If you need to store a large amount of text data in a table, you may run into the problem of needing to store a character string larger than the maximum number or characters allowed for the CHARACTER (or VARCHAR) data type. Suppose, for example, that you had a HUMAN_RESOURCES table and one of the columns was RESUME. If you are using MS-SQL Server as your DBMS, you could store only the first 4,000 characters of the resume in the RESUME column of the HUMAN_RESOURCES table. Fortunately, Microsoft has the TEXT data type which, like Oracle's LONG data type, lets you store character strings of up to 2,147,483,647 characters. (If you are storing text strings in Unicode using columns of type NTEXT, you can store only 1,073,741,823 characters. Each Unicode character takes 2 bytes of storage, so you can store only half as many of them.)
It would be wasteful to preallocate 2GB of disk space for each column you declare as type TEXT. As such, MS-SQL Server preallocates only a small portion (8K) of the maximum TEXT space and allocates the remainder in 8K (8,192 byte) increments as you need it. As such, when it is ready to save character 8,193 of a TEXT string to disk, the DBMS allocates another block (page) of 8,192 bytes and creates a link from the page holding the previous 8,192 bytes to the page holding the next 8,192 bytes.
Once the DBMS stores the data in the TEXT column to disk, the entire TEXT block is logically contiguous. This is to say that the DBMS "sees" the TEXT block as one huge character string, even if the individual 8K blocks (pages) that make up the TEXT block are not physically contiguous. As such, you can display the entire contents of a TEXT column using a single SELECT statement such as:
if, for example, HUMAN_RESOURCES were a table defined as:
CREATE TABLE human_resources
(id INTEGER,
name VARCHAR(25),
department_code TINYINT,
data_of_hire DATE,
resume TEXT)
| Note | The actual number of characters of TEXT data displayed by the SELECT statement is limited by the value of the Global Variable @@Textsize. If you don't change the value of @@Textsize, MS-SQL Server limits the number of TEXT characters displayed to 64K (64,512) by default. |
