10. A computer-implemented method of providing access to data having a particular schema, comprising: receiving abstract queries composed on the basis of a data abstraction model which models the data having the particular schema and exposes the data to users in a manner that allows the users to compose queries on the basis of the data abstraction model without knowledge of the particular schema; wherein the data abstraction model comprises a plurality of logical field definitions, the plurality of logical field definitions being maintained separately from physical queries, each of the logical field definitions comprising: (i) a logical field name; (ii) at least one location attribute identifying a location of physical data corresponding to the logical field name; and (iii) a reference to an access method invoked to access the physical data in response to receiving the abstract query containing the logical field name, wherein at least one of the logical field names is different from a physical field name of the corresponding physical data as defined by the schema; and wherein each abstract query comprises: (i) a user-specified result specification specifying one or more logical field definitions for which results are to be returned; and (ii) at least one user-specified selection criterion specifying conditions to filter the physical data corresponding to the one or more logical field definitions specified by the result specification; and configuring one or more computer processors to perform an operation comprising, in response to receiving each abstract query, wherein each abstract query is expressed according to a syntax of a predefined query language: accessing those logical field definitions of the data abstraction model that correspond to logical field names included in the abstract query; on the basis accessed logical field definitions, transforming, by operation of the one or more computer processors, the abstract query into physical query consistent with the particular schema; and providing the physical query to a query execution environment for execution.
10. A computer-implemented method of providing access to data having a particular schema, comprising: receiving abstract queries composed on the basis of a data abstraction model which models the data having the particular schema and exposes the data to users in a manner that allows the users to compose queries on the basis of the data abstraction model without knowledge of the particular schema; wherein the data abstraction model comprises a plurality of logical field definitions, the plurality of logical field definitions being maintained separately from physical queries, each of the logical field definitions comprising: (i) a logical field name; (ii) at least one location attribute identifying a location of physical data corresponding to the logical field name; and (iii) a reference to an access method invoked to access the physical data in response to receiving the abstract query containing the logical field name, wherein at least one of the logical field names is different from a physical field name of the corresponding physical data as defined by the schema; and wherein each abstract query comprises: (i) a user-specified result specification specifying one or more logical field definitions for which results are to be returned; and (ii) at least one user-specified selection criterion specifying conditions to filter the physical data corresponding to the one or more logical field definitions specified by the result specification; and configuring one or more computer processors to perform an operation comprising, in response to receiving each abstract query, wherein each abstract query is expressed according to a syntax of a predefined query language: accessing those logical field definitions of the data abstraction model that correspond to logical field names included in the abstract query; on the basis accessed logical field definitions, transforming, by operation of the one or more computer processors, the abstract query into physical query consistent with the particular schema; and providing the physical query to a query execution environment for execution. 13. The method of claim 10 , wherein the different access methods types include at least a composed access method type; wherein composed access methods, of respective logical field definitions, define an expression for the respective logical definitions, the expression being defined on the basis of a different logical field definition.