A Character Large OBject (or CLOB) is part of the SQL:1999 standard data types. It is a collection of
character data
In the pursuit of knowledge, data (; ) is a collection of discrete Value_(semiotics), values that convey information, describing quantity, qualitative property, quality, fact, statistics, other basic units of meaning, or simply sequences of sy ...
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 gigabytes. The tradeoff for the capacity is usually limited access methods. In particular, some database systems 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.
Implementations
PostgreSQL 15.0: unsupported but it can be easily defined as a synonym to the text type with:
create domain CLOB as TEXT;
MariaDB 10.8: supported
MySQL 8.0: support unknown. There used
to be a CLOB mentioned in the manual but the data type is no longer mentioned in later manuals.
See also
*
Binary large object
A binary large object (BLOB or blob) is a collection of binary data stored as a single entity. Blobs are typically images, audio or other multimedia objects, though sometimes binary executable code is stored as a blob. They can exist as persistent ...
*
Binary blob
References
Data types
{{Database-stub