#Partial Index Implementation
A partial index is a type of index built over a subset of a map; the subset is defined by a conditional expression (called the predicate of the partial index). The index contains entries for only those table rows that satisfy the predicate. A major motivation for partial indexes is to avoid indexing common values. Since a query searching for a common value (one that accounts for more than a few percent of all the map entries) will not use the index anyway, there is no point in keeping those rows in the index at all. This reduces the size of the index, which will speed up queries that do use the index. It will also speed up many write operations (IMap.put, IMap.set etc.) because the index does not need to be updated in all cases. (benchmarks) [1]
[Wikipedia] (http://en.wikipedia.org/wiki/Partial_index)
[Postgresql] (http://www.postgresql.org/docs/8.0/static/indexes-partial.html)
[In case you may want to look at some related unit tests] (https://github.com/buremb