external data structures1 file organization and indexes on databases

External data structures 1

External data structures

File organization and indexes on databases


Primary vs. Secondary storage

• Primary storage– Main memory (RAM)– Volatile– Small capacity– Fast– You read/write a single

byte

• Secondary storage– Hard disk– Non-volatile– Large capacity– Slow– You read/write a

record (several bytes)


Databases

• External data structures are used by Database Management Systems (DBMS) to store data on the hard disk

• Requirements– Add new data (fast)– Find the data (fast)– Update the data (fast)


File organizationsHow to organize database records inside a file on

the hard disk• Heap (unordered) file

– Records are placed on disk in no particular order

• Sequential (ordered) file– Records are ordered by the value of a

specified field

• Hash files– Records are placed on disk according to a

hash function


Heap files

• Records placed in the file in insertion order.

• Access methods– Insert: Fast– Find: Slow (linear search)– Delete: Mark record as deleted


Ordered files

• Records in the file are sorted by 1 (or more) fields forming the key of the data set

• We can use binary search among records and inside records– If file is ordered by id

• select ... where id = 14– No binary search on non-ordered fields

• select ... where name = ’Anders’

• Insert / delete: Slow– We have to keep the sequence.– Solution: Overflow file with new elements

• Finding elements is a little slower


Hash files

• Hash function calculates the address on the record

• Hash field– Usually the key

• Collision– Problem

• Different records hashes to same address

– Solution• Buckets can hold multiple records


Indexes

• Index in a book– Auxiliary structure

• Not really part of the book– Used when searching for an item in the book

• To avoid linear search

• Index in a database– Auxiliary structure

• Not really part of the database tables– Used when searching for a record in the database

• To avoid linear search– Data file

• Contains records (the data)– Index file

• Contains the index, ordered according to some field


Types of indexes

• Primary index– Ordering in the data file using a key field

• Clustering index– Ordering in the data file using a non-key field

• Called the clustering attribute

– At most 1 primary or clustering index

• Secondary index– Auxiliary data structure– Fast finding on non-prime/-clustering attributes


Pros and cons of secondary indexes

• Benefits– Fast search (select)

• Liabilities– Slow updates (insert, update, delete)

• The data + the index must be updated


Multilevel indexes

• Problem– Large indexes => slow search in the index

• Solution– Indexes of more levels

• Index on the index!


References

• Connolly & Begg Database Systems, 4th edition, Addison Wesley 2004– Appendix C File

organizations and indexes

external data structures1 file organization and indexes on databases

Documents