How to index when the fields used in the query is unpredictable?
How do you shop for groceries when the meal being cooked is unpredictable? - You don't. Or you spend a ton of money buying every grocery item proactively.
For a thin table (only a few columns), it may be possible to index every realistic combination of fields to be searched on. But most times for most tables it's not reasonably possible. And almost always it's not necessary.
Indexes are needed for a query on this collection. The filter, or
$match, stage of this query may be done using any combination of the 100 fields.
This is an unusual request. Even large scale companies like Facebook (to relate an example to your other posts) don't search across so many fields at a time. When you enter a search term in their search box, it's only searching a few fixed fields such as
Tag, etc. It's not searching things like
Gender, etc, etc.
Even if 64 indexes are created, it will at most only be able to cover 64 fields.
Not exactly. A single index can cover multiple combinations of fields at a time. E.g. a single index on the fields
(FirstName, LastName, Tag) would cover predicates on only
FirstName or on
Tag. So depending on the realistic uses, one index can cover multiple use cases.
I know it sounds like you're getting a lot of redundant answers that don't seem to help you out on each of your recent questions, but that's because optimization for a particular situation is very fact specific. Unfortunately only this generalized information can be provided with the generic details you've provided so far.
If you wanted to provide the exact use cases you have, including what is the system you're working on, what are the objects involved, how are they structured, some example data, and how you're trying to search against them, then maybe a more specific set of optimization approaches can be provided, which will likely be oriented around design implementation.