PostgreSQL 8.1.8 Documentation
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# 8.9. Bit String Types

Bit strings are strings of 1's and 0's. They can be used to store or visualize bit masks. There are two SQL bit types: `bit(`n`)` and `bit varying(`n`)`, where `n` is a positive integer.

`bit` type data must match the length `n` exactly; it is an error to attempt to store shorter or longer bit strings. `bit varying` data is of variable length up to the maximum length `n`; longer strings will be rejected. Writing `bit` without a length is equivalent to `bit(1)`, while `bit varying` without a length specification means unlimited length.

> Note: If one explicitly casts a bit-string value to `bit(`n`)`, it will be truncated or zero-padded on the right to be exactly `n` bits, without raising an error. Similarly, if one explicitly casts a bit-string value to `bit varying(`n`)`, it will be truncated on the right if it is more than `n` bits.

> Note: Prior to PostgreSQL 7.2, `bit` data was always silently truncated or zero-padded on the right, with or without an explicit cast. This was changed to comply with the SQL standard.

Refer to Section 4.1.2.3 for information about the syntax of bit string constants. Bit-logical operators and string manipulation functions are available; see Section 9.6.

Example 8-3. Using the bit string types

    CREATE TABLE test (a BIT(3), b BIT VARYING(5));
    INSERT INTO test VALUES (B'101', B'00');
    INSERT INTO test VALUES (B'10', B'101');
    ERROR:  bit string length 2 does not match type bit(3)
    INSERT INTO test VALUES (B'10'::bit(3), B'101');
    SELECT * FROM test;
      a  |  b
    -----+-----
     101 | 00
     100 | 101

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