Data IndependenceThe three-schema architecture can be used to further explain the concept of dataindependence, which can be defined as the capacity to change the schema at onelevel of a database system without having to change the schema at the next higherlevel. We can define two types of data independence:1. Logical data independence is the capacity to change the conceptual schemawithout having to change external schemas or application programs. Wemay change the conceptual schema to expand the database (by adding arecord type or data item), to change constraints, or to reduce the database(by removing a record type or data item). In the last case, external schemasthat refer only to the remaining data should not be affected. For example, theexternal schema of Figure 1.5(a) should not be affected by changing theGRADE_REPORT file (or record type) shown in Figure 1.2 into the oneshown in Figure 1.6(a). Only the view definition and the mappings need tobe changed in a DBMS that supports logical data independence. After theconceptual schema undergoes a logical reorganization, application pro-grams that reference the external schema constructs must work as before.36 Chapter 2 Database System Concepts and ArchitectureChanges to constraints can be applied to the conceptual schema withoutaffecting the external schemas or application programs.2. Physical data independence is the capacity to change the internal schemawithout having to change the conceptual schema. Hence, the externalschemas need not be changed as well. Changes to the internal schema may beneeded because some physical files were reorganized—for example, by creat-ing additional access structures—to improve the performance of retrieval orupdate. If the same data as before remains in the database, we should nothave to change the conceptual schema. For example, providing an accesspath to improve retrieval speed of section records (Figure 1.2) by semesterand year should not require a query such as list all sections offered in fall 2008to be changed, although the query would be executed more efficiently by theDBMS by utilizing the new access path.Generally, physical data independence exists in most databases and file environ-ments where physical details such as the exact location of data on disk, and hard-ware details of storage encoding, placement, compression, splitting, merging ofrecords, and so on are hidden from the user. Applications remain unaware of thesedetails. On the other hand, logical data independence is harder to achieve because itallows structural and constraint changes without affecting application programs—amuch stricter requirement.Whenever we have a multiple-level DBMS, its catalog must be expanded to includeinformation on how to map requests and data among the various levels. The DBMSuses additional software to accomplish these mappings by referring to the mappinginformation in the catalog. Data independence occurs because when the schema ischanged at some level, the schema at the next higher level remains unchanged; onlythe mapping between the two levels is changed. Hence, application programs refer-ring to the higher-level schema need not be changed.The three-schema architecture can make it easier to achieve true data indepen-dence, both physical and logical. However, the two levels of mappings create anoverhead during compilation or execution of a query or program, leading to ineffi-ciencies in the DBMS. Because of this, few DBMSs have implemented the full three-schema architecture.