1. A method at a host organization for optimizing database queries in a database system of the host organization, the database system having one or more processors coupled with memory and including both a relational data store and a non-relational data store therein, the method comprising: operating the database system within the host organization; receiving a request at the host organization, the request specifying data for retrieval from the database system; retrieving, based on the request received at the host organization, one or more locations of the data for retrieval; generating, at the host organization, a database query based on the request, wherein the database query specifies a plurality of data elements for retrieval, the plurality of data elements including one or more data elements residing within the non-relational data store and one or more other data elements residing within the relational data store; optimizing the database query to include at least a sub-query or a pre-query; executing the optimized database query against the database system to retrieve the data; wherein optimizing the database query comprises: (i) identifying via a first sub-query to a table within relational data store, the first portion of the data for retrieval, (ii) identifying via a second sub-query to the non-relational data store, the second portion of the data for retrieval, (iii) identifying a data delta between the first sub-query that identifies the first portion of the data retrieved within the relational data store and the second sub-query that identifies the second portion of the data retrieved within the non-relational data store, and (iv) executing a third sub-query replicating missing data from the relational data store to the non-relational data store according to the data delta; wherein the database query comprises a join operation performed by initiating non-relational database queries on the non-relational data store where one or more foreign key parents are stored in the relational data store; and wherein the join operation comprises a plurality of sub-queries which are generated based on query optimizations available via an optimizer agent of the host organization, the query optimizations selected from the group comprising: a specified ordering for the plurality of sub-queries; a target data store for execution of a corresponding sub-query; one or more pre-query assessments based on the data for retrieval; a replication order from the relational data store to the non-relational data store; and an in-memory join operation specifying at least one or more of the plurality of data elements for retrieval from each of the relational data store and the non-relational data store and placed into memory accessible to the optimizer agent and a corresponding sub-query to retrieve the at least one or more of the plurality of data elements from the memory accessible to the optimizer agent in fulfillment of the request.