LOB Introduction

Char and Varchar2 data types are used to store text columns in an Oracle database. While for the normal requirements they are OK, but they are not as suitable when it comes to store large objects. By large objects I mean storing of entire PDF documents for example or storing of an image. Traditionally Oracle provided two data types for these situations namely as RAW and Long RAW. But the use of these data types was inherent to some limitations. The first one was that you cannot have more one column of RAW or Long RAW data types in one table. The storage capacity was limited to 2GB. And last you cannot perform SQL operations on the subset of data stored as RAW or Long RAW.

In Oracle 10g Release 1 and above (not sure of 9i) Oracle introduced Large Objects or LOB for short. The introduction of LOB data types culminates all the restrictions that were present in RAW and Long RAW data types. LOB itself is not a data type. Rather it is a category of four data types which are divided into two distinct categories. The categorization of LOB data types is based on their storage. The two categories are:

1. Internal

2. External

Internally stored LOB objects are those which are stored inside database. They are treated just like any other object inside database. And all SQL operations are valid for them that are valid for other data types. There are three data types in this category:

1. CLOB: It is used to store large documents and stands for Character Large Objects.
2. NCLOB: It is also used to store large documents which are in character format. But it stores these documents using a different format namely as National Character Set.
3. BLOB: This data type is used to store large objects which are in binary format. For example an image.

Externally stored large objects are those objects which are stored outside database but whose reference is maintained inside database. The storage of these objects is inherent to the limitations of Operating System files. Normal SQL operations performed on other internally stored objects will not be applicable to these externally stored objects. The only data type in this category is BFILE. The column of BFILE data type will be used only to store the location of large object file.