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.