RangeIndex - Uses a ConcurrentNavigableMap to store a key to store a RegionEntryToValuesMap. A RegionEntryToValuesMap is a map that uses the entry as the key and a struct as the value An example of the struct (notice the index iter naming associated with the struct and how the struct is a combination of portfolio, position): struct(index_iter1:Portfolio [ID=8 status=active type=type2 pkid=8 XYZ:Position secId=XYZ out=100.0 type=a id=7 mktValue=8.0, AOL:Position secId=AOL out=5000.0 type=a id=5 mktValue=6.0, APPL:Position secId=APPL out=6000.0 type=a id=6 mktValue=7.0, P1:Position secId=MSFT out=4000.0 type=a id=4 mktValue=5.0, P2:null ],index_iter2:Position secId=APPL out=6000.0 type=a id=6 mktValue=7.0)
CompactRangeIndex - A memory efficient but slightly restricted version of RangeIndex. Will be preferred by the engine over range index if possible. Uses a ConcurrentNavigableMap to store a key and value pair, where the value can either be a RegionEntry, an IndexElemArray that contains RegionEntries or a IndexConcurrentHashSet that contains RegionEntries. The ConcurrentNavigableMap also is passed a Comparator that allows Indexes to match across different Numeric types.
MapRangeIndex - This index contains a map where the key is the map key and the value is a range indexes. So for example an portfolio.positions'key' = 'IBM' The map range index would have a map with a key of 'key' and the value would be a range index. The range index would have another map where the key is 'IBM' and the value would be RegionEntryToValuesMap. The RegionEntryToValuesMap would be a map where the key is the entry itself and the value is 'IBM'
CompactMapRangeIndex - Similar to MapRangeIndex but a map of CompactRangeIndexes instead. Similar restrictions to those between CompactRangeIndex and RangeIndex.
HashIndex - Is a memory savings index that does not store key values and instead extracts the key from the object and uses the hash of the key to slot the RegionEntry into an array
PrimaryKeyIndex - The primary key index is a very lightweight index that hints to the query engine that it should do a a region.get(key)
PartitionedIndex - The partition index is a collection of indexes which are the buckets of the region.
For more details about this topic, please have a look at Querying and Indexes Internals.