Database System Concepts
Database System Concepts
7th Edition
ISBN: 9780078022159
Author: Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher: McGraw-Hill Education
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We are learning the Select Algorithm for the algorithm of order statistics and the images are our notes. if you could explain the algorithm a bit more that would be good, particularly the part I circled in red. Why are the numbers in that order? It doesn't seem to me like they match up to the correct numbers. What does each Select(#, S(#)) point to? Thanks.

Example of SELECT (k, S) algorithm
S = < 1, 10, 8, 5, 6, 3, 5, 4, 2, 4, 7, 5, 8, 8, 9, 24, 6, 16, 20, 11, 4, 3, 14, 19, 2 >
Divide into subsequences of 5 and sort each:
1
2
5
6
2
5
3
7
11
3
6
4
8
16
4
M, the sequence of medians
8
4
8
10
10
5
9
220
14
24
19
Find the median m of M, and rearrange the columns:
2
2
1
5
6
3
3
5
7
11
4
4
6
8
16
m, the median of M
4
14
8
8
5
19
10
9
220
20
24
S1 = < 1, 5, 3, 5, 4, 2, 4, 5, 4, 3, 2>
S2 = < 6,6>
S3 = <10, 8, 7, 8, 8, 9, 24, 16, 20, 11, 14, 19 >
SELECT (5, S)
SELECT (5, S1)
SELECT (12, S)
SELECT (13, S)
SELECT (17, S)
SELECT (20, S)
answer is 6
answer is 6
SELECT (4, S3)
SELECT (7, S3)
etc.
|S1|= 11
| S2 |
= 2
|s3| = 12
expand button
Transcribed Image Text:Example of SELECT (k, S) algorithm S = < 1, 10, 8, 5, 6, 3, 5, 4, 2, 4, 7, 5, 8, 8, 9, 24, 6, 16, 20, 11, 4, 3, 14, 19, 2 > Divide into subsequences of 5 and sort each: 1 2 5 6 2 5 3 7 11 3 6 4 8 16 4 M, the sequence of medians 8 4 8 10 10 5 9 220 14 24 19 Find the median m of M, and rearrange the columns: 2 2 1 5 6 3 3 5 7 11 4 4 6 8 16 m, the median of M 4 14 8 8 5 19 10 9 220 20 24 S1 = < 1, 5, 3, 5, 4, 2, 4, 5, 4, 3, 2> S2 = < 6,6> S3 = <10, 8, 7, 8, 8, 9, 24, 16, 20, 11, 14, 19 > SELECT (5, S) SELECT (5, S1) SELECT (12, S) SELECT (13, S) SELECT (17, S) SELECT (20, S) answer is 6 answer is 6 SELECT (4, S3) SELECT (7, S3) etc. |S1|= 11 | S2 | = 2 |s3| = 12
2
2
IA
IɅ
N≤3 ≤ +
4
IA
3
N≤M≤ <
4
IɅ
VI
ΙΛ
15516)
IɅ
5
7
8
00
6
11
16
elements known to be less than
or equal to m
4
14
8
8
20
VI
5
19
10
9
24
16
elements known to be greater than
or equal to m
20
2
2
1
5
6
3
3
5
7
11
4
4
6
VI
8
VI
IA
ΙΛ
4
14
8
8
ΙΛ
IɅ
5
19
10
9
6≤2≤2
24
Those elements in the green box are known to be less than or equal to m, the number of such
elements is at least (3/5) x (1/2) x n = (3/10)n. Contrapositively, the number of elements greater
than m is at most n - (3/10) n = (7/10) n.
Therefore, the recursive call at Line 12, SELECT (k - |S1| - |S2, S3) takes time at most T(7n/10).
Similarly, the elements in the blue box are guaranteed to be greater than or equal to m, and the
number of such elements is at least (3/10)n. So the number of elements less than m is at most
(7/10)n, and the recursive call at Line 10, SELECT (k, S1) also takes time at most T(7n/10).
Note that depend on the choice for the length of the subsequences (in the algorithm discussed in
class and the example above, the subsequences are of length 5) and n (number of elements in S),
the two boxes may not be of equal size, in which case the recursive calls at Line 12 and Line 10
(observe that only one of the two may be executed) will have different complexities, and the
larger one will dominate the expression for T(n).
expand button
Transcribed Image Text:2 2 IA IɅ N≤3 ≤ + 4 IA 3 N≤M≤ < 4 IɅ VI ΙΛ 15516) IɅ 5 7 8 00 6 11 16 elements known to be less than or equal to m 4 14 8 8 20 VI 5 19 10 9 24 16 elements known to be greater than or equal to m 20 2 2 1 5 6 3 3 5 7 11 4 4 6 VI 8 VI IA ΙΛ 4 14 8 8 ΙΛ IɅ 5 19 10 9 6≤2≤2 24 Those elements in the green box are known to be less than or equal to m, the number of such elements is at least (3/5) x (1/2) x n = (3/10)n. Contrapositively, the number of elements greater than m is at most n - (3/10) n = (7/10) n. Therefore, the recursive call at Line 12, SELECT (k - |S1| - |S2, S3) takes time at most T(7n/10). Similarly, the elements in the blue box are guaranteed to be greater than or equal to m, and the number of such elements is at least (3/10)n. So the number of elements less than m is at most (7/10)n, and the recursive call at Line 10, SELECT (k, S1) also takes time at most T(7n/10). Note that depend on the choice for the length of the subsequences (in the algorithm discussed in class and the example above, the subsequences are of length 5) and n (number of elements in S), the two boxes may not be of equal size, in which case the recursive calls at Line 12 and Line 10 (observe that only one of the two may be executed) will have different complexities, and the larger one will dominate the expression for T(n).
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