array.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

<programlisting>
UPDATE sal_emp SET pay_by_quarter[4] = 15000
    WHERE name = 'Bill';
</programListing>

  or updated in a slice:

<programlisting>
UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}'
    WHERE name = 'Carol';
</programlisting>

 </para>

 <para>
  A stored array value can be enlarged by assigning to an element adjacent to
  those already present, or by assigning to a slice that is adjacent
  to or overlaps the data already present.  For example, if array
  <literal>myarray</> currently has 4 elements, it will have five
  elements after an update that assigns to <literal>myarray[5]</>.
  Currently, enlargement in this fashion is only allowed for one-dimensional
  arrays, not multidimensional arrays.
 </para>

 <para>
  Array slice assignment allows creation of arrays that do not use one-based
  subscripts.  For example one might assign to <literal>myarray[-2:7]</> to
  create an array with subscript values running from -2 to 7.
 </para>

 <para>
  New array values can also be constructed by using the concatenation operator,
  <literal>||</literal>.
<programlisting>
SELECT ARRAY[1,2] || ARRAY[3,4];
 ?column?
-----------
 {1,2,3,4}
(1 row)

SELECT ARRAY[5,6] || ARRAY[[1,2],[3,4]];
      ?column?
---------------------
 {{5,6},{1,2},{3,4}}
(1 row)
</programlisting>
 </para>

 <para>
  The concatenation operator allows a single element to be pushed on to the
  beginning or end of a one-dimensional array. It also accepts two
  <replaceable>N</>-dimensional arrays, or an <replaceable>N</>-dimensional
  and an <replaceable>N+1</>-dimensional array.
 </para>

 <para>
  When a single element is pushed on to the beginning of a one-dimensional
  array, the result is an array with a lower bound subscript equal to
  the right-hand operand's lower bound subscript, minus one. When a single
  element is pushed on to the end of a one-dimensional array, the result is
  an array retaining the lower bound of the left-hand operand. For example:
<programlisting>
SELECT array_dims(1 || ARRAY[2,3]);
 array_dims
------------
 [0:2]
(1 row)

SELECT array_dims(ARRAY[1,2] || 3);
 array_dims
------------
 [1:3]
(1 row)
</programlisting>
 </para>

 <para>
  When two arrays with an equal number of dimensions are concatenated, the
  result retains the lower bound subscript of the left-hand operand's outer
  dimension. The result is an array comprising every element of the left-hand
  operand followed by every element of the right-hand operand. For example:
<programlisting>
SELECT array_dims(ARRAY[1,2] || ARRAY[3,4,5]);
 array_dims
------------
 [1:5]
(1 row)

SELECT array_dims(ARRAY[[1,2],[3,4]] || ARRAY[[5,6],[7,8],[9,0]]);
 array_dims
------------
 [1:5][1:2]
(1 row)
</programlisting>
 </para>

 <para>
  When an <replaceable>N</>-dimensional array is pushed on to the beginning
  or end of an <replaceable>N+1</>-dimensional array, the result is
  analogous to the element-array case above. Each <replaceable>N</>-dimensional
  sub-array is essentially an element of the <replaceable>N+1</>-dimensional
  array's outer dimension. For example:
<programlisting>
SELECT array_dims(ARRAY[1,2] || ARRAY[[3,4],[5,6]]);
 array_dims
------------
 [0:2][1:2]
(1 row)
</programlisting>
 </para>

 <para>
  An array can also be constructed by using the functions
  <function>array_prepend</function>, <function>array_append</function>,
  or <function>array_cat</function>. The first two only support one-dimensional
  arrays, but <function>array_cat</function> supports multidimensional arrays.

  Note that the concatenation operator discussed above is preferred over
  direct use of these functions. In fact, the functions are primarily for use
  in implementing the concatenation operator. However, they may be directly
  useful in the creation of user-defined aggregates. Some examples:

<programlisting>
SELECT array_prepend(1, ARRAY[2,3]);
 array_prepend
---------------
 {1,2,3}
(1 row)

SELECT array_append(ARRAY[1,2], 3);
 array_append
--------------
 {1,2,3}
(1 row)

SELECT array_cat(ARRAY[1,2], ARRAY[3,4]);
 array_cat
-----------
 {1,2,3,4}
(1 row)

SELECT array_cat(ARRAY[[1,2],[3,4]], ARRAY[5,6]);
      array_cat
---------------------
 {{1,2},{3,4},{5,6}}
(1 row)

SELECT array_cat(ARRAY[5,6], ARRAY[[1,2],[3,4]]);
      array_cat
---------------------
 {{5,6},{1,2},{3,4}}
</programlisting>
 </para>
 </sect2>

 <sect2>
  <title>Searching in Arrays</title>

 <para>
  To search for a value in an array, you must check each value of the
  array. This can be done by hand, if you know the size of the array.
  For example:

<programlisting>
SELECT * FROM sal_emp WHERE pay_by_quarter[1] = 10000 OR
                            pay_by_quarter[2] = 10000 OR
                            pay_by_quarter[3] = 10000 OR
                            pay_by_quarter[4] = 10000;
</programlisting>

  However, this quickly becomes tedious for large arrays, and is not
  helpful if the size of the array is uncertain. An alternative method is
  described in <xref linkend="functions-comparisons">. The above
  query could be replaced by:

<programlisting>
SELECT * FROM sal_emp WHERE 10000 = ANY (pay_by_quarter);
</programlisting>

  In addition, you could find rows where the array had all values
  equal to 10000 with:

<programlisting>
SELECT * FROM sal_emp WHERE 10000 = ALL (pay_by_quarter);
</programlisting>

 </para>

 <tip>
  <para>
   Arrays are not sets; searching for specific array elements
   may be a sign of database misdesign.  Consider
   using a separate table with a row for each item that would be an
   array element.  This will be easier to search, and is likely to
   scale up better to large numbers of elements.
  </para>
 </tip>
 </sect2>

 <sect2>
  <title>Array Input and Output Syntax</title>

  <para>
   The external text representation of an array value consists of items that
   are interpreted according to the I/O conversion rules for the array's
   element type, plus decoration that indicates the array structure.
   The decoration consists of curly braces (<literal>{</> and <literal>}</>)
   around the array value plus delimiter characters between adjacent items.
   The delimiter character is usually a comma (<literal>,</>) but can be
   something else: it is determined by the <literal>typdelim</> setting
   for the array's element type.  (Among the standard data types provided
   in the <productname>PostgreSQL</productname> distribution, type
   <literal>box</> uses a semicolon (<literal>;</>) but all the others
   use comma.)  In a multidimensional array, each dimension (row, plane,
   cube, etc.) gets its own level of curly braces, and delimiters
   must be written between adjacent curly-braced entities of the same level.
  </para>

  <para>
   The array output routine will put double quotes around element values
   if they are empty strings or contain curly braces, delimiter characters,
   double quotes, backslashes, or white space.  Double quotes and backslashes
   embedded in element values will be backslash-escaped.  For numeric
   data types it is safe to assume that double quotes will never appear, but
   for textual data types one should be prepared to cope with either presence
   or absence of quotes.  (This is a change in behavior from pre-7.2
   <productname>PostgreSQL</productname> releases.)
  </para>

  <para>
   By default, the lower bound index value of an array's dimensions is
   set to one. If any of an array's dimensions has a lower bound index not
   equal to one, an additional decoration that indicates the actual
   array dimensions will precede the array structure decoration.
   This decoration consists of square brackets (<literal>[]</>)
   around each array dimension's lower and upper bounds, with
   a colon (<literal>:</>) delimiter character in between. The
   array dimension decoration is followed by an equal sign (<literal>=</>).
   For example:
<programlisting>
SELECT 1 || ARRAY[2,3] AS array;

     array
---------------
 [0:2]={1,2,3}
(1 row)

SELECT ARRAY[1,2] || ARRAY[[3,4]] AS array;

          array
--------------------------
 [0:1][1:2]={{1,2},{3,4}}
(1 row)
</programlisting>
  </para>

  <para>
   This syntax can also be used to specify non-default array subscripts
   in an array literal. For example:
<programlisting>
SELECT f1[1][-2][3] AS e1, f1[1][-1][5] AS e2
 FROM (SELECT '[1:1][-2:-1][3:5]={{{1,2,3},{4,5,6}}}'::int[] AS f1) AS ss;

 e1 | e2
----+----
  1 |  6
(1 row)
</programlisting>
  </para>

  <para>
   As shown previously, when writing an array value you may write double
   quotes around any individual array element. You <emphasis>must</> do so
   if the element value would otherwise confuse the array-value parser.
   For example, elements containing curly braces, commas (or whatever the
   delimiter character is), double quotes, backslashes, or leading or trailing
   whitespace must be double-quoted.  To put a double quote or backslash in a
   quoted array element value, precede it with a backslash. Alternatively, you
   can use backslash-escaping to protect all data characters that would
   otherwise be taken as array syntax.
  </para>

  <para>
   You may write whitespace before a left brace or after a right
   brace. You may also write whitespace before or after any individual item
   string. In all of these cases the whitespace will be ignored. However,
   whitespace within double-quoted elements, or surrounded on both sides by
   non-whitespace characters of an element, is not ignored.
  </para>

 <note>
  <para>
   Remember that what you write in an SQL command will first be interpreted
   as a string literal, and then as an array.  This doubles the number of
   backslashes you need.  For example, to insert a <type>text</> array
   value containing a backslash and a double quote, you'd need to write
<programlisting>
INSERT ... VALUES ('{"\\\\","\\""}');
</programlisting>
   The string-literal processor removes one level of backslashes, so that
   what arrives at the array-value parser looks like <literal>{"\\","\""}</>.
   In turn, the strings fed to the <type>text</> data type's input routine
   become <literal>\</> and <literal>"</> respectively.  (If we were working
   with a data type whose input routine also treated backslashes specially,
   <type>bytea</> for example, we might need as many as eight backslashes
   in the command to get one backslash into the stored array element.)
   Dollar quoting (see <xref linkend="sql-syntax-dollar-quoting">) may be
   used to avoid the need to double backslashes.
  </para>
 </note>

 <tip>
  <para>
   The <literal>ARRAY</> constructor syntax (see
   <xref linkend="sql-syntax-array-constructors">) is often easier to work
   with than the array-literal syntax when writing array values in SQL
   commands. In <literal>ARRAY</>, individual element values are written the
   same way they would be written when not members of an array.
  </para>
 </tip>
 </sect2>

</sect1>