datatype.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

     The sequence created for a <type>serial</type> column is
     automatically dropped when the owning column is dropped, and
     cannot be dropped otherwise.  (This was not true in
     <productname>PostgreSQL</productname> releases before 7.3.  Note
     that this automatic drop linkage will not occur for a sequence
     created by reloading a dump from a pre-7.3 database; the dump
     file does not contain the information needed to establish the
     dependency link.) Furthermore, this dependency between sequence
     and column is made only for the <type>serial</> column itself. If
     any other columns reference the sequence (perhaps by manually
     calling the <function>nextval</> function), they will be broken
     if the sequence is removed. Using a <type>serial</> column's sequence
     in such a fashion is considered bad form; if you wish to feed several
     columns from the same sequence generator, create the sequence as an
     independent object.
    </para>
   </sect2>
  </sect1>

  <sect1 id="datatype-money">
   <title>Monetary Types</title>

   <note>
    <para>
     The <type>money</type> type is deprecated. Use
     <type>numeric</type> or <type>decimal</type> instead, in
     combination with the <function>to_char</function> function.
    </para>
   </note>

   <para>
    The <type>money</type> type stores a currency amount with a fixed
    fractional precision; see <xref
    linkend="datatype-money-table">.
    Input is accepted in a variety of formats, including integer and
    floating-point literals, as well as <quote>typical</quote>
    currency formatting, such as <literal>'$1,000.00'</literal>.
    Output is generally in the latter form but depends on the locale.
   </para>

    <table id="datatype-money-table">
     <title>Monetary Types</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Storage Size</entry>
        <entry>Description</entry>
        <entry>Range</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>money</entry>
        <entry>4 bytes</entry>
        <entry>currency amount</entry>
        <entry>-21474836.48 to +21474836.47</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
  </sect1>


  <sect1 id="datatype-character">
   <title>Character Types</title>

   <indexterm zone="datatype-character">
    <primary>character string</primary>
    <secondary>data types</secondary>
   </indexterm>

   <indexterm>
    <primary>string</primary>
    <see>character string</see>
   </indexterm>

   <indexterm zone="datatype-character">
    <primary>character</primary>
   </indexterm>

   <indexterm zone="datatype-character">
    <primary>character varying</primary>
   </indexterm>

   <indexterm zone="datatype-character">
    <primary>text</primary>
   </indexterm>

   <indexterm zone="datatype-character">
    <primary>char</primary>
   </indexterm>

   <indexterm zone="datatype-character">
    <primary>varchar</primary>
   </indexterm>

    <table id="datatype-character-table">
     <title>Character Types</title>
     <tgroup cols="2">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><type>character varying(<replaceable>n</>)</type>, <type>varchar(<replaceable>n</>)</type></entry>
        <entry>variable-length with limit</entry>
       </row>
       <row>
        <entry><type>character(<replaceable>n</>)</type>, <type>char(<replaceable>n</>)</type></entry>
        <entry>fixed-length, blank padded</entry>
       </row>
       <row>
        <entry><type>text</type></entry>
        <entry>variable unlimited length</entry>
       </row>
     </tbody>
     </tgroup>
    </table>

   <para>
    <xref linkend="datatype-character-table"> shows the
    general-purpose character types available in
    <productname>PostgreSQL</productname>.
   </para>

   <para>
    <acronym>SQL</acronym> defines two primary character types:
    <type>character varying(<replaceable>n</>)</type> and
    <type>character(<replaceable>n</>)</type>, where <replaceable>n</>
    is a positive integer.  Both of these types can store strings up to
    <replaceable>n</> characters in length.  An attempt to store a
    longer string into a column of these types will result in an
    error, unless the excess characters are all spaces, in which case
    the string will be truncated to the maximum length. (This somewhat
    bizarre exception is required by the <acronym>SQL</acronym>
    standard.) If the string to be stored is shorter than the declared
    length, values of type <type>character</type> will be space-padded;
    values of type <type>character varying</type> will simply store the
    shorter
    string.
   </para>

    <para>
     If one explicitly casts a value to <type>character
     varying(<replaceable>n</>)</type> or
     <type>character(<replaceable>n</>)</type>, then an over-length
     value will be truncated to <replaceable>n</> characters without
     raising an error. (This too is required by the
     <acronym>SQL</acronym> standard.)
    </para>

   <note>
    <para>
     Prior to <productname>PostgreSQL</> 7.2, strings that were too long were
     always truncated without raising an error, in either explicit or
     implicit casting contexts.
    </para>
   </note>

   <para>
    The notations <type>varchar(<replaceable>n</>)</type> and
    <type>char(<replaceable>n</>)</type> are aliases for <type>character
    varying(<replaceable>n</>)</type> and
    <type>character(<replaceable>n</>)</type>, respectively.
    <type>character</type> without length specifier is equivalent to
    <type>character(1)</type>. If <type>character varying</type> is used
    without length specifier, the type accepts strings of any size. The
    latter is a <productname>PostgreSQL</> extension.
   </para>

   <para>
    In addition, <productname>PostgreSQL</productname> provides the
    <type>text</type> type, which stores strings of any length.
    Although the type <type>text</type> is not in the
    <acronym>SQL</acronym> standard, several other SQL database
    management systems have it as well.
   </para>

   <para>
    Values of type <type>character</type> are physically padded
    with spaces to the specified width <replaceable>n</>, and are
    stored and displayed that way.  However, the padding spaces are
    treated as semantically insignificant.  Trailing spaces are
    disregarded when comparing two values of type <type>character</type>,
    and they will be removed when converting a <type>character</type> value
    to one of the other string types.  Note that trailing spaces
    <emphasis>are</> semantically significant in
    <type>character varying</type> and <type>text</type> values.
   </para>

   <para>
    The storage requirement for data of these types is 4 bytes plus the
    actual string, and in case of <type>character</type> plus the
    padding. Long strings are compressed by the system automatically, so
    the physical requirement on disk may be less. Long values are also
    stored in background tables so they do not interfere with rapid
    access to the shorter column values. In any case, the longest
    possible character string that can be stored is about 1 GB. (The
    maximum value that will be allowed for <replaceable>n</> in the data
    type declaration is less than that. It wouldn't be very useful to
    change this because with multibyte character encodings the number of
    characters and bytes can be quite different anyway. If you desire to
    store long strings with no specific upper limit, use
    <type>text</type> or <type>character varying</type> without a length
    specifier, rather than making up an arbitrary length limit.)
   </para>

   <tip>
    <para>
     There are no performance differences between these three types,
     apart from the increased storage size when using the blank-padded
     type.  While <type>character(<replaceable>n</>)</type> has performance
     advantages in some other database systems, it has no such advantages in
     <productname>PostgreSQL</productname>.  In most situations
     <type>text</type> or <type>character varying</type> should be used
     instead.
    </para>
   </tip>

   <para>
    Refer to <xref linkend="sql-syntax-strings"> for information about
    the syntax of string literals, and to <xref linkend="functions">
    for information about available operators and functions. The
    database character set determines the character set used to store
    textual values; for more information on character set support,
    refer to <xref linkend="multibyte">.
   </para>

   <example>
    <title>Using the character types</title>

<programlisting>
CREATE TABLE test1 (a character(4));
INSERT INTO test1 VALUES ('ok');
SELECT a, char_length(a) FROM test1; -- <co id="co.datatype-char">
<computeroutput>
  a   | char_length
------+-------------
 ok   |           2
</computeroutput>

CREATE TABLE test2 (b varchar(5));
INSERT INTO test2 VALUES ('ok');
INSERT INTO test2 VALUES ('good      ');
INSERT INTO test2 VALUES ('too long');
<computeroutput>ERROR:  value too long for type character varying(5)</computeroutput>
INSERT INTO test2 VALUES ('too long'::varchar(5)); -- explicit truncation
SELECT b, char_length(b) FROM test2;
<computeroutput>
   b   | char_length
-------+-------------
 ok    |           2
 good  |           5
 too l |           5
</computeroutput>
</programlisting>
    <calloutlist>
     <callout arearefs="co.datatype-char">
      <para>
       The <function>char_length</function> function is discussed in
       <xref linkend="functions-string">.
      </para>
     </callout>
    </calloutlist>
   </example>

   <para>
    There are two other fixed-length character types in
    <productname>PostgreSQL</productname>, shown in <xref
    linkend="datatype-character-special-table">. The <type>name</type>
    type exists <emphasis>only</emphasis> for storage of identifiers
    in the internal system catalogs and is not intended for use by the general user. Its
    length is currently defined as 64 bytes (63 usable characters plus
    terminator) but should be referenced using the constant
    <symbol>NAMEDATALEN</symbol>. The length is set at compile time (and
    is therefore adjustable for special uses); the default maximum
    length may change in a future release. The type <type>"char"</type>
    (note the quotes) is different from <type>char(1)</type> in that it
    only uses one byte of storage. It is internally used in the system
    catalogs as a poor-man's enumeration type.
   </para>

    <table id="datatype-character-special-table">
     <title>Special Character Types</title>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Name</entry>
        <entry>Storage Size</entry>
        <entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><type>"char"</type></entry>
        <entry>1 byte</entry>
        <entry>single-character internal type</entry>
       </row>
       <row>
        <entry><type>name</type></entry>
        <entry>64 bytes</entry>
        <entry>internal type for object names</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  </sect1>

 <sect1 id="datatype-binary">
  <title>Binary Data Types</title>

  <indexterm zone="datatype-binary">
   <primary>binary data</primary>
  </indexterm>

  <indexterm zone="datatype-binary">
   <primary>bytea</primary>
  </indexterm>

   <para>
    The <type>bytea</type> data type allows storage of binary strings;
    see <xref linkend="datatype-binary-table">.
   </para>

   <table id="datatype-binary-table">
    <title>Binary Data Types</title>
    <tgroup cols="3">
     <thead>
      <row>
       <entry>Name</entry>
       <entry>Storage Size</entry>
       <entry>Description</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry><type>bytea</type></entry>
       <entry>4 bytes plus the actual binary string</entry>
       <entry>variable-length binary string</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    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 <quote>non-printable</quote>
    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 <quote>raw bytes</>, whereas character
    strings are appropriate for storing text.
   </para>

   <para>
    When entering <type>bytea</type> values, octets of certain values
    <emphasis>must</emphasis> be escaped (but all octet values
    <emphasis>can</emphasis> be escaped) when used as part of a string
    literal in an <acronym>SQL</acronym> 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. <xref linkend="datatype-binary-sqlesc"> shows the
    characters that must be escaped, and gives the alternate escape
    sequences where applicable.
   </para>