Indexing1.pptxnnnnñnnnnnnnnnnnnnnnnnnnnnn

Views
 A view in SQL terminology is a single table that is
derived from other tables
 These other tables can be base tables or
previously defined views.
 A view does not necessarily exist in physical
form; it is considered to be a virtual table, in
contrast to base tables, whose tuples are always
physically stored in the database
Slide 16- 2

Example
Slide 16- 3

 A view is supposed to be always up-to-date; if we
modify the tuples in the base tables on which the
view is defined, the view must automatically
reflect these changes.
Slide 16- 4

 V1A: DROP VIEW WORKS_ON1;
Slide 16- 5

Indexing Structures for Files and
Physical Database Design

Indexing
 Indexing refers to a data structure technique that
is used for quickly retrieving entries from
database files using some attributes that have
been indexed.
 In database systems, indexing is comparable to
indexing in books.
Slide 16- 7

Introduction
 Indexes used to speed up record retrieval in
response to certain search conditions
 Index structures provide secondary access paths
 Any field can be used to create an index
 Multiple indexes can be constructed
 Most indexes based on ordered files
Slide 17- 9

Primary Indexes
Slide 17- 12

Primary Indexes
 Ordered file with two fields
 Primary key, K(i)
 Pointer to a disk block, P(i)
 One index entry in the index file for each block in
the data file
 Indexes may be dense or sparse
 Dense index has an index entry for every search
key value in the data file
 Sparse index has entries for only some search
values
Slide 17- 13

Primary Indexes (cont’d.)
Slide 17-14
Figure 17.1 Primary index on the ordering key field of the file shown in Figure 16.7

Dense Index
 Every search key value in the data file has an index record in the
dense index. It speeds up the search process. The total number of
records present in the index table and the main table are the same in
this case.
 It requires extra space to hold the index record. A pointer to the actual
record on the disk and the search key are both included in the index
records.
Slide 16- 16

Sparse Index
 Only a few items in the data file have index records. Each and every
item points to a certain block. Rather than pointing to each item in the
main database, the index, in this case, points to the records that are
present in the main table that is in a gap.
Slide 16- 17

Clustering Indexes
 Clustering field
 File records are physically ordered on a nonkey
field without a distinct value for each record
 Ordered file with two fields
 Same type as clustering field
 Disk block pointer
Slide 17- 18

Clustering Indexes (cont’d.)
Slide 17-19
Figure 17.2 A clustering index on the Dept_number ordering
nonkey field of an EMPLOYEE file

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