The id cache loads the ids of all documents. If we only load the parent ids, then we can reduce the memory usage of the parent/child support.

Update: The amount of memory that will be reduced when upgrading to version 0.90.1 depends on the amount of child documents in an index. For example if you have one child doc for every parent doc, then it the memory usage of the id cache should be reduced by around half. The more child docs per parent doc the larger the difference.