record modifications n.
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Record Modifications

Record Modifications

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Record Modifications

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  1. Record Modifications Chapter 13 Section 13.8 Neha Samant CS 257 (Section II) Id 222

  2. Modification type:Insertion 2 • Insertion of records without order Records can be placed in a block with empty space or in a new block. Insertion of records in fixed order • Space available in the block • No space available in the block (outside the block) Structured address Pointer to a record from outside the block.

  3. Insertion in fixed order Offset table header unused Record 4 Record 3 Record 2 Record 1 3 Space available within the block • Use of an offset table in the header of each block with pointers to the location of each record in the block. • The records are slid within the block and the pointers in the offset table are adjusted.

  4. Insertion in fixed order Block B Overflow block for B No space available within the block (outside the block) • Find space on a “nearby” block. • In case of no space available on a block, look at the following block in sorted order of blocks. • If space is available in that block ,move the highest records of first block 1 to block 2 and slide the records around on both blocks. • Create an overflow block • Records can be stored in overflow block. • Each block has place for a pointer to an overflow block in its header. • The overflow block can point to a second overflow block as shown below. 4

  5. Modification type: Deletion 5 • Recover space after deletion • When using an offset table, the records can be slid around the block so there will be an unused region in the center that can be recovered. • In case we cannot slide records, an available space list can be maintained in the block header. • The list head goes in the block header and available regions hold the links in the list.

  6. Deletion • Use of tombstone • The tombstone is placed in a record in order to avoid pointers to the deleted record to point to new records. • The tombstone is permanent until the entire database is reconstructed. • If pointers go to fixed locations from which the location of the record is found then we put the tombstone in that fixed location. (See examples) • Where a tombstone is placed depends on the nature of the record pointers. • Map table is used to translate logical record address to physical address. 6

  7. Record 1 Record 2 Deletion • Use of tombstone • If we need to replace records by tombstones, place the bit that serves as the tombstone at the beginning of the record. • This bit remains the record location and subsequent bytes can be reused for another record Record 1 can be replaced, but the tombstone remains, record 2 has no tombstone and can be seen when we follow a pointer to it. 7

  8. Modification type: Update 8 • Fixed Length update No effect on storage system as it occupies same space as before update. • Variable length update • Longer length • Short length

  9. Update 9 Variable length update (longer length) • Stored on the same block: • Sliding records • Creation of overflow block. • Stored on another block • Move records around that block • Create a new block for storing variable length fields.

  10. Update 10 Variable length update (Shorter length) • Same as deletion • Recover space • Consolidate space.