Creating a Database in Software

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Creating a Database
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In the sample database, these columns are good candidates for additional indexes: The COMPANY column in the CUSTOMERS table should be indexed if customer data is often retrieved by company name. The NAME column in the SALESREPS table should be indexed if data about salespeople is often retrieved by salesperson name. The REP column in the ORDERS table should be indexed if orders are frequently retrieved based on the salesperson who took them. The CUST column in the ORDERS table should similarly be indexed if orders are frequently retrieved based on the customer who placed them. The MFR and PRODUCT columns, together, in the ORDERS table should be indexed if orders are frequently retrieved based on the product ordered. The SQL standard doesn t mention indexes or how to create them. It treats database indexes as an implementation detail, which is outside of the core, standardized SQL. However, the use of indexes is essential to achieve adequate performance in any sizeable enterprise-class database. In practice, most popular DBMS brands (including Oracle, Microsoft SQL Server, MySQL, Informix, Sybase, and DB2) support indexes through some form of the CREATE INDEX statement, shown in Figure 13-7. The statement assigns a name to the index and specifies the table for which the index is created. The statement also specifies the column(s) to be indexed and whether they should be indexed in ascending or descending order. The DB2 version of the CREATE INDEX statement, shown in Figure 13-7, is the most straightforward. Its only option is the keyword UNIQUE, which is used to specify that the combination of columns being indexed must contain a unique value for every row of the table. The following is an example of a CREATE INDEX statement that builds an index for the ORDERS table based on the MFR and PRODUCT columns and that requires combinations of columns to have a unique value: Create a unique index for the OFFICES table.
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CREATE UNIQUE INDEX OFC_MGR_IDX ON OFFICES (MGR);
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CREATE UNIQUE
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INDEX index-name ON table-name
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column-name ASC DESC ,
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FIGURE 13-7
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Basic CREATE INDEX statement syntax diagram
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Database Structure
Create an index for the ORDERS table.
CREATE INDEX ORD_PROD_IDX ON ORDERS (MFR, PRODUCT);
In most major DBMS products, the CREATE INDEX statement includes additional DBMS-specific clauses that specify the disk location for the index and for performancetuning parameters. Typical performance parameters include the size of the index pages, the percentage of free space that the index should allow for new rows, the type of index to be created, whether it should be clustered (an arrangement that places the table s physical data rows on the disk medium in the same sequence as the index), and so on. These options make the CREATE INDEX statement quite DBMS-specific in actual use. Some DBMS products support two or more different types of indexes, which are optimized for different types of database access. For example, a B-tree index uses a tree structure of index entries and index blocks (groups of index entries) to organize the data values that it contains into ascending or descending order. This type of index, which is the default type in nearly all DBMS products, provides efficient searching for a single value or for a range of values, such as the search required for an inequality comparison operator or a range test (BETWEEN) operation. A different type of index, a hash index, uses a randomizing technique to place all of the possible data values into a moderate number of buckets within the index. For example, if there are 10 million possible data values, an index with 500 hash buckets might be appropriate. Since a given data value is always placed into the same bucket, the DBMS can search for that value simply by locating the appropriate bucket and searching within it. With 500 buckets, the number of items to be searched is reduced, on average, by a factor of 500. This makes hash indexes very fast when searching for an exact match of a data value. But the assignment of values to buckets does not preserve the order of data values, so a hash index cannot be used for inequality or range searches. Other types of indexes are appropriate for other specific DBMS situations, including T-tree index A variation of the B-tree index that is optimized for in-memory databases Bitmap index Useful when there is a relatively small number of possible data values Index-organized table A relatively new option that stores the entire table in the index. This is useful for tables that have few columns other than the primary key, such as code lookup tables that typically have only a code (such as a department code) and a description (such as a department name). When a DBMS supports multiple index types, the CREATE INDEX statement not only defines and creates the index, but also defines its type. If you create an index for a table and later decide that it is not needed, the DROP INDEX statement removes the index from the database. The statement removes the index created in the previous example: Drop the index created earlier.
DROP INDEX ORD_PROD_IDX;
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