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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 ...

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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 ...

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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...

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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...

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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...

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Modifying Large Objects ■ If the application requires insert/delete of bytes from specified regions of an object: ● B+­tree file organization (described later in Chapter 12) can be modified to represent large objects ● Each leaf page of the tree stores between half and 1 page worth of data from the object ■ Special­purpose application program...

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XML Application: Web Services ■ The Simple Object Access Protocol (SOAP) standard: ● Invocation of procedures across applications with distinct databases ● XML used to represent procedure input and output ■ A Web service is a site providing a collection of SOAP procedures ● Described using the Web Services Description Language (WSDL) ● Direct...

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Comparison of O­O and O­R Databases ■ Relational systems ● simple data types, powerful query languages, high protection. ■ Persistent­programming­language­based OODBs ● complex data types, integration with programming language, high performance. ■ Object­relational systems ● complex data types, powerful query languages, high protection. ■ No...

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Digital Certificates ■ Digital certificates are used to verify authenticity of public keys. ■ Problem: when you communicate with a web site, how do you know if you are talking with the genuine web site or an imposter? ● Solution: use the public key of the web site ● Problem: how to verify if the public key itself is genuine? ■ Solution: ● Ev...

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