1

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


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

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

Logical address
Logical Physical
Physical address
5


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

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


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

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


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

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

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


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

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


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


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


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


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

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