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Three Phase Commit (3PC) ■ Assumptions: ● No network partitioning ● At any point, at least one site must be up. ● At most K sites (participants as well as coordinator) can fail ■ Phase 1: Obtaining Preliminary Decision: Identical to 2PC Phase 1. ● Every site is ready to commit if instructed to do so ■ Phase 2 of 2PC is split into 2 phases, P...
93 trang | Chia sẻ: vutrong32 | Ngày: 17/10/2018 | Lượt xem: 1058 | Lượt tải: 0
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...
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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...
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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...
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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: 1004 | 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: 1033 | 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: 995 | 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...
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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: 1023 | 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: 1044 | Lượt tải: 0