9.5. Binary String Functions and Operators
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## 9.5. Binary String Functions and Operators

This section describes functions and operators for examining and manipulating binary strings, that is values of type `bytea`. Many of these are equivalent, in purpose and syntax, to the text-string functions described in the previous section.

SQL defines some string functions that use key words, rather than commas, to separate arguments. Details are in Table 9.11. PostgreSQL also provides versions of these functions that use the regular function invocation syntax (see Table 9.12).

Table 9.11. SQL Binary String Functions and Operators

Function/Operator Description Example(s)
`bytea` `||` `bytea` → `bytea` Concatenates the two binary strings. `'\x123456'::bytea || '\x789a00bcde'::bytea` → `\x123456789a00bcde`
`bit_length` ( `bytea` ) → `integer` Returns number of bits in the binary string (8 times the `octet_length`). `bit_length('\x123456'::bytea)` → `24`
`octet_length` ( `bytea` ) → `integer` Returns number of bytes in the binary string. `octet_length('\x123456'::bytea)` → `3`
`overlay` ( `bytes` `bytea` `PLACING` `newsubstring` `bytea` `FROM` `start` `integer` [ `FOR` `count` `integer` ] ) → `bytea` Replaces the substring of `bytes` that starts at the `start`'th byte and extends for `count` bytes with `newsubstring`. If `count` is omitted, it defaults to the length of `newsubstring`. `overlay('\x1234567890'::bytea placing '\002\003'::bytea from 2 for 3)` → `\x12020390`
`position` ( `substring` `bytea` `IN` `bytes` `bytea` ) → `integer` Returns first starting index of the specified `substring` within `bytes`, or zero if it's not present. `position('\x5678'::bytea in '\x1234567890'::bytea)` → `3`
`substring` ( `bytes` `bytea` [ `FROM` `start` `integer` ] [ `FOR` `count` `integer` ] ) → `bytea` Extracts the substring of `bytes` starting at the `start`'th byte if that is specified, and stopping after `count` bytes if that is specified. Provide at least one of `start` and `count`. `substring('\x1234567890'::bytea from 3 for 2)` → `\x5678`
`trim` ( [ `BOTH` ] `bytesremoved` `bytea` `FROM` `bytes` `bytea` ) → `bytea` Removes the longest string containing only bytes appearing in `bytesremoved` from the start and end of `bytes`. `trim('\x9012'::bytea from '\x1234567890'::bytea)` → `\x345678`
`trim` ( [ `BOTH` ] [ `FROM` ] `bytes` `bytea`, `bytesremoved` `bytea` ) → `bytea` This is a non-standard syntax for `trim()`. `trim(both from '\x1234567890'::bytea, '\x9012'::bytea)` → `\x345678`

Additional binary string manipulation functions are available and are listed in Table 9.12. Some of them are used internally to implement the SQL-standard string functions listed in Table 9.11.

Table 9.12. Other Binary String Functions

Function Description Example(s)
`btrim` ( `bytes` `bytea`, `bytesremoved` `bytea` ) → `bytea` Removes the longest string containing only bytes appearing in `bytesremoved` from the start and end of `bytes`. `btrim('\x1234567890'::bytea, '\x9012'::bytea)` → `\x345678`
`get_bit` ( `bytes` `bytea`, `n` `bigint` ) → `integer` Extracts n'th bit from binary string. `get_bit('\x1234567890'::bytea, 30)` → `1`
`get_byte` ( `bytes` `bytea`, `n` `integer` ) → `integer` Extracts n'th byte from binary string. `get_byte('\x1234567890'::bytea, 4)` → `144`
`length` ( `bytea` ) → `integer` Returns the number of bytes in the binary string. `length('\x1234567890'::bytea)` → `5`
`length` ( `bytes` `bytea`, `encoding` `name` ) → `integer` Returns the number of characters in the binary string, assuming that it is text in the given `encoding`. `length('jose'::bytea, 'UTF8')` → `4`
`md5` ( `bytea` ) → `text` Computes the MD5 hash of the binary string, with the result written in hexadecimal. `md5('Th\000omas'::bytea)` → `8ab2d3c9689aaf18​b4958c334c82d8b1`
`set_bit` ( `bytes` `bytea`, `n` `bigint`, `newvalue` `integer` ) → `bytea` Sets n'th bit in binary string to `newvalue`. `set_bit('\x1234567890'::bytea, 30, 0)` → `\x1234563890`
`set_byte` ( `bytes` `bytea`, `n` `integer`, `newvalue` `integer` ) → `bytea` Sets n'th byte in binary string to `newvalue`. `set_byte('\x1234567890'::bytea, 4, 64)` → `\x1234567840`
`sha224` ( `bytea` ) → `bytea` Computes the SHA-224 hash of the binary string. `sha224('abc'::bytea)` → `\x23097d223405d8228642a477bda2​55b32aadbce4bda0b3f7e36c9da7`
`sha256` ( `bytea` ) → `bytea` Computes the SHA-256 hash of the binary string. `sha256('abc'::bytea)` → `\xba7816bf8f01cfea414140de5dae2223​b00361a396177a9cb410ff61f20015ad`
`sha384` ( `bytea` ) → `bytea` Computes the SHA-384 hash of the binary string. `sha384('abc'::bytea)` → `\xcb00753f45a35e8bb5a03d699ac65007​272c32ab0eded1631a8b605a43ff5bed​8086072ba1e7cc2358baeca134c825a7`
`sha512` ( `bytea` ) → `bytea` Computes the SHA-512 hash of the binary string. `sha512('abc'::bytea)` → `\xddaf35a193617abacc417349ae204131​12e6fa4e89a97ea20a9eeee64b55d39a​2192992a274fc1a836ba3c23a3feebbd​454d4423643ce80e2a9ac94fa54ca49f`
`substr` ( `bytes` `bytea`, `start` `integer` [, `count` `integer` ] ) → `bytea` Extracts the substring of `bytes` starting at the `start`'th byte, and extending for `count` bytes if that is specified. (Same as `substring(`bytes` from `start` for `count`)`.) `substr('\x1234567890'::bytea, 3, 2)` → `\x5678`

Functions `get_byte` and `set_byte` number the first byte of a binary string as byte 0. Functions `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.

For historical reasons, the function `md5` returns a hex-encoded value of type `text` whereas the SHA-2 functions return type `bytea`. Use the functions `encode` and `decode` to convert between the two. For example write `encode(sha256('abc'), 'hex')` to get a hex-encoded text representation, or `decode(md5('abc'), 'hex')` to get a `bytea` value.

Functions for converting strings between different character sets (encodings), and for representing arbitrary binary data in textual form, are shown in Table 9.13. For these functions, an argument or result of type `text` is expressed in the database's default encoding, while arguments or results of type `bytea` are in an encoding named by another argument.

Table 9.13. Text/Binary String Conversion Functions

Function Description Example(s)
`convert` ( `bytes` `bytea`, `src_encoding` `name`, `dest_encoding` `name` ) → `bytea` Converts a binary string representing text in encoding `src_encoding` to a binary string in encoding `dest_encoding` (see Section 23.3.4 for available conversions). `convert('text_in_utf8', 'UTF8', 'LATIN1')` → `\x746578745f696e5f75746638`
`convert_from` ( `bytes` `bytea`, `src_encoding` `name` ) → `text` Converts a binary string representing text in encoding `src_encoding` to `text` in the database encoding (see Section 23.3.4 for available conversions). `convert_from('text_in_utf8', 'UTF8')` → `text_in_utf8`
`convert_to` ( `string` `text`, `dest_encoding` `name` ) → `bytea` Converts a `text` string (in the database encoding) to a binary string encoded in encoding `dest_encoding` (see Section 23.3.4 for available conversions). `convert_to('some_text', 'UTF8')` → `\x736f6d655f74657874`
`encode` ( `bytes` `bytea`, `format` `text` ) → `text` Encodes binary data into a textual representation; supported `format` values are: `base64`, `escape`, `hex`. `encode('123\000\001', 'base64')` → `MTIzAAE=`
`decode` ( `string` `text`, `format` `text` ) → `bytea` Decodes binary data from a textual representation; supported `format` values are the same as for `encode`. `decode('MTIzAAE=', 'base64')` → `\x3132330001`

The `encode` and `decode` functions support the following textual formats:

base64

  The `base64` format is that of RFC 2045 Section 6.8. As per the RFC, encoded lines are broken at 76 characters. However instead of the MIME CRLF end-of-line marker, only a newline is used for end-of-line. The `decode` function ignores carriage-return, newline, space, and tab characters. Otherwise, an error is raised when `decode` is supplied invalid base64 data — including when trailing padding is incorrect.

escape

  The `escape` format converts zero bytes and bytes with the high bit set into octal escape sequences (`\``nnn`), and it doubles backslashes. Other byte values are represented literally. The `decode` function will raise an error if a backslash is not followed by either a second backslash or three octal digits; it accepts other byte values unchanged.

hex

  The `hex` format represents each 4 bits of data as one hexadecimal digit, `0` through `f`, writing the higher-order digit of each byte first. The `encode` function outputs the `a`-`f` hex digits in lower case. Because the smallest unit of data is 8 bits, there are always an even number of characters returned by `encode`. The `decode` function accepts the `a`-`f` characters in either upper or lower case. An error is raised when `decode` is given invalid hex data — including when given an odd number of characters.

See also the aggregate function `string_agg` in Section 9.21 and the large object functions in Section 34.4.

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