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 | E'\\000' |
SELECT E'\\000'::bytea; |
\000 |
39 | single quote |
'''' or E'\\047'
|
SELECT E'\''::bytea; |
' |
92 | backslash |
E'\\\\' or E'\\134'
|
SELECT E'\\\\'::bytea; |
\\ |
0 to 31 and 127 to 255 | “non-printable” octets |
E'\\ (octal value) |
SELECT E'\\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 (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 E'\\001'
becomes \001
after passing through the
escape string 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
single-quote 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 E'\\134'::bytea; |
\\ |
0 to 31 and 127 to 255 | “non-printable” octets |
\ (octal value) |
SELECT E'\\001'::bytea; |
\001 |
32 to 126 | “printable” octets | client character set representation | SELECT E'\\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.