Overview

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# Overview - PowerPoint PPT Presentation

Database systems the complete book 13.8. record modification By Ahmad Yazdankhah ( ID 219) CS 257 P rof.: T. Y. Lin San José State University SPRING 2011. Overview. What problems we would have during: Insertion Deletion Update. 13.8.1 . Insertion. What would we do for record insertion?

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Database systemsthe complete book13.8. record modificationByAhmad Yazdankhah(ID 219)CS 257Prof.: T. Y. LinSan José State UniversitySPRING 2011

Overview

What problems we would have during:

• Insertion
• Deletion
• Update
13.8.1. Insertion
• What would we do for record insertion?
• Find a block with enough empty space
• Get a new block
• But what would happen if we should insert the record in a specific block?
13.8.1. Insertion (cont’d)
• There are two possibilities:
• There is enough room in the target block
• There is not enough room in the target block
• In the 1st case, insert and update the offset table
• In the 2nd case, there are two approaches:
• Find space on a nearby block and slide the records
• Create an overflow block
13.8.1. Insertion (cont’d)
• What is an overflow block?
• It is another block in which we put the records that theoretically belong to this block.
• In block’s header, there is a pointer to an overflow.
13.8.2. Deletion
• The usual technique is to put a tombstone in place of the record.
• This is not the case that we delete the record physically.
• This is a permanent tombstone until the database is reconstructed.
13.8.2. Deletion (cont’d)
• Where to put the sign of deletion?
• If using offset table in the block header, put null in the pointer’s place
• If using a map table, put null in physical address
13.8.3. Update
• No problem if fixed length records
• The problems arise for variable length records
• We have all problems mentioned for insertion and deletion
13.8.3. Update (cont’d)
• If the new record is shorter:
• Slide down the records to consolidate the free spaces
• If the new record is longer:
• Slide down the records to make room
• Create an overflow block