PostgreSQL 9.0.4 Documentation
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# 9.5. Binary String Functions and Operators

This section describes functions and operators for examining and manipulating values of type `bytea`.

SQL defines some string functions that use key words, rather than commas, to separate arguments. Details are in Table 9-8. PostgreSQL also provides versions of these functions that use the regular function invocation syntax (see Table 9-9).

Table 9-8. SQL Binary String Functions and Operators

Function | Return Type | Description | Example | Result
--- | --- | --- | --- | ---
``string` `||` `string`` | `bytea` | String concatenation | `E'\\\\Post'::bytea || E'\\047gres\\000'::bytea` | `\\Post'gres\000`
``octet_length(`string`)`` | `int` | Number of bytes in binary string | `octet_length(E'jo\\000se'::bytea)` | `5`
``overlay(`string` placing `string` from `int` [for `int`])`` | `bytea` | Replace substring | `overlay(E'Th\\000omas'::bytea placing E'\\002\\003'::bytea from 2 for 3)` | `T\\002\\003mas`
``position(`substring` in `string`)`` | `int` | Location of specified substring | `position(E'\\000om'::bytea in E'Th\\000omas'::bytea)` | `3`
``substring(`string` [from `int`] [for `int`])`` | `bytea` | Extract substring | `substring(E'Th\\000omas'::bytea from 2 for 3)` | `h\000o`
``trim([both] `bytes` from `string`)`` | `bytea` | Remove the longest string containing only the bytes in `bytes` from the start and end of `string` | `trim(E'\\000'::bytea from E'\\000Tom\\000'::bytea)` | `Tom`

Additional binary string manipulation functions are available and are listed in Table 9-9. Some of them are used internally to implement the SQL-standard string functions listed in Table 9-8.

Table 9-9. Other Binary String Functions

Function | Return Type | Description | Example | Result
--- | --- | --- | --- | ---
``btrim(`string` `bytea`, `bytes` `bytea`)`` | `bytea` | Remove the longest string consisting only of bytes in `bytes` from the start and end of `string` | `btrim(E'\\000trim\\000'::bytea, E'\\000'::bytea)` | `trim`
``decode(`string` `text`, `type` `text`)`` | `bytea` | Decode binary string from `string` previously encoded with `encode`. Parameter type is same as in `encode`. | `decode(E'123\\000456', 'escape')` | `123\000456`
``encode(`string` `bytea`, `type` `text`)`` | `text` | Encode binary string to ASCII-only representation. Supported types are: `base64`, `hex`, `escape`. | `encode(E'123\\000456'::bytea, 'escape')` | `123\000456`
``get_bit(`string`, `offset`)`` | `int` | Extract bit from string | `get_bit(E'Th\\000omas'::bytea, 45)` | `1`
``get_byte(`string`, `offset`)`` | `int` | Extract byte from string | `get_byte(E'Th\\000omas'::bytea, 4)` | `109`
``length(`string`)`` | `int` | Length of binary string | `length(E'jo\\000se'::bytea)` | `5`
``md5(`string`)`` | `text` | Calculates the MD5 hash of `string`, returning the result in hexadecimal | `md5(E'Th\\000omas'::bytea)` | `8ab2d3c9689aaf18 b4958c334c82d8b1`
``set_bit(`string`, `offset`, `newvalue`)`` | `bytea` | Set bit in string | `set_bit(E'Th\\000omas'::bytea, 45, 0)` | `Th\000omAs`
``set_byte(`string`, `offset`, `newvalue`)`` | `bytea` | Set byte in string | `set_byte(E'Th\\000omas'::bytea, 4, 64)` | `Th\000o@as`

`get_byte` and `set_byte` number the first byte of a binary string as byte 0. `get_bit` and `set_bit` number bits from the right within each byte; for example bit 0 is the least significant bit of the first byte, and bit 15 is the most significant bit of the second byte.

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