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Production databases are often organized like the copy of the sample database shown in Figure 13-5, with all of their major tables collected together and owned by an application ID or database administrator. The database administrator gives other users permission to access the tables, using the SQL security scheme described in 15. Recall, however, that you must use qualified table names to refer to another user s tables. In practice,
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Database George s tables Sam s tables OFFICES DBA s tables PRODUCTS
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FIGURE 13-5
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Typical organization of a production database
this means that every query against the major tables in Figure 13-5 must use qualified table names, which makes queries like the following one long and tedious to type: List the name, sales, of ce, and of ce sales for everyone.
SELECT NAME, OP_ADMIN.SALESREPS.SALES, OFFICE, OP_ADMIN.OFFICES.SALES FROM OP_ADMIN.SALESREPS, OP_ADMIN.OFFICES;
To address this problem, many SQL DBMS products provide an alias or synonym capability. A synonym is a name that you define that stands for the name of some other table. In DB2, you create an alias using the CREATE ALIAS statement. (Older versions of DB2 actually used a CREATE SYNONYM statement, and Oracle and SQL Server still use this form of the statement, but it has the same effect as the CREATE ALIAS statement.) If you were the user named George in Figure 13-5, for example, you might use this pair of CREATE ALIAS statements: Create aliases for two tables owned by another user.
CREATE ALIAS REPS FOR OP_ADMIN.SALESREPS; CREATE ALIAS OFFICES FOR OP_ADMIN.OFFICES;
Once you have defined a synonym or alias, you can use it just like a table name in SQL queries. The previous query thus becomes
SELECT NAME, REPS.SALES, OFFICE, OFFICES.SALES FROM REPS, OFFICES;
The use of aliases doesn t change the meaning of the query, and you must still have permission to access the other users tables. Nonetheless, synonyms simplify the SQL statements you use and make it appear as if the tables were your own. If you decide later that you no longer want to use the synonyms, they can be removed with the DROP ALIAS statement:
13:
Creating a Database
Drop the aliases created earlier.
DROP ALIAS REPS; DROP ALIAS OFFICES;
Synonyms or aliases are supported by DB2, Oracle, SQL Server, and Informix. However, they are not specified by the ANSI/ISO SQL standard.
Indexes (CREATE/DROP INDEX)
One of the physical storage structures provided by most SQL-based database management systems is an index, which is a structure that provides rapid access to the rows of a table based on the values of one or more columns. Figure 13-6 shows the PRODUCTS table and two indexes that have been created for it. One of the indexes provides access based on the
PRODUCTS Table MFR_ID IMM ACI ACI PRODUCT_ID DESCRIPTION PRICE QTY_ON_HAND
779c 41003 41004
900-1b Brace $1,875.00 Size 3 Widget $107.00 Size 4 Widget $117.00
9 207 139
PART IV
INDEX
INDEX
ACI 41003 ACI 41004
900-1b Brace
Size 3 Widget IMM 779C Size 4 Widget
FIGURE 13-6
Two indexes on the PRODUCTS table
Part IV:
Database Structure
DESCRIPTION column. The other provides access based on the primary key of the table, which is a combination of the MFR_ID and PRODUCT_ID columns. The DBMS uses the index as you might use the index of a book. The index stores data values and pointers to the rows where those data values occur. In the index the data values are arranged in ascending or descending order, so that the DBMS can quickly search the index to find a particular value. It can then follow the pointer to locate the row containing the value. The presence or absence of an index is completely transparent to the SQL user who accesses a table. For example, consider this SELECT statement: Find the quantity and price for size 4 widgets.
SELECT QTY_ON_HAND, PRICE FROM PRODUCTS WHERE DESCRIPTION = 'Size 4 Widget';
The statement doesn t say whether there is an index on the DESCRIPTION column, and the DBMS will carry out the query in either case. If there were no index for the DESCRIPTION column, the DBMS would be forced to process the query by sequentially scanning the PRODUCTS table, row by row, examining the DESCRIPTION column in each row. To make sure it had found all of the rows that satisfied the search condition, it would have to examine every row in the table. For a large table with millions of rows, the scan of the table could take minutes or hours. With an index for the DESCRIPTION column, the DBMS can locate the requested data with much less effort. It searches the index to find the requested value ( Size 4 Widget ) and then follows the pointer to find the requested row(s) of the table. The index search is very rapid because the index is sorted and its rows are very small. Moving from the index to the row(s) is also very rapid because the index tells the DBMS where on the disk the row(s) are located. As this example shows, the advantage of having an index is that it greatly speeds the execution of SQL statements with search conditions that refer to the indexed column(s). One disadvantage of having an index is that it consumes additional disk space. Another disadvantage is that the index must be updated every time a row is added to the table and every time the indexed column is updated in an existing row. This imposes additional overhead on INSERT and UPDATE statements for the table. In general, it s a good idea to create an index for columns that are used frequently in search conditions. In addition, an index on foreign key columns can often enhance the performance of joins. Indexing is also more appropriate when queries against a table are more frequent than inserts and updates. Most DBMS products always establish an index for the primary key of a table, because they anticipate that access to the table will most frequently be via the primary key. The primary key index also helps the DBMS quickly check for duplicate values as new rows are inserted into the table. Most DBMS products also automatically establish an index for any column (or column combination) defined with a uniqueness constraint. As with the primary key, the DBMS must check the value of such a column in any new row to be inserted, or in any update to an existing row, to make certain that the value does not duplicate a value already contained in the table. Without an index on the column(s), the DBMS would have to sequentially search through every row of the table to check the constraint. With an index, the DBMS can simply use the index to find a row (if it exists) with the value in question, which is a much faster operation than a sequential search.
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