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Representing Data Elements. Fields, Records, Blocks Variable-length Data Modifying Records. Source: our textbook. Overview. Attributes are represented by sequences of bytes, called fields Tuples are represented by collections of fields, called records

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Representing data elements l.jpg

Representing Data Elements

Fields, Records, Blocks

Variable-length Data

Modifying Records

Source: our textbook


Overview l.jpg
Overview

  • Attributes are represented by sequences of bytes, called fields

  • Tuples are represented by collections of fields, called records

  • Relations are represented by collections of records, called files

  • Files are stored in blocks, using specialized data structures to support efficient modification and querying


Representing sql data types l.jpg
Representing SQL Data Types

  • integers and reals: built-in

  • CHAR(n): array of n bytes

  • VARCHAR(n): array of n+1 bytes (extra byte is either string length or null char)

  • dates and times: fixed length strings

  • etc.


Representing tuples l.jpg

30

286

287

297

0

address

VARCHAR(255)

256 bytes

birthdate

DATE

10 bytes

name

CHAR(30)

30 bytes

gender

CHAR(1)

1 byte

Representing Tuples

  • For now, assume all attributes (fields) are fixed length.

  • Concatenate the fields

  • Store the offset of each field in schema


More on tuples l.jpg

32

288

292

304

0

address

VARCHAR(255)

256 bytes

birthdate

DATE

10 bytes

+ 2

name

CHAR(30)

30 bytes

+ 2

gender

CHAR(1)

1 byte

+ 3

More on Tuples

  • Due to hardware considerations, certain types of data need to start at addresses that are multiples of 4 or 8

  • Previous example becomes:


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Record Headers

  • Often it is convenient to keep some "header" information in each record:

    • a pointer to schema information (attributes/fields, types, their order in the tuple, constraints)

    • length of the record/tuple

    • timestamp of last modification


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Packing Records into Blocks

  • Start with block header:

    • timestamp of last modification/access

    • offset of each record in the block, etc.

  • Follow with sequence of records

  • May end with some unused space

header

block 1

block 2

block n-1

block n


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Representing Addresses

  • Often addresses (pointers) are part of records:

    • the application data in object-oriented databases

    • as part of indexes and other data structures supporting the DBMS

  • Every data item (block, record, etc.) has two addresses:

    • database address: address on the disk

      (typically 8-16 bytes)

    • memory address, if the item is in virtual memory (typically 4 bytes)


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Translation Table

  • Provides mapping from database addresses to memory addresses for all blocks currently in memory

  • Later we'll discuss how to implement it


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Pointer Swizzling

  • When a block is moved from disk into main memory, change all the disk addresses that point to items in this block into main memory addresses.

  • Need a bit for each address to indicate if it is a disk address or a memory address.

  • Why? Faster to follow memory pointers (only uses a single machine instruction).


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Example of Swizzling

Disk

Main Memory

read into

main memory

Block 1

Block 2


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Swizzling Policies

  • Automatic swizzling: as soon as block is brought into memory, swizzle all relevant pointers

  • Swizzling on demand: only swizzle a pointer if and when it is actually followed

  • No swizzling

  • Programmer control


Automatic swizzling l.jpg
Automatic Swizzling

  • Locating all pointers within a block:

    • refer to the schema, which will indicate where addresses are in the records

    • for index structures, pointers are at known locations

  • Update translation table with memory addresses of items in the block

  • Update pointers in the block (in memory) with memory addresses, when possible, as obtained from translation table


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Unswizzling

  • When a block is moved from memory back to disk, all pointers must go back to database (disk) addresses

  • Use translation table again

  • Important to have an efficient data structure for the translation table


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Pinned Records and Blocks

  • A block in memory is pinned if it cannot be safely written back to disk

  • Indicate with a bit in the block header

  • Reasons for pinning:

    • related to failure recovery (more later)

    • because of pointer swizzling

  • If block B1 has swizzled pointer to an item in block B2, then B2 is pinned.


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Unpinning a Block

  • Consider each item in the block to be unpinned

  • Keep in the translation table the places in memory holding swizzled pointers to that item (e.g., with a linked list)

  • Unswizzle those pointers (i.e., use translation table to replace the memory addresses with database (disk) addresses


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Variable Length Data

  • Data items with varying size (e.g., if maximum size of a field is large but most of the time the values are small)

  • Variable-format records (e.g., NULLs method for representing a hierarchy of entity sets as relations)

  • Records that do not fit in a block (e.g., an MPEG of a movie)


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Variable-Length Fields

  • Store the fixed-length fields before the variable-length fields in each record

  • Keep in the record header

    • record length

    • pointers to the beginnings of all the variable-length fields

  • Book discusses variations on this idea


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Variable-Format Records

  • Represent by a sequence of tagged fields

  • Each tagged field contains

    • name

    • type

    • length, if not deducible from the type

    • value


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Splitting Records Across Blocks

  • Called spanned records

  • Useful when

    • record size exceeds block size

    • putting an integral number of records in a block wastes a lot of the block (e.g., record size is 51% of block size)

  • Each record or fragment header contains

    • bit indicating if it is a fragment

    • if fragment then pointers to previous and next fragments of the record (i.e., a linked list)


Record modification l.jpg
Record Modification

  • Modifications to records:

    • insert

    • delete

    • update

  • issues even with fixed-length records and fields

  • even more involved with variable-length data


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Inserting New Records

  • If records need not be any particular order, then just find a block with enough empty space

  • Later we'll see how to keep track of all the tuples of a given relation

  • But what if blocks should be kept in a certain order, such as sorted on primary key?


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Insertion in Order

If there is space in the block, then add the record

(going right to left), add a pointer to it (going left

to right) and rearrange the pointers as needed.


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What if Block is Full?

  • Records are stored in several blocks, in sorted order

  • One approach: keep a linked list of "overflow" blocks for each block in the main sequence

  • Another approach is described in the book


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Deleting Records

  • Try to reclaim space made available after a record is deleted

  • If using an offset table, then rearrange the records to fill in any hole that is left behind and adjust the pointers

  • Additional mechanisms are based on keeping a linked list of available space and compacting when possible


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Tombstones

  • What about pointers to deleted records?

  • We place a tombstone in place of each deleted record

  • Tombstone is permanent

  • Issue of where to place the tombstone

  • Keep a tombstone bit in each record header: if this is a tombstone, then no need to store additional data


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Updating Records

  • For fixed-length records, there is no effect on the storage system

  • For variable-length records:

    • if length increases, like insertion

    • if length decreases, like deletion except tombstones are not necessary