Transcript Storage : Records and Files

Records and Files
Storage Technology: Topic 3
Introduction to Database Systems
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Record Formats: Fixed Length
F1
F2
F3
F4
L1
L2
L3
L4
Base address (B)
Address = B+L1+L2
Information about field types stored in
system catalogs.
 Direct access to i’th field.

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Record Formats: Variable Length

Two alternative formats (# fields is fixed):
F1
4
Field
Count
F2
$
F3
$
F4
$
$
Fields Delimited by Special Symbols
F1
F2
F3
F4
Array of Field Offsets
 Second offers direct access to i’th field, efficient storage
of nulls (special don’t know value); small directory overhead.
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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 (specified using record id)
– scan all records (possibly with some conditions on
the records to be retrieved)
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Page Formats: Fixed Length Records
Slot 1
Slot 2
Slot 1
Slot 2
Free
Space
...
Slot N
...
Slot N
Slot M
N
PACKED
1 . . . 0 1 1M
number
of records
M ... 3 2 1
UNPACKED, BITMAP
number
of slots
 Record
id = <page id, slot #>. In first
alternative, moving records for free space
management changes rid; may not be acceptable.
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Page Formats: Variable Length Records
Rid = (i,N)
Page i
Rid = (i,2)
Rid = (i,1)
20
N
...
16
2
SLOT DIRECTORY
24
1
Pointer
to start
of free
space
 Can
move records on page without changing rid;
so, attractive for fixed-length records too.
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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
Pages with
Free Space
Header
Page
Data
Page

Data
Page
Data
Page
Full Pages
Each page contains 2 `pointers’ plus data.
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Heap File Using a Page Directory
Data
Page 1
Header
Page
Data
Page 2
DIRECTORY
Data
Page N
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.

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Indexes

A Heap file allows us to retrieve records:
– by specifying the rid, or
– by scanning all records sequentially

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 students with a gpa > 3

Indexes enable us to answer value-based
(associative) queries efficiently.
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Alternative File Organizations
Many alternatives exist, each ideal for some
situation , and not so good in others:
– Heap files: Suitable when typical access is a file
scan retrieving all records.
– Sorted Files: Best if records must be retrieved in
some order, or only a `range’ of records is needed.
– Hashed Files: Good for equality selections.
 File is a collection of buckets. Bucket = primary
page plus zero or more overflow pages.
 Hashing function h: h(r) = bucket in which
record r belongs. h looks at only some of the
fields of r, called the search fields.
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Cost Model for Analysis
Ignore CPU costs, for simplicity:
–
–
–
–
B: The number of data pages
R: Number of records per page
D: (Average) time to read or write disk page
Measuring number of page I/O’s ignores gains of
sequential I/O; thus, even I/O cost is only
approximated.
– Average-case analysis; based on several simplistic
assumptions.
 Good enough to show the overall trends!
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Cost of Operations
Scan all recs
Heap
File
BD
Sorted
File
BD
Hashed
File
1.25 BD
Equality Search
0.5 BD
D log2B
D
Range Search
BD
Insert
2D
D (log2B + # of 1.25 BD
pages with
matches)
Search + BD
2D
Delete
Search + D Search + BD
2D
 Several assumptions underlie these (rough) estimates!
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Assumptions
Single record insert and delete.
 Heap Files:

– Equality selection on key; exactly one match.
– Insert always at end of file.

Sorted Files:
– Files compacted after deletions.
– Selections on sort field(s).

Hashed Files:
– No overflow buckets, 80% page occupancy.
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Summary
Variable length record format with field offset
directory offers support for direct access to
i’th field and null values.
 Slotted page format supports variable length
records and allows records to move on page.
 File layer keeps track of pages in a file, and
supports abstraction of a collection of records.

– Linked list or directory data structure
– Sorted and hashed files for query processing
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