Tài liệu, ebook Công Nghệ Thông Tin

Design of Parallel Systems (Cont.) ■ On­line reorganization of data and schema changes must be supported. ● For example, index construction on terabyte databases can take hours or days even on a parallel system.  Need to allow other processing (insertions/deletions/updates) to be performed on relation even as index is being constructed. ● Ba...

43 trang | Chia sẻ: vutrong32 | Ngày: 17/10/2018 | Lượt xem: 1189 | Lượt tải: 0

Networks Types (Cont.) ■ WANs with continuous connection (e.g. the Internet) are needed for implementing distributed database systems ■ Groupware applications such as Lotus notes can work on WANs with discontinuous connection: ● Data is replicated. ● Updates are propagated to replicas periodically. ● Copies of data may be updated independent...

37 trang | Chia sẻ: vutrong32 | Ngày: 17/10/2018 | Lượt xem: 1175 | Lượt tải: 0

Web Directories ■ A Web directory is just a classification directory on Web pages ● E.g. Yahoo! Directory, Open Directory project ● Issues:  What should the directory hierarchy be?  Given a document, which nodes of the directory are categories relevant to the document ● Often done manually  Classification of documents into a hierarchy ma...

25 trang | Chia sẻ: vutrong32 | Ngày: 17/10/2018 | Lượt xem: 1188 | Lượt tải: 0

Other Types of Mining ■ Text mining: application of data mining to textual documents ● cluster Web pages to find related pages ● cluster pages a user has visited to organize their visit history ● classify Web pages automatically into a Web directory ■ Data visualization systems help users examine large volumes of data and detect patterns visu...

52 trang | Chia sẻ: vutrong32 | Ngày: 17/10/2018 | Lượt xem: 1097 | Lượt tải: 0

Show Paging (Cont.) ■ Advantages of shadow­paging over log­based schemes ● no overhead of writing log records ● recovery is trivial ■ Disadvantages : ● Copying the entire page table is very expensive  Can be reduced by using a page table structured like a B+­tree – No need to copy entire tree, only need to copy paths in the tree that lead ...

81 trang | Chia sẻ: vutrong32 | Ngày: 17/10/2018 | Lượt xem: 1138 | Lượt tải: 0

SI In Oracle and PostgreSQL ■ Warning: SI used when isolation level is set to serializable, by Oracle and PostgreSQL ● PostgreSQL’s implementation of SI described in Section 26.4.1.3 ● Oracle implements “first updater wins” rule (variant of “first committer wins”)  concurrent writer check is done at time of write, not at commit time  Allows ...

79 trang | Chia sẻ: vutrong32 | Ngày: 17/10/2018 | Lượt xem: 1105 | Lượt tải: 0

Implementation of Isolation ■ Schedules must be conflict or view serializable, and recoverable, for the sake of database consistency, and preferably cascadeless. ■ A policy in which only one transaction can execute at a time generates serial schedules, but provides a poor degree of concurrency. ■ Concurrency­control schemes tradeoff between th...

46 trang | Chia sẻ: vutrong32 | Ngày: 17/10/2018 | Lượt xem: 1281 | Lượt tải: 0

Multiquery Optimization ■ Example Q1: select * from (r natural join t) natural join s Q2: select * from (r natural join u) natural join s ● Both queries share common subexpression (r natural join s) ● May be useful to compute (r natural join s) once and use it in both queries  But this may be more expensive in some situations – e.g. (r natu...

69 trang | Chia sẻ: vutrong32 | Ngày: 17/10/2018 | Lượt xem: 1129 | Lượt tải: 0

Complex Joins ■ Join involving three relations: loan depositor customer ■ Strategy 1. Compute depositor customer; use result to compute loan (depositor customer) ■ Strategy 2. Computer loan depositor first, and then join the result with customer. ■ Strategy 3. Perform the pair of joins at once. Build and index on loan for loan­number, and on...

55 trang | Chia sẻ: vutrong32 | Ngày: 17/10/2018 | Lượt xem: 1136 | Lượt tải: 0

Grid Files (Cont.) ■ During insertion, if a bucket becomes full, new bucket can be created if more than one cell points to it. ● Idea similar to extendable hashing, but on multiple dimensions ● If only one cell points to it, either an overflow bucket must be created or the grid size must be increased ■ Linear scales must be chosen to uniforml...

84 trang | Chia sẻ: vutrong32 | Ngày: 17/10/2018 | Lượt xem: 1123 | Lượt tải: 0