Download presentation

Presentation is loading. Please wait.

Published byEstefania Rasor Modified over 2 years ago

1
15.8 Algorithms using more than two passes Presented By: Seungbeom Ma (ID 125) Professor: Dr. T. Y. Lin Computer Science Department San Jose State University

2
Multipass Algorithms Previously, most of algorithms are required two passes. There is a case that we need more than two passes. Case : Data is too big to store in main memory. We have to hash or sort the relation with multipass algorithms.

3
Agenda 1. Multipass Sort-Based Algorithm 2. Multipass Hash-Based Algorithm

4
Multipass sort-based algorithm. M: Number of Memory Buffers R: Relation B(R) : Number of blocks for holding relation. BASIS: 1. If R fits in M block (B (R) <= M). 2. Reading R into main memory. 3. Sorting R in the main memory with any sorting algorithm. 4. Write the sorted relation to disk.

5
Multipass sort-based algorithm. INDUCTION: (B(R)> M) 1. If R does not fit into main memory then partitioning the blocks hold R into M groups, which call R 1, R 2, …, R M 2.Recursively sorting R i from i =1 to M 3.Once sorting is done, the algorithm merges the M sorted sub- lists.

7
Performance: Multipass Sort-Based Algorithms 1) Each pass of a sorting algorithm: 1.Reading data from the disk. 2. Sorting data with any sorting algorithms 3. Writing data back to the disk. 2-1) (k)-pass sorting algorithm needs 2k B(R) disk I/O’s 2-2)To calculate (Multipass)-pass sorting algorithm needs = > A+ B A: 2(K-1 ) (B(R) + B(S) ) [ disk I/O operation to sort the sublists] B: B(R) + B(S)[ disk I/O operation to read the sorted the sublists in the final pass] Total: (2k-1)(B(R)+B(S)) disk I/O’s

8
Multipass Hash-Based Algorithms 1. Hashing the relations into M-1 buckets, where M is number of memory buffers. 2. Unary case: It applies the operation to each bucket individually. 1.Duplicate elimination ( δ ) and grouping ( γ ). 1) Grouping: Min, Max, Count, Sum, AVG, which can group the data in the table 2) Duplicate elimination: Distinct Basis: If the relation fits in M memory block, -> Reading relation into memory and perform the operations. 3. Binary case: It applies the operation to each corresponding pair of buckets. Query operations: union, intersection, difference, and join If either relations fits in M-1 memory blocks, -> Reading that relation into main memory M-1 blocks -> Reading next relation to 1 block at a time into the M th block Then performing the operations.

9
INDUCTION If Unary and Binary relation does not fit into the main memory buffers. 1.Hashing each relation into M-1 buckets. 2.Recursively performing the operation on each bucket or corresponding pair of buffers. 3.Accumulating the output from each buckets or pair.

10
Hash-Based Algorithms : Unary Operatiors

11
Perfermance: Hash-Based Algorithms R: Realtion. Operations are like δ and γ M: Buffers U(M, k): Number of blocks in largest relation with k-pass hashing algorithm.

12
Performance: Induction Induction: 1. Assuming that the first step divides relation R into M-1 equal buckets. 2. The buckets for the next pass must be small enough to handle in k-1 passes 3.Since R is divided into M-1 buckets, we need to have (M-1)u(M, k-1).

13
Sort-Based VS Hash-Based 1. Sort-based can produce output in sorted order. It might be helpful to reduce rotational latency or seek time 2. Hash-based depends on buckets being of equal size. For binary operations, hash-based only limits size of smaller relation. Therefore, hash-based can be faster than sort-based for small size of relation.

14
THANKS

Similar presentations

OK

1 Query Processing Two-Pass Algorithms Source: our textbook.

1 Query Processing Two-Pass Algorithms Source: our textbook.

© 2018 SlidePlayer.com Inc.

All rights reserved.

Ads by Google

Ppt on plasma arc welding Ppt on information technology management Ppt on machine translation service Ppt on water our lifeline phone Ppt on land resources and development Ppt on marketing plan Ppt on input devices and output devices Ppt on web services testing Ppt online downloader for games Ppt on blind faith