A data type used in CREATE TABLE and ALTER TABLE statements.

Length: 32,767 bytes. Do not use any length constraint when declaring STRING columns, as you might be familiar with from VARCHAR, CHAR, or similar column types from relational database systems.

Character sets: For full support in all Impala subsystems, restrict string values to the ASCII character set. UTF-8 character data can be stored in Impala and retrieved through queries, but UTF-8 strings containing non-ASCII characters are not guaranteed to work properly with string manipulation functions, comparison operators, or the ORDER BY clause. For any national language aspects such as collation order or interpreting extended ASCII variants such as ISO-8859-1 or ISO-8859-2 encodings, Impala does not include such metadata with the table definition. If you need to sort, manipulate, or display data depending on those national language characteristics of string data, use logic on the application side.

Conversions:

• Impala does not automatically convert STRING to any numeric type. Impala does automatically convert STRING to TIMESTAMP if the value matches one of the accepted TIMESTAMP formats; see TIMESTAMP Data Type for details.

• You can use CAST() to convert STRING values to TINYINT, SMALLINT, INT, BIGINT, FLOAT, DOUBLE, or TIMESTAMP.

• You cannot directly cast a STRING value to BOOLEAN. You can use a CASE expression to evaluate string values such as 'T', 'true', and so on and return Boolean true and false values as appropriate.

• You can cast a BOOLEAN value to STRING, returning '1' for true values and '0' for false values.

Partitioning:

Although it might be convenient to use STRING columns for partition keys, even when those columns contain numbers, for performance and scalability it is much better to use numeric columns as partition keys whenever practical. Although the underlying HDFS directory name might be the same in either case, the in-memory storage for the partition key columns is more compact, and computations are faster, if partition key columns such as YEAR, MONTH, DAY and so on are declared as INT, SMALLINT, and so on.

Zero-length strings: For purposes of clauses such as DISTINCT and GROUP BY, Impala considers zero-length strings (""), NULL, and space to all be different values.

Related information: Literals, String Functions, Date and Time Functions