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  Should I index every field, to make my queries 100% optimizable?:

  No.  It is not necessary, nor practical, to index every field in your
  database, to achieve high performance queries.  Create indexes on fields
  that are generally used the most for reporting.  For example, if you TOTAL
  invoices for a specific date range, say each week, or month, then maintain
  an index on that date.  Keep in mind that you want your indexes, to
  produce the smallest set of records to be visited by Mach SIx to test the
  non-indexed portion of the query.  You should be able to achieve this
  goal, without reproducing the majority of your database in indexes.