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2. 1. Database address space 2. Virtual address space 3. Map table 4. Translation table 5. Swizzling and UnSwizzling 6. Pinned Blocks 2

3.  A database system is usually client-server architecture  The client applications use a conventional virtual address space  Server’s data is structured as database address space 3

4.  Physical Address: each block and record has a physical address as:  Logical Address: each block can have a shorter address (4 bytes) as logical address.  A map table can translate logical-to-physical 4 Host Disk ID Cylind. Num. Track #Block #Offset

5. 5 LogicalPhysical Logical address Physical address

6.  Why we use map table?  Answer: flexibility in data manipulation  How?  All pointers to the records refer to the map table  When moving records around or deleting, we can update the map table. 6

7.  How to combine physical and logical addresses?  With the physical address, reach the block and using the key value to find the record  Keep an offset table in the block header 7 Host Disk ID Cylind. Num. Track #Block # Key Value Offset Table Unused Record 4 Record 3 Record 2 Record 1

8.  To move a record in a block, just change the offset table entry  To delete a record, just leave a tombstone in the offset table  To move to another block, just put the new block address in the offset table (forwarding address) 8

9.  There are some cases that records contain pointers  When records represent objects (and not tuples)  Object-Relational databases  Index blocks (next chapter) 9

10.  Here are some facts:  We know that all data manipulations are done in main memory.  To refer a data item on HD, we use database address space.  To refer a data item in memory, we use memory address. 10

11. Question: what would happen to the pointers when we move the block from the secondary storage to memory? 11

12.  So, we need a translation table to map this correspondence. 12 Database Address Memory Address Database Address Memory address

13.  The techniques that change the pointer addresses is known as Pointer Swizzling.  The idea is that we should change the pointers content in order to point to the right locations. 13

14.  When we move a block to memory, we put an entry for each pointer it has into the translation table. 14

15.  In this technique, we swizzle the pointers when we need them.  In this way we save the time because some records won’t be invoked at all. 15

16.  In this technique, we just create the translation table for the blocks and don’t swizzle the pointers inside the blocks.  It means that the pointers always use database address space. 16

17.  Question: what would happen to the pointers when we move the block from memory to the secondary storage?  The pointers should be changed to database address space. This is called Unswizzling!  The translation table can be used in both direction. 17

18.  If the block in the memory cannot be written back to the disk safely, we call it Pinned.  Example: Block B1 has a swizzled pointer to a record in Block B2. What would happen if we move the B2?  In this situation, we set the pin bit of B2 to prevent moving of B2. 18

19.  One possible approach is to keep track of all references to a block in a linked list. 19

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