Database Systems: The Complete Book- P7

Database Systems: The Complete Book- P7: Database Systems and Database Design and Application courses offered at the junior, senior and graduate levels in Computer Science departments. Written by well-known computer scientists, this introduction to database systems offers a comprehensive approach, focusing on database design, database use, and implementation of database applications and database management systems
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Database Systems: The Complete Book- P7 CHAPTER 12. REPRESENTING DATA EL.EMENTS 12.3. REPRESENTING BLOCK AAiD RECORD ADDRESSES 581 logical physical do not know in advance how many records the block will hold, and we do not Logical address have to allocate a fixed amount of the block header to the table initially. - offset table-) header -- unused -+ address record record 4 record 3 record 1 Figure 12.6: A map table translates logical to physical addresses t t I to reserve some bytes to represent the host, others to represent the storage unit, and so on, a rational address notation would use considerably more than Figure 12.7: A block with a table of offsets telling us the position of each record 10 bytes for a system of this scale. within the block The address of a record is now the physical address of its block plus the offset 12.3.2 Logical and Structured Addresses of the entry in the blocks offset table for that record. This level of indirection within the block offers many of the advantages of logical addresses, without the One might wonder what the purpose of logical addresses could be. All the infor- need for a global map table. mation needed for a physical address is found in the map table, and following logical pointers to records requires consulting the map table and then going 1 can move the record around within the block, and all we have to do % to the physical address. However, the level of indirection involved in the map is change the records entry in the offset table; pointers to the record will table allows us considerable flexibility. For example, many data organizations still be able to find it. require us to move records around, either within a block or from block to block. If we use a map table, then all pointers to the record refer to this map table, We can even allow the record to move to another block, if the offset table and all we have to do when ure move or delete the record is to change the entry entries are large enough to hold a .forwarding address for the record. for that record in the table. Many combinations of logical and physical addresses are possible as well, Finally, we have an option, should the record be deleted, of leaving in its yielding structured address schemes. For instance, one could use a physical offset-table entry a tombstone, a special value that indicates the record has address for the block (but not the offset within the block), and add the key value been deleted. Prior to its deletion, pointers to this record may have been for the record being referred to. Then, to find a record given this structured stored a t various places in the database. After record deletio ...