Slideshow:
Terminology: Sequential file
Terminology: Ordered index (file)
Usage: use binary search in (smaller) index file to find search key
Then follow the database address (pointer) to find the record
Terminology: Primary index (file)
Terminology: Index sequential file
Terminology: Secondary index (file)
Terminology: Dense index
Example of a Dense index
Terminology: Sparse index
Example: Sparse index
Example on using
a Sparse index
How do we find the data record using the sparse index ?
Example on using
a Sparse index
Look up search key ≤ 40 in sparse index file
Example on using
a Sparse index
Follow database address to data block (or next level index !!)
Example on using
a Sparse index
Search key in data block
-
Sequential file:
- Sequential file = a file whose records are sorted by some attribute(s) (usually its primary key)
Example: a sequential file
-
Sort key:
- Sort key = field(s) whose values are used to sort/order the records in a sequential file
-
Ordered index:
- Ordered index = an index file where the index entries are sorted (in the order of the search key)
Example: an ordered index
-
Primary index
- Primary index = an ordered index whose search key is also the sort key used for the sequential file
Example: a primary index
-
Secondary index
- Secondary index = an ordered index whose search key is NOT the sort key used for the sequential file
Example: a secondary index
-
Index sequential file
- Index sequential file = a sequential file that has a primary index
Example: an index sequential file
-
Dense index
- Dense index =
the index file contains an
index entry:
+-----------+-----------------------+ | Search key | block/record pointer | +-----------+-----------------------+for every value of the search key in the data file
Example: a dense index
Note:
- A
secondary index
is always
a dense index:
- Dense index =
the index file contains an
index entry:
-
Sparse index
- Sparse index =
the index file's
search entries
+-----------+-----------------------+ | Search key | block/record pointer | +-----------+-----------------------+does not contain every value of the search key of the data file
-
Requirement for
a sparse index:
- The search key of a
(sparse) index
must be
the sort key of
the data file !!!
I.e.: only a primary index can be a sparse index !!!
More on the sparse index next....
- The search key of a
(sparse) index
must be
the sort key of
the data file !!!
- Sparse index =
the index file's
search entries
- A sparse index contains:
- one index entry for each (data) block
Organization of a sparse index:
-
How to use a sparse index:
Example:
- Look up the record with search key = 40
Procedure:
- Find the
largest search key
that is ≤ 40:
We found key = 30
- Search in the
data block for
search key 40:
- Comments:
- Sparse indices are
uncommon
Because:
- Only a
primary index
can be
a sparse index
This requires that the file is sorted on the search key !!!
- Only a
primary index
can be
a sparse index
- Most commonly used
indexes is:
secondary index
Very flexible:
- The file does not need to be sorted
- Sparse indices are
uncommon