Spinoff from java-dev thread "Sorting with little memory: A suggestion", started by Toke Eskildsen.
When sorting by SortField.STRING we currently ask FieldCache for a StringIndex on that field.
This can consumes tons of RAM, when the values are mostly unique (eg a title field), as it populates both int ords as well as String values.
But, if the index is only one segment, and the search sets fillFields=false, we don't need the String values, just the int ords. If the app needs to show the fields it can pull them (for the 1 page) from stored fields.
This can be a potent optimization – alot of RAM saved – for optimized indexes.
When fixing this we must take care to share the int ords if some queries do fillFields=true and some =false... ie, FieldCache will be called twice and it should share the int ords across those invocations.