VARCHAR Data Type (CDH 5.2 or higher only)
A variable-length character type, truncated during processing if necessary to fit within the specified length.
In the column definition of a CREATE TABLE statement:
The maximum length you can specify is 65,535.
Partitioning: This type can be used for partition key columns. Because of the efficiency advantage of numeric values over character-based values, if the partition key is a string representation of a number, prefer to use an integer type with sufficient range (INT, BIGINT, and so on) where practical.
HBase considerations: This data type cannot be used with HBase tables.
- This type can be read from and written to Parquet files.
- There is no requirement for a particular level of Parquet.
- Parquet files generated by Impala and containing this type can be freely interchanged with other components such as Hive and MapReduce.
- Parquet data files can contain values that are longer than allowed by the VARCHAR(n) length limit. Impala ignores any extra trailing characters when it processes those values during a query.
Text table considerations:
Text data files can contain values that are longer than allowed by the VARCHAR(n) length limit. Any extra trailing characters are ignored when Impala processes those values during a query.
The Avro specification allows string values up to 2**64 bytes in length. Impala queries for Avro tables use 32-bit integers to hold string lengths. In CDH 5.7 / Impala 2.5 and higher, Impala truncates CHAR and VARCHAR values in Avro tables to 2**32 bytes. If a query encounters a STRING value longer than 2**32 bytes in an Avro table, the query fails. In earlier releases, encountering such long values in an Avro table could cause a crash.
Schema evolution considerations:
You can use ALTER TABLE ... CHANGE to switch column data types to and from VARCHAR. You can convert from STRING to VARCHAR(n), or from VARCHAR(n) to STRING, or from CHAR(n) to VARCHAR(n), or from VARCHAR(n) to CHAR(n). When switching back and forth between VARCHAR and CHAR, you can also change the length value. This schema evolution works the same for tables using any file format. If a table contains values longer than the maximum length defined for a VARCHAR column, Impala does not return an error. Any extra trailing characters are ignored when Impala processes those values during a query.
This type is available on CDH 5.2 or higher.
Internal details: Represented in memory as a byte array with the minimum size needed to represent each value.
Added in: CDH 5.2.0 (Impala 2.0.0)
Column statistics considerations: Because the values of this type have variable size, none of the column statistics fields are filled in until you run the COMPUTE STATS statement.
All data in CHAR and VARCHAR columns must be in a character encoding that is compatible with UTF-8. If you have binary data from another database system (that is, a BLOB type), use a STRING column to hold it.
The following examples show how long and short VARCHAR values are treated. Values longer than the maximum specified length are truncated by CAST(), or when queried from existing data files. Values shorter than the maximum specified length are represented as the actual length of the value, with no extra padding as seen with CHAR values.
create table varchar_1 (s varchar(1)); create table varchar_4 (s varchar(4)); create table varchar_20 (s varchar(20)); insert into varchar_1 values (cast('a' as varchar(1))), (cast('b' as varchar(1))), (cast('hello' as varchar(1))), (cast('world' as varchar(1))); insert into varchar_4 values (cast('a' as varchar(4))), (cast('b' as varchar(4))), (cast('hello' as varchar(4))), (cast('world' as varchar(4))); insert into varchar_20 values (cast('a' as varchar(20))), (cast('b' as varchar(20))), (cast('hello' as varchar(20))), (cast('world' as varchar(20))); select * from varchar_1; +---+ | s | +---+ | a | | b | | h | | w | +---+ select * from varchar_4; +------+ | s | +------+ | a | | b | | hell | | worl | +------+ [localhost:21000] > select * from varchar_20; +-------+ | s | +-------+ | a | | b | | hello | | world | +-------+ select concat('[',s,']') as s from varchar_20; +---------+ | s | +---------+ | [a] | | [b] | | [hello] | | [world] | +---------+
The following example shows how identical VARCHAR values compare as equal, even if the columns are defined with different maximum lengths. Both tables contain 'a' and 'b' values. The longer 'hello' and 'world' values from the VARCHAR_20 table were truncated when inserted into the VARCHAR_1 table.
select s from varchar_1 join varchar_20 using (s); +-------+ | s | +-------+ | a | | b | +-------+
The following examples show how VARCHAR values are freely interchangeable with STRING values in contexts such as comparison operators and built-in functions:
select length(cast('foo' as varchar(100))) as length; +--------+ | length | +--------+ | 3 | +--------+ select cast('xyz' as varchar(5)) > cast('abc' as varchar(10)) as greater; +---------+ | greater | +---------+ | true | +---------+
UDF considerations: This type cannot be used for the argument or return type of a user-defined function (UDF) or user-defined aggregate function (UDA).