using a union graph with two different backend graph, how can jena knows when to query a particular backend?

[刑天]

Sorry for my poor written English...


My question is simple:
I have a unioned default graph which backed by two different graphs: one for 
tdb, and one for memory. I only made queries using default graph, how can jena 
knows when to query a tdb, and when to query a memory db. Is jena loads 
everything in memory for this situation?


this is the assembler:


@prefix :      <http://base/#> .
@prefix rdf:   <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix tdb:   <http://jena.hpl.hp.com/2008/tdb#> .
@prefix tdb2:  <http://jena.apache.org/2016/tdb#> .
@prefix ja:    <http://jena.hpl.hp.com/2005/11/Assembler#> .
@prefix rdfs:  <http://www.w3.org/2000/01/rdf-schema#> .
@prefix fuseki: <http://jena.apache.org/fuseki#> .


:service_tdb_all  a                   fuseki:Service ;
        rdfs:label                    "TDB2 test" ;
        fuseki:dataset                :dataset ;
        fuseki:name                   "test" ;
        fuseki:serviceQuery           "query" , "sparql" ;
        fuseki:serviceReadGraphStore  "get" ;
        fuseki:serviceReadWriteGraphStore
                "data" ;
        fuseki:serviceUpdate          "update" ;
        fuseki:serviceUpload          "upload" .


:dataset a ja:RDFDataset ;
  ja:defaultGraph :unioned_model ;
  .


:unioned_model
  a            ja:UnionModel ;
  ja:subModel <#graph> ;
  ja:subModel <#ontology> ;
  .


<#graph> rdf:type tdb:GraphTDB ;
  tdb:dataset :tdb_dataset_readwrite ;
  .


:tdb_dataset_readwrite
        a              tdb:DatasetTDB ;
        tdb:location  "/Users/gaozhixin/example_tdb" .


<#ontology> rdf:type ja:MemoryModel ;
     ja:content [ja:externalContent <file:/Users/gaozhixin/example.ttl> ] ;