What are indexes in a database, how to create, alter, drop and view indexes?

1. Index

2. What is Index
 Indexes are optional structures associated with
tables and clusters that allow SQL statements to
execute more quickly against a table. Just as the
index in this manual helps you locate information
faster than if there were no index, an Oracle
Database index provides a faster access path to table
data. You can use indexes without rewriting any
queries. Your results are the same, but you see them
more quickly.
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

3.  Indexes are logically and physically independent of
the data in the associated table. Being independent
structures, they require storage space. You can create
or drop an index without affecting the base tables,
database applications, or other indexes. The
database automatically maintains indexes when you
insert, update, and delete rows of the associated
table. If you drop an index, all applications continue
to work. However, access to previously indexed data
might be slower.

4. Create an Index
 CREATE [UNIQUE] INDEX index_name
ON table_name (column1[, column2, . column_n])
[ COMPUTE STATISTICS ];
 UNIQUE indicates that the combination of values in the indexed
columns must be unique.
 COMPUTE STATISTICS 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.

5.  create an index on the supplier table called supplier_idx. It consists of only one field - the supplier_name field
 CREATE INDEX supplier_idx
ON supplier (supplier_name);
 create an index with more than one field
 CREATE INDEX supplier_idx
ON supplier (supplier_name, city);
 Create unique index with one field
 CREATE UNIQUE INDEX supplier_u_idx
ON supplier (supplier_name);
 Create unique index with more than one field
 CREATE UNIQUE INDEX supplier_u_idx
ON supplier (supplier_name, city);

6. Rename an Index
 ALTER INDEX index_name
RENAME TO new_index_name;
 ALTER INDEX supplier_idx
RENAME TO supplier_index_name;

7. Drop an Index
 DROP INDEX index_name;
 DROP INDEX supplier_idx;

8. More on Indexes (user_indexes,
user_ind_columns)
 To see all indexes made on a table
SELECT index_name , index_type, table_name FROM
user_indexes WHERE table_name=‘MY_TABLE’;
 To see columns on which indexes have been made
SELECT index_name, table_name, column_name
FROM user_ind_columns WHERE
table_name=‘MY_TABLE’;