13. A system comprising: a processor and a memory; a communications interface to a database system having a relational data store and a non-relational data store implemented therein; a request processor to receive a request at the host system for a database system, the request specifying data for retrieval from the database system; a customer schema processor to retrieve, based on the request, one or more locations of the data for retrieval; a query layer agent to generate 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 a non-relational data store and one or more other data elements residing within a relational data store; a query optimizer to optimize the database query to include at least a sub-query or a pre-query; wherein the query layer agent to execute the optimized database query against the database system to retrieve the data; wherein the query optimizer to further: (i) identify via a first sub-query to a table within relational data store, the first portion of the data for retrieval, (ii) identify via a second sub-query to the non-relational data store, the second portion of the data for retrieval, (iii) identify 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) execute a third sub-query to replicate 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.
13. A system comprising: a processor and a memory; a communications interface to a database system having a relational data store and a non-relational data store implemented therein; a request processor to receive a request at the host system for a database system, the request specifying data for retrieval from the database system; a customer schema processor to retrieve, based on the request, one or more locations of the data for retrieval; a query layer agent to generate 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 a non-relational data store and one or more other data elements residing within a relational data store; a query optimizer to optimize the database query to include at least a sub-query or a pre-query; wherein the query layer agent to execute the optimized database query against the database system to retrieve the data; wherein the query optimizer to further: (i) identify via a first sub-query to a table within relational data store, the first portion of the data for retrieval, (ii) identify via a second sub-query to the non-relational data store, the second portion of the data for retrieval, (iii) identify 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) execute a third sub-query to replicate 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. 15. The system of claim 13 : wherein the database query comprises the plurality of sub-queries; wherein at least one of the plurality of sub-queries are directed toward retrieving the one or more data elements residing within the non-relational data store from the non-relational data store; and wherein at least a second one of the plurality of sub-queries are directed toward retrieving the one or more other data elements residing within the relational data store from the relational data store.