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Locating blocks/records on disks and in memory using its database address

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• Previously: use of database address
  • Fact:

    Database address (physical and logical) are similar to pointers to a block/record

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• Usages of database addresses
  • Usages of database addresses:

    Database addresses are part of a record and is used to reference another record: Index files need to use database addresses to locate the records:

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• Problem of block/record addressing
  • Recall:

    A database object (block/record) is identified by: the database address of the database object (= the logical/physical address) when the database object is on disk the virtual memory address of the database object in memory must be used when (a copy of) the database object resides in memory

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  • Therefore:

    The DBMS needs a management system (software) to perform the following conversion: Database address <-----> Virtual memory address

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• Observations
  • Fact:

    When a database object (block/record) is only stored on disk, we can only refer to (and access) the database object using its database address When a database object (block/record) is read into main memory: we must use the database object in memory for manipulation purposes (access using its virtual memory address BUT: we also need its database address when we want to write the database object back to disk

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• A dilemma....
  • Consider the following record that references another record using the database address xyz:

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  • If the block that contains the referenced record (xyz) is not in memory:

    We must use the database address xyz to find it

    Graphically:

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  • BUT, if the block that contains the referenced record (xyz) is stored in memory

    We must not use the database address xyz Instead, we must (find and) use its virtual address !!!

    Graphically:

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  • Questions:

    How can we distinguish these 2 cases ??? How can we find the virtual address if the record/block is in memory ???

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• A simple technique to locate database objects
  • Fact:

    The 100% sure way to identify a database object is using: the database address of the database object (Because every database object has a database address; only some database objects has a virtual memory address)

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  • Translation Table:

    Translation table is used to translate: Database Address ----> Virtual Memory Address Given a database address: If database address is found in translation table: Then use the corresponding virtual memory address to access database object in memory If database address is not found in translation table: You must first read the database object into memory (The read operation must update the translation table with the virtual memory address of the database object in memory) Translation Table is organized as a Hash Table because: Relatively few database objects will be present in memory

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  • Example 1: access a record that is (already) in memory:

    The following scenario depicts the situation when we access a record that is in memory: We are currently processing a record in Block 1: The record contains a database address to the record xyz The database address is found in the translation table: We will use the copy in memory (given by the virtual address in the translation table)

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  • Example 2: access a record that is not presently in memory:

    The following scenario depicts the situation when we access a record that is not in memory: We are currently processing a record in Block 1: The record contains a database address to the record xyz The database address is not found in the translation table: This will trigger a read operation to bring the database object into memory (BTW: we know the database address of the database object)