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Illustration

Illustration. All of databases can take advantage of indexing to increase their speed. An index allows a set of table rows matching some criterion to be quickly located.

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Illustration

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  1. Illustration • All of databases can take advantage of indexing to increase their speed. • An index allows a set of table rows matching some criterion to be quickly located. • Most relational DBMSs have the advantage that indexes can be created or dropped without changing existing applications making use of them. • Indexes act transparently to the application or end-user querying the database; while they affect performance, any SQL command will run with or without indexes to compute the result of an SQL statement.

  2. Illustration • All of databases can take advantage of indexing to increase their speed. • An index allows a set of table rows matching some criterion to be quickly located. • Most relational DBMSs have the advantage that indexes can be created or dropped without changing existing applications making use of them. • Indexes act transparently to the application or end-user querying the database; while they affect performance, any SQL command will run with or without indexes to compute the result of an SQL statement.

  3. Illustration • Selection Operation: The selection, or restriction, operation retrieves tuples from a relation, limiting the results to only those that meet a specific criteria, i.e. a subset in terms of set theory. The SQL equivalent of selection is the SELECT query statement with a WHERE clause. • The projection operation is essentially a selection operation in which duplicate tuples are removed from the result. The SQL GROUP BY clause, or the DISTINCT keyword implemented by some SQL dialects, can be used to remove duplicates from a result set. • The join operation defined for relational databases is often referred to as a natural join. In this type of join, two relations are connected by their common attributes. SQL's approximation of a natural join is the INNER JOIN join operator.

  4. illustration • Databases may store relational tables/indexes in memory or on hard disk in one of many forms: • ordered/unordered flat files • ISAM (Indexed Sequential Access Method) • hash buckets • B+ trees

  5. ISAM • In an ISAM system, data is organized into records which are composed of fixed length fields. • Records are stored sequentially, originally to speed access on a tape system. • Indexes contain "pointers" into the tables, allowing individual records to be retrieved without having to search the entire data set.

  6. Hash Tables • a hash table or hash map is a data structure that uses a hash function to efficiently map certain identifiers or keys (e.g., person names) to associated values (e.g., their telephone numbers). • The hash function is used to transform the key into the index (the hash) of an array element (the slot or bucket) where the corresponding value is to be sought.

  7. B+ Trees • A B+ tree (BplusTree) is a type of tree which represents sorted data in a way that allows for efficient insertion, retrieval and removal of records, each of which is identified by a key. • In a B+ tree, in contrast to a B-tree, all records are stored at the leaf level of the tree; only keys are stored in interior nodes.

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