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\[email protected]

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.

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