column indexes.
There are not any special restrictions around index on managed fields that I am aware of, and if there are they are obscure enough to likely be irrelevant for most people.
You can only index fields that are allowed normally, according to 000006007:
A Custom Index can't be created on these types of fields:
- Multi-select Picklists.
- Currency fields in a Multicurrency Organization.
- Long text fields.
- Binary fields (fields of type blob, file, or encrypted text.).
Note: New data types, typically complex ones, may be added to Salesforce and fields of these types may not allow custom indexing.
Perhaps Support was trying to communicate that a particular field could not be indexed because of its data type? If so, you'll probably need to eventually refactor your code and objects to use a data type that can be indexed.