Large objects
The term large object and the generic acronym LOB are used to refer to any CLOB, DBCLOB, or BLOB data type.
Manipulating large objects with locators
Since LOB values can be very large, the transfer of these values from the database server to client application program variables can be time consuming. Also, application programs typically process LOB values a piece at a time, rather than as a whole. For these cases, the application can reference a LOB value via a large object locator (LOB locator). 1
A large object locator or LOB locator is a variable with a value that represents a single LOB value in the database server. LOB locators were developed to provide users with a mechanism by which they could easily manipulate very large objects in application programs without requiring them to store the entire LOB value on the client machine where the application program may be running.
For example, when selecting a LOB value, an application program could select the entire LOB value and place it into an equally large variable (which is acceptable if the application program is going to process the entire LOB value at once), or it could instead select the LOB value into a LOB locator. Then, using the LOB locator, the application program can issue subsequent database operations on the LOB value by supplying the locator value as input. The resulting output of the locator operation, for example the amount of data assigned to a client variable, would then typically be a small subset of the input LOB value.
LOB locators may also represent more than just base values; they can also represent the value associated with a LOB expression. For example, a LOB locator might represent the value associated with:
SUBSTR(lob_value_1 CONCAT lob_value_2 CONCAT lob_value_3, 42, 6000000)For non-locator-based host variables in an application program, when a null value is selected into that host variable, the indicator variable is set to -1, signifying that the value is null. In the case of LOB locators, however, the meaning of indicator variables is slightly different. Since a LOB locator host variable itself can never be null, a negative indicator variable value indicates that the LOB value represented by the LOB locator is null. The null information is kept local to the client by virtue of the indicator variable value -- the server does not track null values with valid LOB locators.
It is important to understand that a LOB locator represents a value, not a row or location in the database. Once a value is selected into a LOB locator, there is no operation that one can perform on the original row or table that will affect the value which is referenced by the LOB locator. The value associated with a LOB locator is valid until the transaction ends, or until the LOB locator is explicitly freed, whichever comes first.
A LOB locator is only a mechanism used to refer to a LOB value during a transaction; it does not persist beyond the transaction in which it was created. Also, it is not a database type; it is never stored in the database and, as a result, cannot participate in views or check constraints. However, since a locator is a representation of a LOB type, there are SQLTYPEs for LOB locators so that they can be described within an SQLDA structure that is used by FETCH, OPEN, CALL, and EXECUTE statements.
For the restrictions that apply to the use of LOB strings, see Limitations on use of strings.