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><DIV
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><H1
CLASS="SECT1"
><A
NAME="DATATYPE-BINARY"
>8.4. Binary Data Types</A
></H1
><P
> The <TT
CLASS="TYPE"
>bytea</TT
> data type allows storage of binary strings;
see <A
HREF="datatype-binary.html#DATATYPE-BINARY-TABLE"
>Table 8-6</A
>.
</P
><DIV
CLASS="TABLE"
><A
NAME="DATATYPE-BINARY-TABLE"
></A
><P
><B
>Table 8-6. Binary Data Types</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Storage Size</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="TYPE"
>bytea</TT
></TD
><TD
>1 or 4 bytes plus the actual binary string</TD
><TD
>variable-length binary string</TD
></TR
></TBODY
></TABLE
></DIV
><P
> 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 <SPAN
CLASS="QUOTE"
>"non-printable"</SPAN
>
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 <SPAN
CLASS="QUOTE"
>"raw bytes"</SPAN
>, whereas character
strings are appropriate for storing text.
</P
><P
> The <TT
CLASS="TYPE"
>bytea</TT
> type supports two external formats for
input and output: <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>'s historical
<SPAN
CLASS="QUOTE"
>"escape"</SPAN
> format, and <SPAN
CLASS="QUOTE"
>"hex"</SPAN
> format. Both
of these are always accepted on input. The output format depends
on the configuration parameter <A
HREF="runtime-config-client.html#GUC-BYTEA-OUTPUT"
>bytea_output</A
>;
the default is hex. (Note that the hex format was introduced in
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 9.0; earlier versions and some
tools don't understand it.)
</P
><P
> The <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> standard defines a different binary
string type, called <TT
CLASS="TYPE"
>BLOB</TT
> or <TT
CLASS="TYPE"
>BINARY LARGE
OBJECT</TT
>. The input format is different from
<TT
CLASS="TYPE"
>bytea</TT
>, but the provided functions and operators are
mostly the same.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN5374"
>8.4.1. <TT
CLASS="TYPE"
>bytea</TT
> Hex Format</A
></H2
><P
> The <SPAN
CLASS="QUOTE"
>"hex"</SPAN
> format encodes binary data as 2 hexadecimal digits
per byte, most significant nibble first. The entire string is
preceded by the sequence <TT
CLASS="LITERAL"
>\x</TT
> (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
<TT
CLASS="LITERAL"
>\x</TT
> sequence).
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.
</P
><P
> Example:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT E'\\xDEADBEEF';</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN5383"
>8.4.2. <TT
CLASS="TYPE"
>bytea</TT
> Escape Format</A
></H2
><P
> The <SPAN
CLASS="QUOTE"
>"escape"</SPAN
> format is the traditional
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> format for the <TT
CLASS="TYPE"
>bytea</TT
>
type. It
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. So this format should probably be avoided
for most new applications.
</P
><P
> When entering <TT
CLASS="TYPE"
>bytea</TT
> values in escape format,
octets of certain
values <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>must</I
></SPAN
> be escaped, while all octet
values <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>can</I
></SPAN
> 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 value 92) can alternatively be represented by
double backslashes.
<A
HREF="datatype-binary.html#DATATYPE-BINARY-SQLESC"
>Table 8-7</A
>
shows the characters that must be escaped, and gives the alternative
escape sequences where applicable.
</P
><DIV
CLASS="TABLE"
><A
NAME="DATATYPE-BINARY-SQLESC"
></A
><P
><B
>Table 8-7. <TT
CLASS="TYPE"
>bytea</TT
> Literal Escaped Octets</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><COL><COL><THEAD
><TR
><TH
>Decimal Octet Value</TH
><TH
>Description</TH
><TH
>Escaped Input Representation</TH
><TH
>Example</TH
><TH
>Output Representation</TH
></TR
></THEAD
><TBODY
><TR
><TD
>0</TD
><TD
>zero octet</TD
><TD
><TT
CLASS="LITERAL"
>E'\\000'</TT
></TD
><TD
><TT
CLASS="LITERAL"
>SELECT E'\\000'::bytea;</TT
></TD
><TD
><TT
CLASS="LITERAL"
>\000</TT
></TD
></TR
><TR
><TD
>39</TD
><TD
>single quote</TD
><TD
><TT
CLASS="LITERAL"
>''''</TT
> or <TT
CLASS="LITERAL"
>E'\\047'</TT
></TD
><TD
><TT
CLASS="LITERAL"
>SELECT E'\''::bytea;</TT
></TD
><TD
><TT
CLASS="LITERAL"
>'</TT
></TD
></TR
><TR
><TD
>92</TD
><TD
>backslash</TD
><TD
><TT
CLASS="LITERAL"
>E'\\\\'</TT
> or <TT
CLASS="LITERAL"
>E'\\134'</TT
></TD
><TD
><TT
CLASS="LITERAL"
>SELECT E'\\\\'::bytea;</TT
></TD
><TD
><TT
CLASS="LITERAL"
>\\</TT
></TD
></TR
><TR
><TD
>0 to 31 and 127 to 255</TD
><TD
><SPAN
CLASS="QUOTE"
>"non-printable"</SPAN
> octets</TD
><TD
><TT
CLASS="LITERAL"
>E'\\<TT
CLASS="REPLACEABLE"
><I
>xxx'</I
></TT
></TT
> (octal value)</TD
><TD
><TT
CLASS="LITERAL"
>SELECT E'\\001'::bytea;</TT
></TD
><TD
><TT
CLASS="LITERAL"
>\001</TT
></TD
></TR
></TBODY
></TABLE
></DIV
><P
> The requirement to escape <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>non-printable</I
></SPAN
> 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 <A
HREF="datatype-binary.html#DATATYPE-BINARY-SQLESC"
>Table 8-7</A
> was exactly one octet in
length, even though the output representation is sometimes
more than one character.
</P
><P
> The reason multiple backslashes are required, as shown
in <A
HREF="datatype-binary.html#DATATYPE-BINARY-SQLESC"
>Table 8-7</A
>, is that an input
string written as a string literal must pass through two parse
phases in the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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
<TT
CLASS="TYPE"
>bytea</TT
> input function as starting either a three
digit octal value or escaping another backslash. For example,
a string literal passed to the server as <TT
CLASS="LITERAL"
>E'\\001'</TT
>
becomes <TT
CLASS="LITERAL"
>\001</TT
> after passing through the
escape string parser. The <TT
CLASS="LITERAL"
>\001</TT
> is then sent
to the <TT
CLASS="TYPE"
>bytea</TT
> 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 <TT
CLASS="TYPE"
>bytea</TT
>,
so it follows the normal rules for string literals. (See also
<A
HREF="sql-syntax-lexical.html#SQL-SYNTAX-STRINGS"
>Section 4.1.2.1</A
>.)
</P
><P
> <TT
CLASS="TYPE"
>Bytea</TT
> octets are sometimes escaped when output. In general, each
<SPAN
CLASS="QUOTE"
>"non-printable"</SPAN
> octet is converted into
its equivalent three-digit octal value and preceded by one backslash.
Most <SPAN
CLASS="QUOTE"
>"printable"</SPAN
> octets are represented by their standard
representation in the client character set. The octet with decimal
value 92 (backslash) is doubled in the output.
Details are in <A
HREF="datatype-binary.html#DATATYPE-BINARY-RESESC"
>Table 8-8</A
>.
</P
><DIV
CLASS="TABLE"
><A
NAME="DATATYPE-BINARY-RESESC"
></A
><P
><B
>Table 8-8. <TT
CLASS="TYPE"
>bytea</TT
> Output Escaped Octets</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><COL><COL><THEAD
><TR
><TH
>Decimal Octet Value</TH
><TH
>Description</TH
><TH
>Escaped Output Representation</TH
><TH
>Example</TH
><TH
>Output Result</TH
></TR
></THEAD
><TBODY
><TR
><TD
>92</TD
><TD
>backslash</TD
><TD
><TT
CLASS="LITERAL"
>\\</TT
></TD
><TD
><TT
CLASS="LITERAL"
>SELECT E'\\134'::bytea;</TT
></TD
><TD
><TT
CLASS="LITERAL"
>\\</TT
></TD
></TR
><TR
><TD
>0 to 31 and 127 to 255</TD
><TD
><SPAN
CLASS="QUOTE"
>"non-printable"</SPAN
> octets</TD
><TD
><TT
CLASS="LITERAL"
>\<TT
CLASS="REPLACEABLE"
><I
>xxx</I
></TT
></TT
> (octal value)</TD
><TD
><TT
CLASS="LITERAL"
>SELECT E'\\001'::bytea;</TT
></TD
><TD
><TT
CLASS="LITERAL"
>\001</TT
></TD
></TR
><TR
><TD
>32 to 126</TD
><TD
><SPAN
CLASS="QUOTE"
>"printable"</SPAN
> octets</TD
><TD
>client character set representation</TD
><TD
><TT
CLASS="LITERAL"
>SELECT E'\\176'::bytea;</TT
></TD
><TD
><TT
CLASS="LITERAL"
>~</TT
></TD
></TR
></TBODY
></TABLE
></DIV
><P
> Depending on the front end to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> you use,
you might have additional work to do in terms of escaping and
unescaping <TT
CLASS="TYPE"
>bytea</TT
> strings. For example, you might also
have to escape line feeds and carriage returns if your interface
automatically translates these.
</P
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