9.5. 位元字串函式及運算子

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) 8ab2d3c9689aaf18​b4958c334c82d8b1
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') \x23097d223405d8228642a477bda2​55b32aadbce4bda0b3f7e36c9da7
sha256(bytea) bytea SHA-256 hash sha256('abc') \xba7816bf8f01cfea414140de5dae2223​b00361a396177a9cb410ff61f20015ad
sha384(bytea) bytea SHA-384 hash sha384('abc') \xcb00753f45a35e8bb5a03d699ac65007​272c32ab0eded1631a8b605a43ff5bed​8086072ba1e7cc2358baeca134c825a7
sha512(bytea) bytea SHA-512 hash sha512('abc') \xddaf35a193617abacc417349ae204131​12e6fa4e89a97ea20a9eeee64b55d39a​2192992a274fc1a836ba3c23a3feebbd​454d4423643ce80e2a9ac94fa54ca49f

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.