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Lecture6(2) - Lecture 6 on Physical Database DBMS has a...

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01/11/12 Lecture 6 on Physical Database DBMS has a view of the database as a collection of stored records, and that view is supported by the file manager which has a view of the database as a collection of pages, and that view is supported by the disk manager. The physical database design is initiated to a certain extent in the “logical” design. The physical organization is determined largely by the need for operational efficiency, fast response times, and cost minimization. Most data storage devices record data as a stream of bits. The groups of bits which we can read with one machine instruction are called physical records. The physical records are stored at locations which are identified by a means of machine addresses. A program identifies a logical record or sequent by means of a key.
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01/11/12 DBMS File manager Disk Manager Stored database The DBMS, file manager and disk manager Request stored record Request stored page Disk I/O operation Store record returned Stored page return Data read from disk
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01/11/12 Sequential file With the physical sequential access method, the physical records are stored in logical sequence. If the storage medium to be used is a tape, the programmer has to present the physical records in a logical sequence. If the storage medium is a direct access one, the system will interconnect the physical records so that they are in logical sequence, even if they were not presented in logical sequence. The records must be read in a fixed sequence from begin to end sequential.
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01/11/12 Overflow area With ISAM files the records are grouped so as to fit onto physical disk tracks, and one track on each cylinder contains an index to the records stored in that cylinder. When new records are inserted after the original sequential file has been set up these are stored in an overflow area. The index track contains pointers both to the prime data area and to the overflow area.
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01/11/12
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01/11/12 Indexed File: B+-Tree A common scheme for extremely large files is to induce a hierarchy of indices that follow the hierarchical nature of the secondary storages devices on which the file resides. We can view the hierarchy of indices as a tree. The benefit of index file is efficient space utilization in searching indexes.
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01/11/12 Third-Level Index (Index Block) (Root) Second-Level Index (Index Block) First -Level Index (Index Block ) Main File (Leaf Block) Multilevel Index B+ Tree
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01/11/12 B+ Tree File keyword Address 385 678 805 7 8 9 keyword Address 150 385 1 2 keyword Address 536 678 3 4 keyword Address 785 805 5 6 001 003 : : :150 251 : : : 385 455 : : : 536 605 : : : 678 705 : : : 785 791 : : : 805 Index File Block 10 Index File Block 7 Index File Block 8 Index File Block 9 First level Index Second level Index Data File Block 1 Data File Block 2 Data File Block 3 Data File Block 4 Data File Block 5 Data File Block 6
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01/11/12 B+-tree parameters
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