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13.8 Record Modifications

13.8 Record Modifications. CS257 Lok Kei Leong ( 108 ). Outline. Record Insertion Record Deletion Record Update. Insertion. Insert new records into a relation - records of a relation in no particular order - record of a relation in fixed order (e.g. sorted by primary key)

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13.8 Record Modifications

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  1. 13.8 Record Modifications CS257 Lok Kei Leong ( 108 )

  2. Outline • Record Insertion • Record Deletion • Record Update

  3. Insertion • Insert new records into a relation- records of a relation in no particular order- record of a relation in fixed order (e.g. sorted by primary key) • A pointer to a record from outside the block is a “structured address” Offeset table header unused Record 2 Record 3 Record 4 Record 1

  4. What If The Block is Full? • If we need to insert the record in a particular block but the block is full. What should we do? • Find room outside the Block • There are 2 solutions • Find Space on Nearby Block • Create an Overflow Block

  5. Insertion (solution 1) • Find space on a “nearby” block • Block B1 has no space • If space available on block B2 move records of B1 to B2 • If there are external pointers to records of B1 moved to B2 leave forwarding address in offset table of B1

  6. Insertion (solution 2) • Create an overflow block • Each block B has its header pointer to an overflow block where additional blocks of B can be placed Block B Overflow block for B

  7. Deletion • Slide around the block • Cannot slide records- maintain an available-space list in the block headerto keep track of space available • Avoid dangle or wind up pointing to a new record

  8. Tombstone • What about pointer to deleted records ? • A tombstone is placed in place of each deleted record • A tombstone is a bit placed at first byte of deleted record to indicate the record was deleted ( 0 – Not Deleted 1 – Deleted) • A tombstone is permanent Record 1 Record 2

  9. Update • For Fixed-Length Records, there is no effect on the storage system • For variable length records: • associated with insertion and deletion(never create a tombstone for old record) • Longer updated recordcreate more space on its block- sliding records - create an overflow block

  10. Question?

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