8.4. Binary Data Types
bytea data type allows storage of binary strings;
see Table 8.6.
Table 8.6. Binary Data Types
|1 or 4 bytes plus the actual binary string||variable-length binary string|
A binary string is a sequence of octets (or bytes). Binary strings are distinguished from character strings in two ways. First, binary strings specifically allow storing octets of value zero and other “non-printable” octets (usually, octets outside the decimal range 32 to 126). Character strings disallow zero octets, and also disallow any other octet values and sequences of octet values that are invalid according to the database's selected character set encoding. Second, operations on binary strings process the actual bytes, whereas the processing of character strings depends on locale settings. In short, binary strings are appropriate for storing data that the programmer thinks of as “raw bytes”, whereas character strings are appropriate for storing text.
bytea type supports two
formats for input and output: “hex” format
and PostgreSQL's historical
“escape” format. Both
of these are always accepted on input. The output format depends
on the configuration parameter bytea_output;
the default is hex. (Note that the hex format was introduced in
PostgreSQL 9.0; earlier versions and some
tools don't understand it.)
The SQL standard defines a different binary
string type, called
OBJECT. The input format is different from
bytea, but the provided functions and operators are
mostly the same.
bytea Hex Format
The “hex” format encodes binary data as 2 hexadecimal digits
per byte, most significant nibble first. The entire string is
preceded by the sequence
\x (to distinguish it
from the escape format). In some contexts, the initial backslash may
need to be escaped by doubling it, in the same cases in which backslashes
have to be doubled in escape format; details appear below.
The hexadecimal digits can
be either upper or lower case, and whitespace is permitted between
digit pairs (but not within a digit pair nor in the starting
The hex format is compatible with a wide
range of external applications and protocols, and it tends to be
faster to convert than the escape format, so its use is preferred.
bytea Escape Format
The “escape” format is the traditional
PostgreSQL format for the
takes the approach of representing a binary string as a sequence
of ASCII characters, while converting those bytes that cannot be
represented as an ASCII character into special escape sequences.
If, from the point of view of the application, representing bytes
as characters makes sense, then this representation can be
convenient. But in practice it is usually confusing because it
fuzzes up the distinction between binary strings and character
strings, and also the particular escape mechanism that was chosen is
somewhat unwieldy. Therefore, this format should probably be avoided
for most new applications.
bytea values in escape format,
octets of certain
values must be escaped, while all octet
values can be escaped. In
general, to escape an octet, convert it into its three-digit
octal value and precede it
by a backslash (or two backslashes, if writing the value as a
literal using escape string syntax).
Backslash itself (octet decimal value 92) can alternatively be represented by
shows the characters that must be escaped, and gives the alternative
escape sequences where applicable.
bytea Literal Escaped Octets
|Decimal Octet Value||Description||Escaped Input Representation||Example||Output Representation|
|0 to 31 and 127 to 255||“non-printable” octets|
The requirement to escape non-printable octets varies depending on locale settings. In some instances you can get away with leaving them unescaped. Note that the result in each of the examples in Table 8.7 was exactly one octet in length, even though the output representation is sometimes more than one character.
The reason multiple backslashes are required, as shown
in Table 8.7, is that an input
string written as a string literal must pass through two parse
phases in the PostgreSQL server.
The first backslash of each pair is interpreted as an escape
character by the string-literal parser (assuming escape string
syntax is used) and is therefore consumed, leaving the second backslash of the
pair. (Dollar-quoted strings can be used to avoid this level
of escaping.) The remaining backslash is then recognized by the
bytea input function as starting either a three
digit octal value or escaping another backslash. For example,
a string literal passed to the server as
\001 after passing through the
escape string parser. The
\001 is then sent
bytea input function, where it is converted
to a single octet with a decimal value of 1. Note that the
single-quote character is not treated specially by
so it follows the normal rules for string literals. (See also
Bytea octets are output in
format by default. If you change bytea_output
“non-printable” octet are converted to
equivalent three-digit octal value and preceded by one backslash.
Most “printable” octets are output by their standard
representation in the client character set, e.g.:
SET bytea_output = 'escape'; SELECT 'abc \153\154\155 \052\251\124'::bytea; bytea ---------------- abc klm *\251T
The octet with decimal value 92 (backslash) is doubled in the output. Details are in Table 8.8.
bytea Output Escaped Octets
|Decimal Octet Value||Description||Escaped Output Representation||Example||Output Result|
|0 to 31 and 127 to 255||“non-printable” octets|
|32 to 126||“printable” octets||client character set representation|
Depending on the front end to PostgreSQL you use,
you might have additional work to do in terms of escaping and
bytea strings. For example, you might also
have to escape line feeds and carriage returns if your interface
automatically translates these.