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 9.12. PostgreSQL also provides versions of these functions that use the regular function invocation syntax (see Table 9.13).

Note

The sample results shown on this page assume that the server parameter bytea_output is set to escape (the traditional PostgreSQL format).

Table 9.12. SQL Binary String Functions and Operators

Function	Return Type	Description	Example	Result
`string \|\| string`	`bytea`	String concatenation	`'\\Post'::bytea \|\| '\047gres\000'::bytea`	`\\Post'gres\000`
`octet_length(string)`	`int`	Number of bytes in binary string	`octet_length('jo\000se'::bytea)`	`5`
`overlay(string placing string from int [for int])`	`bytea`	Replace substring	`overlay('Th\000omas'::bytea placing '\002\003'::bytea from 2 for 3)`	`T\\002\\003mas`
`position(substring in string)`	`int`	Location of specified substring	`position('\000om'::bytea in 'Th\000omas'::bytea)`	`3`
`substring(string [from int] [for int])`	`bytea`	Extract substring	`substring('Th\000omas'::bytea from 2 for 3)`	`h\000o`
`trim([both] bytes from string)`	`bytea`	Remove the longest string containing only bytes appearing in `bytes` from the start and end of `string`	`trim('\000\001'::bytea from '\000Tom\001'::bytea)`	`Tom`

Additional binary string manipulation functions are available and are listed in Table 9.13. Some of them are used internally to implement the SQL-standard string functions listed in Table 9.12.

Table 9.13. Other Binary String Functions

Function	Return Type	Description	Example	Result
`btrim(string bytea, bytes bytea)`	`bytea`	Remove the longest string containing only bytes appearing in `bytes` from the start and end of `string`	`btrim('\000trim\001'::bytea, '\000\001'::bytea)`	`trim`
`decode(string text, format text)`	`bytea`	Decode binary data from textual representation in `string`. Options for `format` are same as in `encode`.	`decode('123\000456', 'escape')`	`123\000456`
`encode(data bytea, format text)`	`text`	Encode binary data into a textual representation. Supported formats are: `base64`, `hex`, `escape`. `escape` converts zero bytes and high-bit-set bytes to octal sequences (`\nnn`) and doubles backslashes.	`encode('123\000456'::bytea, 'escape')`	`123\000456`
`get_bit(string, offset)`	`int`	Extract bit from string	`get_bit('Th\000omas'::bytea, 45)`	`1`
`get_byte(string, offset)`	`int`	Extract byte from string	`get_byte('Th\000omas'::bytea, 4)`	`109`
`length(string)`	`int`	Length of binary string	`length('jo\000se'::bytea)`	`5`
`md5(string)`	`text`	Calculates the MD5 hash of `string`, returning the result in hexadecimal	`md5('Th\000omas'::bytea)`	`8ab2d3c9689aaf18b4958c334c82d8b1`
`set_bit(string, offset, newvalue)`	`bytea`	Set bit in string	`set_bit('Th\000omas'::bytea, 45, 0)`	`Th\000omAs`
`set_byte(string, offset, newvalue)`	`bytea`	Set byte in string	`set_byte('Th\000omas'::bytea, 4, 64)`	`Th\000o@as`
`sha224(bytea)`	`bytea`	SHA-224 hash	`sha224('abc')`	`\x23097d223405d8228642a477bda255b32aadbce4bda0b3f7e36c9da7`
`sha256(bytea)`	`bytea`	SHA-256 hash	`sha256('abc')`	`\xba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad`
`sha384(bytea)`	`bytea`	SHA-384 hash	`sha384('abc')`	`\xcb00753f45a35e8bb5a03d699ac65007272c32ab0eded1631a8b605a43ff5bed8086072ba1e7cc2358baeca134c825a7`
`sha512(bytea)`	`bytea`	SHA-512 hash	`sha512('abc')`	`\xddaf35a193617abacc417349ae20413112e6fa4e89a97ea20a9eeee64b55d39a2192992a274fc1a836ba3c23a3feebbd454d4423643ce80e2a9ac94fa54ca49f`

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.

Note that for historic 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 encode(sha256('abc'), 'hex') to get a hex-encoded text representation.

See also the aggregate function string_agg in Section 9.20 and the large object functions in Section 34.4.

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