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Unformatted text preview: Overview of Query Evaluation 121 2. An access path is a way of retrieving tuples from a table and consists of either a file scan or an index plus a matching selection condition. An index matches a selection condition if the index can be used to retrieve just the tuples that satisfy the condition. An index can match some subset of conjunts in a selection condition even though it does not match the entire condition and we refer to the conjuct that the index matches as the primary conjuncts in the selection. Primary conjuncts are important because they allow us to quickly discard information we do not need and only focus in on searching/sorting the data that more closely matches the selection conditions. 3. Information about relations, indexes, and views is stored in the system catalogs. This includes file names, file sizes, and file structure, the attribute names and data types, lists of keys, and constraints. Some commonly stored statistical information includes: (a) Cardinality - the number of tuples for each relation (b) Size - the number of pages in each relation (c) Index Cardinality - the number of distinct key values for each index (d) Index Size - the number of pages for each index (or number of leaf pages) (e) Index Height - the number of nonleaf levels for each tree index (f) Index Range - the minimum present key value and the maximum present key value for each index....
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This note was uploaded on 01/17/2012 for the course EGN 4302 taught by Professor Dr.vishak during the Fall '12 term at University of Central Florida.
- Fall '12