RecordPage Formats

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Record/Page Formats

• Esezer_at_ata.cs.hun.edu.tr

2
Record Formats

• Organization of records whether field length of
  record
• Fixed
• Variable
• Not Type and number of fields are identical for
  all
  toples

3
Fixed Length Records
F1
F2
F3
F4
L1
L2
L3
L4
Base address (B)
Address BL1L2

• All fields can be placed continous
• Finding ith field address requires adding length
  of previous fields to base address.

4
Variable Length Records

• Two alternative formats ( fields is fixed)

F1 F2 F3
F4
Fields Delimited by Special Symbols
Field Count
F1 F2 F3 F4
Array of Field Offsets

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Variable Length Records(Cont.)

• In first
• All previous fields must be scanned to acces
  desired records
• In Second
• Second offers direct access to i.th field
• Points begin and end of the field
• Efficient storage for nulls
• Small directory overhead

6
Disadvantage of Variable Length

• If field is growth to larger size,
• Subsequent fields must be shifted
• Offsets must be updated
• If changing of field length requires passing to
  another page,
• memory adres of page is changed
• Refernces to old address must be updated
• If changing of field length requires locating on
  more pages
• Chaining must be set up for record

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Page Formats Fixed Length Records
Slot 1
Slot 1
Slot 2
Slot 2
Free Space
. . .
. . .
Slot N
Slot N
Slot M
N
M
1
0
. . .
1
1
M ... 3 2 1
number of records
number of slots
PACKED
UNPACKED, BITMAP

• In first alternative, moving records for free
  space management changes memory address of record
  may not be acceptable.

8
Page Formats Variable Length Records
Rid (i,N)
Page i
Rid (i,2)
Rid (i,1)
N
Pointer to start of free space
20
16
24
N . . . 2 1
slots
SLOT DIRECTORY

• Can move records on page without changing memory
  address of records so, attractive for
  fixed-length records too.

9
Page Formats Var. Leng. Recs.

• Keep a directory for slots that show ltrecord
  offset, record lengthgt
• Keep a pointer to point free space
• For placement a record
• If it is possible, insert in free space
• Reorganize page to combine wasted space then
  insert
• Insert another page
• For deleting a record
• Put 1 to record offset information in directory

10
Files of Records

• Page or block is OK when doing I/O, but higher
  levels of DBMS operate on records, and files of
  records.
• FILE A collection of pages, each containing a
  collection of records. Must support
• insert/delete/modify record
• read a particular record
• scan all records (possibly with some conditions
  on the records to be retrieved)

11
Unordered (Heap) Files

• Simplest file structure contains records in no
  particular order.
• As file grows and shrinks, disk pages are
  allocated and de-allocated.
• To support record level operations, we must
• keep track of the pages in a file
• keep track of free space on pages
• keep track of the records on a page
• There are many alternatives for keeping track of
  this.

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Heap File Implemented as a List
Data Page
Data Page
Data Page
Full Pages
Header Page
Data Page
Data Page
Data Page
Pages with Free Space

• The header page id and Heap file name must be
  stored someplace on disk.
• Each page contains two pointers plus data.

13
Heap File Using a Page Directory

• The entry for a page can include the number of
  free bytes on the page.
• The directory is a collection of pages linked
  list implementation is just one alternative

14
Indexes

• Sometimes, we want to retrieve records by
  specifying the values in one or more fields,
  e.g.,
• Find all students in the CS department
• Find all employees with a age gt 30
• Indexes are file structures that enable us to
  answer such value-based queries efficiently.

15
System Catalogs

• For each index
• structure (e.g., B tree) and search key fields
• For each relation
• name, file name, file structure (e.g., Heap file)
• attribute name and type, for each attribute
• index name, for each index
• integrity constraints
• For each view
• view name and definition
• Plus statistics, authorization, buffer pool size,
  etc.

• Catalogs are themselves stored as tables!

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Attr_Cat(attr_name, rel_name, type, position)