Large Scale Entity Store Database?

[zhuangmz08]

Hi, I dig into the Postgres table, and I find that entities are actually stored 
as JSON-format strings, which seems to use SQL database as a Document database. 
I'm wondering how efficient queries are achieved? I'm going to insert and query 
millions of entities. Have you ever tested the performance? Should I use 
Mongo-support Entity Store instead? Thanks a lot.