Oracle / PLSQL: Indexes

This Oracle tutorial explains how to create, rename and drop indexes in Oracle with syntax and examples.

What is an Index in Oracle?

An index is a performance-tuning method of allowing faster retrieval of records. An index creates an entry for each value that appears in the indexed columns. By default, Oracle creates B-tree indexes.

Create an Index

Syntax

The syntax for creating an index in Oracle/PLSQL is:

CREATE [UNIQUE] INDEX index_name
  ON table_name (column1, column2, ... column_n)
  [ COMPUTE STATISTICS ];

UNIQUE

It indicates that the combination of values in the indexed columns must be unique.

index_name

The name to assign to the index.

table_name

The name of the table in which to create the index.

column1, column2, ... column_n

The columns to use in the index.

COMPUTE STATISTICS

It tells Oracle to collect statistics during the creation of the index. The statistics are then used by the optimizer to choose a "plan of execution" when SQL statements are executed.

Example

Let's look at an example of how to create an index in Oracle/PLSQL.

For example:

CREATE INDEX supplier_idx
  ON supplier (supplier_name);

In this example, we've created an index on the supplier table called supplier_idx. It consists of only one field - the supplier_name field.

We could also create an index with more than one field as in the example below:

CREATE INDEX supplier_idx
  ON supplier (supplier_name, city);

We could also choose to collect statistics upon creation of the index as follows:

CREATE INDEX supplier_idx
  ON supplier (supplier_name, city)
  COMPUTE STATISTICS;

Create a Function-Based Index

In Oracle, you are not restricted to creating indexes on only columns. You can create function-based indexes.

Syntax

The syntax for creating a function-based index in Oracle/PLSQL is:

CREATE [UNIQUE] INDEX index_name
  ON table_name (function1, function2, ... function_n)
  [ COMPUTE STATISTICS ];

UNIQUE

It indicates that the combination of values in the indexed columns must be unique.

index_name

The name to assign to the index.

table_name

The name of the table in which to create the index.

function1, function2, ... function_n

The functions to use in the index.

COMPUTE STATISTICS

It tells Oracle to collect statistics during the creation of the index. The statistics are then used by the optimizer to choose a "plan of execution" when SQL statements are executed.

Example

Let's look at an example of how to create a function-based index in Oracle/PLSQL.

For example:

CREATE INDEX supplier_idx
  ON supplier (UPPER(supplier_name));

In this example, we've created an index based on the uppercase evaluation of the supplier_name field.

However, to be sure that the Oracle optimizer uses this index when executing your SQL statements, be sure that UPPER(supplier_name) does not evaluate to a NULL value. To ensure this, add UPPER(supplier_name) IS NOT NULL to your WHERE clause as follows:

SELECT supplier_id, supplier_name, UPPER(supplier_name)
FROM supplier
WHERE UPPER(supplier_name) IS NOT NULL
ORDER BY UPPER(supplier_name);

Rename an Index

Syntax

The syntax for renaming an index in Oracle/PLSQL is:

ALTER INDEX index_name
  RENAME TO new_index_name;

index_name

The name of the index that you wish to rename.

new_index_name

The new name to assign to the index.

Example

Let's look at an example of how to rename an index in Oracle/PLSQL.

For example:

ALTER INDEX supplier_idx
  RENAME TO supplier_index_name;

In this example, we're renaming the index called supplier_idx to supplier_index_name.

Collect Statistics on an Index

If you forgot to collect statistics on the index when you first created it or you want to update the statistics, you can always use the ALTER INDEX command to collect statistics at a later date.

Syntax

The syntax for collecting statistics on an index in Oracle/PLSQL is:

ALTER INDEX index_name
  REBUILD COMPUTE STATISTICS;

index_name

The index in which to collect statistics.

Example

Let's look at an example of how to collect statistics for an index in Oracle/PLSQL.

For example:

ALTER INDEX supplier_idx
  REBUILD COMPUTE STATISTICS;

In this example, we're collecting statistics for the index called supplier_idx.

Drop an Index

Syntax

The syntax for dropping an index in Oracle/PLSQL is:

DROP INDEX index_name;

index_name

The name of the index to drop.

Example

Let's look at an example of how to drop an index in Oracle/PLSQL.

For example:

DROP INDEX supplier_idx;

In this example, we're dropping an index called supplier_idx.