We want to search crash analysis by signature in our internal dashboard https://findit-for-me.googleplex.com/crash/cracas-dashboard

There are 2 ways to do that:
(1) Indexing ``signature`` property and query it.
Pro: This way is clean and fast 
Con: 
Ndb can only do exact match, which means we have to enter the whole signature, we cannot enter partial signature or regex.
We have to re-save all the existing data to get their index updated.

(2) In memory filter by signature
Pro: Can support partial signature or regex.
Con:
It is slow and requires we fetch a large batch of results to filter.