public IndexMap getIndexMap(String indexName, String indexedExpression, String fromClause) {
    return new IndexMapImpl();
  }
Return an IndexMap that is persisted to the disk store used by this region.

This IndexMap should be used as the backing map for any regions that are using the Soplog
persistence.

Calling this method may create a branch new index map on disk, or it may recover an index map
that was previously persisted, depending on whether the index previously existed.

TODO: none of the parameters are ever used

@param indexName the name of the index
@param indexedExpression the index expression
@param fromClause the from clause.
@return The index map.
@throws IllegalStateException if this region is not using soplog persistence
@throws IllegalStateException if this index was previously persisted with a different
        expression or from clause.
geode-core/src/main/java/org/apache/geode/internal/cache/LocalRegion.java