File Organizations and Indices

1
File Organizations and Indices
2
Unordered (Heap) Files

• Simplest file structure contains records in no
  particular order default in many systems.
• 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.

3
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 the search
  fields of r.
• Clustered Files Tuples from gt 1 relation stored
  together to speed up joins (later.)

4
Index Motivation

• 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 gt 3
• Indexes are structured files that enable us to
  answer such value-based queries efficiently.

5
Index Definitions

• An index on a file speeds up selections on the
  search key fields for the index.
• Any subset of the fields of a relation can be the
  search key for an index on the relation.
• Search key is not the same as key (set of fields
  that uniquely identify a record in a relation).
• An index contains a collection of data entries,
  and supports efficient retrieval of all data
  entries k with a given search key value k.

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Alternatives for Data Entry k in Index

• Three alternatives
• Data record with key value k
• ltk, rid of data record with search key value kgt
• ltk, list of rids of data records with search key
  kgt
• Choice of alternative for data entries is
  orthogonal to the indexing technique used to
  locate data entries with a given key value k.
• Examples of indexing techniques B trees,
  hash-based structures
• Typically, index contains auxiliary information
  that directs searches to the desired data entries

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Index Classification

• Primary vs. secondary If search key contains
  primary key, then called primary index.
• Unique index Search key contains a candidate
  key.
• Clustered vs. unclustered If order of data
  records is the same as, or close to, order of
  data entries, then called clustered index.
• A relation can be clustered on at most one search
  key.
• Cost of retrieving data records through index
  varies greatly based on whether index is
  clustered or not!

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Clustered vs. Unclustered Index

• Suppose that the data records are stored in a
  Heap file.
• To build clustered index, first sort the Heap
  file (with some free space on each page for
  future inserts).
• Overflow pages may be needed for inserts. (Thus,
  order of data recs is close to, but not
  identical to, the sort order.)

Index entries
UNCLUSTERED
CLUSTERED
direct search for
data entries
Data entries
Data entries
(Index File)
(Data file)
Data Records
Data Records
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Index Classification (Contd.)

• Dense vs. Sparse If there is at least one data
  entry per search key value (in some data
  record), then dense.
• Every sparse index is clustered!
• Sparse indexes are smaller however, some useful
  optimizations are based on dense indexes.

Ashby, 25, 3000
22
Basu, 33, 4003
25
Bristow, 30, 2007
30
Ashby
33
Cass, 50, 5004
Cass
Smith
Daniels, 22, 6003
40
Jones, 40, 6003
44
44
Smith, 44, 3000
50
Tracy, 44, 5004
Sparse Index
Dense Index
on
on
Data File
Name
Age
10
Summary

• Many alternative file organizations exist, each
  appropriate in some situation.
• Index is a collection of data entries plus a way
  to quickly find entries with given search key
  values.
• Can have several indexes on a given file of data
  records, each with a different search key.
• Indexes can be classified as clustered vs.
  unclustered, primary vs. secondary, and dense vs.
  sparse. Differences have important consequences
  for utility/performance.