This section describes functions and operators for examining and
    manipulating binary strings, that is values of type bytea.
    Many of these are equivalent, in purpose and syntax, to the
    text-string functions described in the previous section.
   
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).
Table 9.11. SQL Binary String Functions and Operators
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 Description Example(s) | 
|---|
| 
        
        
         Returns the number of bits set in the binary string (also known as “popcount”). 
         | 
| 
        
         Extracts n'th bit from binary string. 
         | 
| 
        
         Extracts n'th byte from binary string. 
         | 
| Returns the number of bytes in the binary string. 
         | 
| 
         
        Returns the number of characters in the binary string, assuming
        that it is text in the given  
         | 
| Computes the MD5 hash of the binary string, with the result written in hexadecimal. 
         | 
| 
        
         
        Sets n'th bit in
        binary string to  
         | 
| 
        
         
        Sets n'th byte in
        binary string to  
         | 
| Computes the SHA-224 hash of the binary string. 
         | 
| Computes the SHA-256 hash of the binary string. 
         | 
| Computes the SHA-384 hash of the binary string. 
         | 
| Computes the SHA-512 hash of the binary string. 
         | 
| 
        
         
        Extracts the substring of  
         | 
   Functions get_byte and set_byte
   number the first byte of a binary string as byte 0.
   Functions 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.
  
   For historical 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 write encode(sha256('abc'),
   'hex') to get a hex-encoded text representation,
   or decode(md5('abc'), 'hex') to get
   a bytea value.
  
   
   
   Functions for converting strings between different character sets
   (encodings), and for representing arbitrary binary data in textual
   form, are shown in
   Table 9.13.  For these
   functions, an argument or result of type text is expressed
   in the database's default encoding, while arguments or results of
   type bytea are in an encoding named by another argument.
  
Table 9.13. Text/Binary String Conversion Functions
| Function Description Example(s) | 
|---|
| 
       
        
       Converts a binary string representing text in
       encoding  
        | 
| 
       
        
       Converts a binary string representing text in
       encoding  
        | 
| 
       
        
       Converts a  
        | 
| 
       
        
       Encodes binary data into a textual representation; supported
        
        | 
| 
       
        
       Decodes binary data from a textual representation; supported
        
        | 
   The encode and decode
   functions support the following textual formats:
   
       The base64 format is that
       of RFC
       2045 Section 6.8.  As per the RFC, encoded lines are
       broken at 76 characters.  However instead of the MIME CRLF
       end-of-line marker, only a newline is used for end-of-line.
       The decode function ignores carriage-return,
       newline, space, and tab characters.  Otherwise, an error is
       raised when decode is supplied invalid
       base64 data — including when trailing padding is incorrect.
      
       The escape format converts zero bytes and
       bytes with the high bit set into octal escape sequences
       (\nnn), and it doubles
       backslashes.  Other byte values are represented literally.
       The decode function will raise an error if a
       backslash is not followed by either a second backslash or three
       octal digits; it accepts other byte values unchanged.
      
       The hex format represents each 4 bits of
       data as one hexadecimal digit, 0
       through f, writing the higher-order digit of
       each byte first.  The encode function outputs
       the a-f hex digits in lower
       case.  Because the smallest unit of data is 8 bits, there are
       always an even number of characters returned
       by encode.
       The decode function
       accepts the a-f characters in
       either upper or lower case.  An error is raised
       when decode is given invalid hex data
       — including when given an odd number of characters.
      
   See also the aggregate function string_agg in
   Section 9.21 and the large object functions
   in Section 33.4.