Download 15.6 Index Based Algorithms

15.6 Index Based Algorithms
Priyank Shah
CS 257
Contents
• Clustering and non-clustering indexes
• Index based Selection
• Joining using an index
• Joining using a sorted index
• A database index is data structure that
improves the speed of data retrieval
operations on a database table at the cost of
slower writes and increased storage space.
• Non clustered index - contains the index keys
in sorted order, with the leaf level of the index
containing the pointer to the record
• A bitmap index is a special kind of index that
stores the bulk of its data as bit arrays
• Algorithms are useful for the selection
operator.
• In a clustered relation tuples are packed
roughly as few blocks, as they can possibly
hold those tuples.
Clustering And Nonclustering Indexes
• Clustering indexes are on an attribute or
attributes such that all the tuples with a fixed
value for the search key of this index appear
on roughly as few blocks as can hold them.
• A relation that isn’t clustered cannot have a
clustering index
Index-based Selection
• For a selection σC(R), suppose C is of the form
a=v, where a is an attribute
• For clustering index R.a:
the number of disk I/O’s will be B(R)/V(R,a)
Index-based Selection
• The actual number may be higher:
1. index is not kept entirely in main
memory
2. they spread over more blocks
3. may not be packed as tightly as possible
into blocks
Example
• B(R)=1000, T(R)=20,000 number of I/O’s
required:
• 1. clustered, not index
1000
• 2. not clustered, not index
20,000
• 3. If V(R,a)=100, index is clustering
10
• 4. If V(R,a)=10, index is nonclustering 2,000
Joining by using an index
• Natural join R(X, Y) S S(Y, Z)
Number of I/O’s to get R
Clustered: B(R)
Not clustered: T(R)
Number of I/O’s to get tuple t of S
Clustered: T(R)B(S)/V(S,Y)
Not clustered: T(R)T(S)/V(S,Y)
Example
• R(X,Y): 1000 blocks S(Y,Z)=500 blocks
Assume 10 tuples in each block,
so T(R)=10,000 and T(S)=5000
V(S,Y)=100
If R is clustered, and there is a clustering index on
Y for S
the number of I/O’s for R is:
1000
the number of I/O’s for S
is10,000*500/100=50,000
Joining Using a Sorted index
• Natural join R(X, Y) S (Y, Z) with index on Y
for either R or S
• Example:
relation R(X,Y) and R(Y,Z) with index on Y for
both relations
search keys (Y-value) for R: 1,3,4,4,5,6
search keys (Y-value) for S: 2,2,4,6,7,8
Joining using a sorted index
• Used when the index is a B-tree, or structure
from which we easily can extract the tuples of
a relation in sorted order.