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CHAPTER 7 DATA DEFINITION, PART II
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the corresponding rows in the table. To further optimize access, indexes are internally organized in a tree structure. (See Oracle Concepts for more details on physical index structures.) If there were such an index on employee names, the optimizer could decide to abandon the full table scan approach and perform an index search instead. The index offers a very efficient access path to all names, returning all row identifiers of employees with a name starting with a Q. This probably would result in a huge performance improvement, because there are only a few database blocks to be visited to produce the query result. For some of your other queries, indexes on department numbers or birth dates could be useful. You can create as many indexes per table as you like. In summary, the performance of your SQL statements can often be improved significantly by creating indexes. Sometimes, it is obvious that an index will help, such as when your tables contain a lot of rows and your queries are very selective (only retrieving a few rows). On the other hand, though, you may find that your application benefits from an index on a singlerow, single-column table. Indexes may speed up queries, but the other side of the index picture is the maintenance overhead. Every additional index slows down data manipulation further, because every INSERT/UPDATE/DELETE statement against a table must immediately be processed against all corresponding indexes to keep the indexes synchronized with the table. Also, indexes occupy additional space in your database. This means that you should carefully consider which columns should be indexed and which ones should not be indexed. These are some suggestions for index candidates: Foreign key columns Columns often used in WHERE clauses Columns often used in ORDER BY and GROUP BY clauses Here, we ll look at the commands for index creation and management.
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Figure 7-11 shows the (simplified) syntax of the CREATE INDEX command.
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Figure 7-11. CREATE INDEX command syntax diagram
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CHAPTER 7 DATA DEFINITION, PART II
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The storage clause allows you to influence various physical index storage attributes, such as the storage location and the space allocation behavior. See Oracle SQL Reference for more details. If the table rows happen to be inserted and stored in index order, you can specify the NOSORT option to speed up index creation. The Oracle DBMS will skip the sort phase (normally needed during index creation), but if the rows turn out to be in the wrong order, the CREATE INDEX command will fail with an error message.
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Unique indexes serve two purposes: they provide additional access paths to improve response times (like nonunique indexes), and they also prevent duplicate values. You create unique indexes by specifying the UNIQUE option of the CREATE INDEX command (see Figure 7-11). Note, however, that it is recommended to ensure uniqueness in your tables using the PRIMARY KEY and UNIQUE constraints, leaving it up to the Oracle DBMS to choose an appropriate physical implementation of those constraints.
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Regular indexes work the best if the corresponding columns contain many different values, resulting in better selectivity. Unique indexes offer the best selectivity, because they contain only different values. This means that every equality search (... WHERE COL = ...) results in at most one row. At the other side of the spectrum, if a column contains only a few values (typical examples are gender, status, and yes/no columns), a regular index is not very useful, because the average selectivity of equality searches will be poor. For such low-cardinality columns, the Oracle DBMS supports bitmap indexes. Bitmap indexes also outperform regular indexes if your WHERE clause is complicated, using many AND, OR, and NOT connectives. You create bitmap indexes by specifying the BITMAP option (see Figure 7-11).
Caution Indexes slow down data manipulation, and bitmap indexes are the most expensive index type
in terms of maintenance. Don t create bitmap indexes on tables with a lot of DML activity.
Function-Based Indexes
As Figure 7-11 shows, you can specify an expression between the parentheses when defining the table columns to be indexed. That means that instead of simply specifying a single column or a comma-separated list of columns, you can choose to specify a more complicated expression in an index definition. Indexes containing such expressions are referred to as function-based indexes. See Listing 7-14 for an example, where we create an index on an expression for the yearly salary.
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