See Also Connector/J, Connector/ODBC, Connector/PHP, prepared statement. 

## CLOB 

 An SQL data type ( TINYTEXT , TEXT , MEDIUMTEXT , or LONGTEXT ) for objects containing any kind of character data, of arbitrary size. Used for storing text-based documents, with associated character set and collation order. The techniques for handling CLOBs within a MySQL application vary with each Connector  and API . MySQL Connector/ODBC defines TEXT  values as LONGVARCHAR . For storing binary data, the equivalent is the BLOB type. 

See Also API, BLOB, connector, Connector/ODBC. 

## clustered index 

 The InnoDB  term for a primary key  index. InnoDB  table storage is organized based on the values of the primary key columns, to speed up queries and sorts involving the primary key columns. For best performance, choose the primary key columns carefully based on the most performance-critical queries. Because modifying the columns of the clustered index is an expensive operation, choose primary columns that are rarely or never updated. 

In the Oracle Database product, this type of table is known as an index-organized table . See Also index, primary key, secondary index. 

## cold backup 

 A backup  taken while the database is shut down. For busy applications and websites, this might not be practical, and you might prefer a warm backup  or a hot backup . 

See Also backup, hot backup, warm backup. 

## column 

 A data item within a row , whose storage and semantics are defined by a data type. Each table  and index  is largely defined by the set of columns it contains. 

Each column has a cardinality  value. A column can be the primary key  for its table, or part of the primary key. A column can be subject to a unique constraint , a NOT NULL constraint , or both. Values in different columns, even across different tables, can be linked by a foreign key  relationship. 

In discussions of MySQL internal operations, sometimes field  is used as a synonym. See Also cardinality, foreign key, index, NOT NULL constraint, primary key, row, table, unique constraint. 

## column index 

 An index  on a single column. See Also composite index, index. 

## column prefix 

 When an index  is created with a length specification, such as CREATE INDEX idx ON t1 (c1(N)) , only the first N characters of the column value are stored in the index. Keeping the index prefix small makes the index compact, and the memory and disk I/O savings help performance. (Although making the index prefix too small can hinder query optimization by making rows with different values appear to the query optimizer to be duplicates.) 

For columns containing binary values or long text strings, where sorting is not a major consideration and storing the entire value in the index would waste space, the index automatically uses the first N (typically 768) characters of the value to do lookups and sorts. See Also index. 

## command interceptor 

 Synonym for statement interceptor . One aspect of the interceptor  design pattern available for both Connector/NET  and Connector/J . What Connector/NET calls a command, Connector/J refers to as a statement. Contrast with exception interceptor . 

