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 9.11.
    PostgreSQL also provides versions of these functions
    that use the regular function invocation syntax
    (see Table 9.12).
   
Note
     The sample results shown on this page assume that the server parameter
     bytea_output is set
     to escape (the traditional PostgreSQL format).
    
Table 9.11. 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(stringplacingstringfromint[forint]) | bytea | Replace substring | overlay('Th\000omas'::bytea placing '\002\003'::bytea from 2 for 3) | T\\002\\003mas | 
| position(substringinstring) | int | Location of specified substring | position('\000om'::bytea in 'Th\000omas'::bytea) | 3 | 
| substring(string[fromint] [forint]) | bytea | Extract substring | substring('Th\000omas'::bytea from 2 for 3) | h\000o | 
| trim([both]
        bytesfromstring) | bytea | Remove the longest string containing only bytes appearing in bytesfrom the start
        and end ofstring | trim('\000\001'::bytea from '\000Tom\001'::bytea) | Tom | 
    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 | Return Type | Description | Example | Result | 
|---|
| btrim(stringbytea,bytesbytea) | bytea | Remove the longest string containing only bytes appearing in bytesfrom the start and end ofstring | btrim('\000trim\001'::bytea, '\000\001'::bytea) | trim | 
| decode(stringtext,formattext) | bytea | Decode binary data from textual representation in string.
       Options forformatare same as inencode. | decode('123\000456', 'escape') | 123\000456 | 
| encode(databytea,formattext) | text | Encode binary data into a textual representation.  Supported
       formats are: base64,hex,escape.escapeconverts 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 35.4.