Character large object

From Wikipedia, the free encyclopedia
Jump to: navigation, search

A Character Large Object (or CLOB) is a collection of character data in a database management system, usually stored in a separate location that is referenced in the table itself. Oracle and IBM DB2 provide a construct explicitly named CLOB, and the majority of other database systems support some form of the concept, often labeled as text, memo or long character fields.

CLOBs usually have very high size-limits, of the order of 2GB or more. The tradeoff for the capacity is usually limited access methods. In particular, some database systems[which?] limit certain SQL clauses and/or functions, such as LIKE or SUBSTRING from being used on CLOBs. Those that permit such operations may perform them very slowly.

Alternative methods of accessing the data are often provided, including means of extracting or inserting ranges of data from the CLOB.

Database systems vary in their storage patterns for CLOBs. Some systems always store CLOBs as a reference to out-of-table data, while others store small CLOBs in-table, changing their storage patterns when the size of the data grows beyond a threshold. Other systems are configurable in their behavior.

CLOBs differ from BLOBs by having a specified character encoding, and also by the capability to permit some string-oriented operations on them.

See also[edit]