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Instructions l i and lj of transactions Ti and Tj respectively, conflict if and only if there exists some item Q accessed by both li and lj, and at least one of these instructions wrote Q. 1. l i = read(Q), lj = read(Q). li and lj don’t conflict. 2. l i = read(Q), lj = write(Q). They conflict. 3. l i = write(Q), lj = read(Q). They conflict...

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Some query optimizers integrate heuristic selection and the generation of alternative access plans. ● Frequently used approach  heuristic rewriting of nested block structure and aggregation  followed by cost­based join­order optimization for each block ● Some optimizers (e.g. SQL Server) apply transformations to entire query and do not depe...

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Worst case estimate: b r ∗ bs + br block transfers + 2 * br seeks ● Each block in the inner relation s is read once for each block in the outer relation (instead of once for each tuple in the outer relation ■ Best case: b r + bs block transfers + 2 seeks. ■ Improvements to nested loop and block nested loop algorithms: ● In block nested­loop,...

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Since the inter­node connections are done by pointers, “logically” close blocks need not be “physically” close. ■ The non­leaf levels of the B+­tree form a hierarchy of sparse indices. ■ The B+­tree contains a relatively small number of levels  Level below root has at least 2*  n/2 values  Next level has at least 2*  n/2 *  n/2 values...

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Level 1 provides much better write performance than level 5 ● Level 5 requires at least 2 block reads and 2 block writes to write a single block, whereas Level 1 only requires 2 block writes ● Level 1 preferred for high update environments such as log disks ■ Level 1 had higher storage cost than level 5 ● disk drive capacities increasing rapid...

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Distinction between subelement and attribute ● In the context of documents, attributes are part of markup, while subelement contents are part of the basic document contents ● In the context of data representation, the difference is unclear and may be confusing  Same information can be represented in two ways –

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To find all books that have the word “database” as a keyword, select title from books where ‘database’ in (unnest(keyword­set )) ■ We can access individual elements of an array by using indices ● E.g.: If we know that a particular book has three authors, we could write: select author­array[1], author­array[2], author­array[3] from books whe...

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A Web server can easily serve as a front end to a variety of information services. ■ The document name in a URL may identify an executable program, that, when run, generates a HTML document. ● When a HTTP server receives a request for such a document, it executes the program, and sends back the HTML document that is generated. ● The Web clie...

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Atomicity is actually a property of how the elements of the domain are used. ● Example: Strings would normally be considered indivisible ● Suppose that students are given roll numbers which are strings of the form CS0012 or EE1127 ● If the first two characters are extracted to find the department, the domain of roll numbers is not atomic. ● ...

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The combination of primary keys of the participating entity sets forms a super key of a relationship set. ● (customer_id, account_number) is the super key of depositor ● NOTE: this means a pair of entity sets can have at most one relationship in a particular relationship set.  Example: if we wish to track all access_dates to each account by ...

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