Database Reference
In-Depth Information
5.
The default algorithm for Oracle indexes is B-tree; however
the user has the option of creating a bit-map index. The bit-
map is like a matrix representing row-IDs and columns. It is
useful in situations where many column values are identical
(for instance where the primary key is composite).
Finally, you can drop an index via the
Drop-Index
statement. The syntax for this
statement is shown in Figure
11-17
.
Figure 11-17.
The Drop-Index Statement
Example 9:
The following statements create indexes on the tables indicated:
Example 10:
The following statement drops the index
CrsX
:
Note
: When a table is dropped, all its related indexes are automatically dropped also.
11.9 Creating and Managing Sequences
A sequence is an Oracle database object that automatically generates unique numbers.
It is typically used to create primary key values, particularly if the primary key is a single
attribute. For composite primary keys, or other alphanumeric codes, the sequence could
still be useful in generating a unique number, which is to be concatenated with some
other data to comprise a code.
A sequence provides two significant advantages:
•
Its use could lead to shorter application code.
•
When sequence values are cached, processing efficiency is
enhanced.
