Slideshow:
- Sample of an
index
(found in the back of
a text book):
- Index:
- Index = a data structure that enable the user to find (= locate) data items efficiently (quickly) using search keys
- Consider
the following query on
relation
stored on
disk:
- Relation R is
stored on
disk:
- Query:
select * from employee where lname = 'Smith';
- Relation R is
stored on
disk:
- Without
further information,
the only way to
process the
above query is:
- Read
all blocks of
the relation employee
- Look for tuples with lname = 'Smith'
- Read
all blocks of
the relation employee
- More efficient processing:
- Maintain
location information
on
specific attributes
(most useful ones, e.g.,
key attributes)
in the relation
Graphically:
We only need to read a setset of the data block
- Maintain
location information
on
specific attributes
(most useful ones, e.g.,
key attributes)
in the relation
- Search key:
- Search key = field(s) used to create the additional search information used to speed up a look up operation
- Index file:
- Index file =
a file that store
records of the
following format:
+------------+----------------------+ | Search key | block/record pointer | +------------+----------------------+Graphically:
Note:
- The size of an index file is usually much smaller than the size of a data file
- Index file =
a file that store
records of the
following format:
- Note:
- The pointer is
usually a
block pointer
- So the index allow you to locate the block that contain the record quickly
- The record is
found by
a search operation
inside the block
(after the block is
read into
main memory)
- Because the search operation only access main memory, the search is relatively quick (because you don't use disk access operations in the search)
- The pointer is
usually a
block pointer