The bytea
data type allows storage of binary strings;
see Table 8.6, “Binary Data Types”.
Table 8.6. Binary Data Types
Name | Storage Size | Description |
---|---|---|
bytea |
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 by two characteristics: First, binary strings specifically allow storing octets of value zero and other “non-printable” octets (usually, octets outside the 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.
When entering bytea
values, octets of certain values
must be escaped (but all octet values
can be escaped) when used as part of a string
literal in an SQL statement. In general, to
escape an octet, it is converted into the three-digit octal number
equivalent of its decimal octet value, and preceded by two
backslashes. Table 8.7, “bytea
Literal Escaped Octets” shows the
characters that must be escaped, and gives the alternate escape
sequences where applicable.
Table 8.7. bytea
Literal Escaped Octets
Decimal Octet Value | Description | Escaped Input Representation | Example | Output Representation |
---|---|---|---|---|
0 | zero octet | '\\000' |
SELECT '\\000'::bytea; |
\000 |
39 | single quote |
'\'' or '\\047'
|
SELECT '\''::bytea; |
' |
92 | backslash |
'\\\\' or '\\134'
|
SELECT '\\\\'::bytea; |
\\ |
0 to 31 and 127 to 255 | “non-printable” octets |
'\\ (octal value) |
SELECT '\\001'::bytea; |
\001 |
The requirement to escape “non-printable” octets actually
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, “bytea
Literal Escaped Octets” was exactly one octet in
length, even though the output representation of the zero octet and
backslash are more than one character.
The reason that you have to write so many backslashes, as shown in
Table 8.7, “bytea
Literal Escaped Octets”, 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 and is therefore consumed, leaving the
second backslash of the pair. 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'
becomes \001
after
passing through the string-literal parser. The
\001
is then sent to the bytea
input function, where it is converted to a single octet with a
decimal value of 1. Note that the apostrophe character is not
treated specially by bytea
, so it follows the normal
rules for string literals. (See also Section 4.1.2.1, “String Constants”.)
Bytea
octets are also escaped in the output. In general, each
“non-printable” octet is converted into
its equivalent three-digit octal value and preceded by one backslash.
Most “printable” octets are represented by their standard
representation in the client character set. The octet with decimal
value 92 (backslash) has a special alternative output representation.
Details are in Table 8.8, “bytea
Output Escaped Octets”.
Table 8.8. bytea
Output Escaped Octets
Decimal Octet Value | Description | Escaped Output Representation | Example | Output Result |
---|---|---|---|---|
92 | backslash | \\ |
SELECT '\\134'::bytea; |
\\ |
0 to 31 and 127 to 255 | “non-printable” octets |
\ (octal value) |
SELECT '\\001'::bytea; |
\001 |
32 to 126 | “printable” octets | client character set representation | SELECT '\\176'::bytea; |
~ |
Depending on the front end to PostgreSQL you use,
you may have additional work to do in terms of escaping and
unescaping bytea
strings. For example, you may also
have to escape line feeds and carriage returns if your interface
automatically translates these.
The SQL standard defines a different binary
string type, called BLOB
or BINARY LARGE
OBJECT
. The input format is different from
bytea
, but the provided functions and operators are
mostly the same.