You can create indexes explicitly (outside of integrity constraints) using the SQL statement CREATE INDEX. The following statement creates an index named emp_ename for the ename column of the emp table:
CREATE INDEX emp_ename ON emp(ename) TABLESPACE users STORAGE (INITIAL 20K NEXT 20k PCTINCREASE 75);
Notice that several storage settings and a tablespace are explicitly specified for the index. If you do not specify storage options (such as INITIAL and NEXT) for an index, the default storage options of the default or specified tablespace are automatically used.
Indexes can be unique or non-unique. Unique indexes guarantee that no two rows of a table have duplicate values in the key column (or columns). Non-unique indexes do not impose this restriction on the column values.
Use the CREATE UNIQUE INDEX statement to create a unique index. The following example creates a unique index:
CREATE UNIQUE INDEX dept_unique_index ON dept (dname) TABLESPACE indx;
Alternatively, you can define UNIQUE integrity constraints on the desired columns. The database enforces UNIQUE integrity constraints by automatically defining a unique index on the unique key. This is discussed in the following section. However, it is advisable that any index that exists for query performance, including unique indexes, be created explicitly.
Oracle Database enforces a UNIQUE key or PRIMARY KEY integrity constraint on a table by creating a unique index on the unique key or primary key. This index is automatically created by the database when the constraint is enabled. No action is required by you when you issue the CREATE TABLE or ALTER TABLE statement to create the index, but you can optionally specify a USING INDEX clause to exercise control over its creation. This includes both when a constraint is defined and enabled, and when a defined but disabled constraint is enabled.
To enable a UNIQUE or PRIMARY KEY constraint, thus creating an associated index, the owner of the table must have a quota for the tablespace intended to contain the index, or the UNLIMITED TABLESPACE system privilege. The index associated with a constraint always takes the name of the constraint, unless you optionally specify otherwise.
You can set the storage options for the indexes associated with UNIQUE and PRIMARY KEY constraints using the USING INDEX clause. The following CREATE TABLE statement enables a PRIMARY KEY constraint and specifies the storage options of the associated index:
CREATE TABLE emp ( empno NUMBER(5) PRIMARY KEY, age INTEGER) ENABLE PRIMARY KEY USING INDEX TABLESPACE users;
Specifying the Index Associated with a Constraint
If you require more explicit control over the indexes associated with UNIQUE and PRIMARY KEY constraints, the database lets you:
These options are specified using the USING INDEX clause. The following statements present some examples.
CREATE TABLE a ( a1 INT PRIMARY KEY USING INDEX (create index ai on a (a1)));
CREATE TABLE b( b1 INT, b2 INT, CONSTRAINT bu1 UNIQUE (b1, b2) USING INDEX (create unique index bi on b(b1, b2)), CONSTRAINT bu2 UNIQUE (b2, b1) USING INDEX bi);
CREATE TABLE c(c1 INT, c2 INT); CREATE INDEX ci ON c (c1, c2); ALTER TABLE c ADD CONSTRAINT cpk PRIMARY KEY (c1) USING INDEX ci;
If a single statement creates an index with one constraint and also uses that index for another constraint, the system will attempt to rearrange the clauses to create the index before reusing it.
Oracle Database provides you with the opportunity to collect statistics at very little resource cost during the creation or rebuilding of an index. These statistics are stored in the data dictionary for ongoing use by the optimizer in choosing a plan for the execution of SQL statements. The following statement computes index, table, and column statistics while building index emp_ename on column ename of table emp:
CREATE INDEX emp_ename ON emp(ename) COMPUTE STATISTICS;
When creating an extremely large index, consider allocating a larger temporary tablespace for the index creation using the following procedure:
Using this procedure can avoid the problem of expanding your usual, and usually shared, temporary tablespace to an unreasonably large size that might affect future performance.
You can create and rebuild indexes online. This enables you to update base tables at the same time you are building or rebuilding indexes on that table. You can perform DML operations while the index build is taking place, but DDL operations are not allowed. Parallel execution is not supported when creating or rebuilding an index online.
The following statements illustrate online index build operations:
CREATE INDEX emp_name ON emp (mgr, emp1, emp2, emp3) ONLINE;
Note: Keep in mind that the time that it takes on online index build to complete is proportional to the size of the table and the number of concurrently executing DML statements. Therefore, it is best to start online index builds when DML activity is low.
Function-based indexes facilitate queries that qualify a value returned by a function or expression. The value of the function or expression is precomputed and stored in the index.
In addition to the prerequisites for creating a conventional index, if the index is based on user-defined functions, then those functions must be marked DETERMINISTIC. Also, you just have the EXECUTE object privilege on any user-defined function(s) used in the function-based index if those functions are owned by another user.
Additionally, to use a function-based index:
CREATE INDEX stores the timestamp of the most recent function used in the function-based index. This timestamp is updated when the index is validated. When performing tablespace point-in-time recovery of a function-based index, if the timestamp on the most recent function used in the index is newer than the timestamp stored in the index, then the index is marked invalid. You must use the ANALYZE INDEX...VALIDATE STRUCTURE statement to validate this index.
To illustrate a function-based index, consider the following statement that defines a function-based index (area_index) defined on the function area(geo):
CREATE INDEX area_index ON rivers (area(geo));
In the following SQL statement, when area(geo) is referenced in the WHERE clause, the optimizer considers using the index area_index.
SELECT id, geo, area(geo), desc FROM rivers WHERE Area(geo) > 5000;
Table owners should have EXECUTE privileges on the functions used in function-based indexes.
Because a function-based index depends upon any function it is using, it can be invalidated when a function changes. If the function is valid, you can use an ALTER INDEX...ENABLE statement to enable a function-based index that has been disabled. The ALTER INDEX...DISABLE statement lets you disable the use of a function-based index. Consider doing this if you are working on the body of the function.
Note: An alternative to creating a function-based index is to add a virtual column to the target table and index the virtual column.
Creating an index using key compression enables you to eliminate repeated occurrences of key column prefix values.
Key compression breaks an index key into a prefix and a suffix entry. Compression is achieved by sharing the prefix entries among all the suffix entries in an index block. This sharing can lead to huge savings in space, allowing you to store more keys for each index block while improving performance.
Key compression can be useful in the following situations:
You enable key compression using the COMPRESS clause. The prefix length (as the number of key columns) can also be specified to identify how the key columns are broken into a prefix and suffix entry. For example, the following statement compresses duplicate occurrences of a key in the index leaf block:
CREATE INDEX emp_ename ON emp(ename) TABLESPACE users COMPRESS 1;
The COMPRESS clause can also be specified during rebuild. For example, during rebuild you can disable compression as follows:
ALTER INDEX emp_ename REBUILD NOCOMPRESS;
Beginning with Release 11g, you can create invisible indexes. An invisible index is an index that is ignored by the optimizer unless you explicitly set the OPTIMIZER_USE_INVISIBLE_INDEXES initialization parameter to TRUE at the session or system level. Making an index invisible is an alternative to making it unusable or dropping it. Using invisible indexes, you can do the following:
Unlike unusable indexes, an invisible index is maintained during DML statements.
To create an invisible index, use the SQL statement CREATE INDEX with the INVISIBLE clause. The following statement creates an invisible index named emp_ename for the ename column of the emp table:
CREATE INDEX emp_ename ON emp(ename) TABLESPACE users STORAGE (INITIAL 20K NEXT 20k PCTINCREASE 75) INVISIBLE;