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[postgresql] / doc / src / sgml / array.sgml

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<chapter id="arrays">
 <title>Arrays</title>

 <indexterm>
  <primary>arrays</primary>
 </indexterm>

 <para>
  <productname>PostgreSQL</productname> allows columns of a table to be
  defined as variable-length multidimensional arrays. Arrays of any
  built-in type or user-defined type can be created.  To illustrate
  their use, we create this table:
<programlisting>
CREATE TABLE sal_emp (
    name            text,
    pay_by_quarter  integer[],
    schedule        text[][]
);
</programlisting>
  As shown, an array data type is named by appending square brackets
  (<literal>[]</>) to the data type name of the array elements.
  The above query will create a table named
  <structname>sal_emp</structname> with a <type>text</type> string
  (<structfield>name</structfield>), a one-dimensional array of type
  <type>integer</type> (<structfield>pay_by_quarter</structfield>),
  which represents the employee's salary by quarter, and a
  two-dimensional array of <type>text</type>
  (<structfield>schedule</structfield>), which represents the
  employee's weekly schedule.
 </para>

 <para>
  Now we do some <command>INSERT</command>s.  Observe that to write an array
  value, we enclose the element values within curly braces and separate them
  by commas.  If you know C, this is not unlike the syntax for
  initializing structures.
     
<programlisting>
INSERT INTO sal_emp
    VALUES ('Bill',
    '{10000, 10000, 10000, 10000}',
    '{{"meeting", "lunch"}, {}}');

INSERT INTO sal_emp
    VALUES ('Carol',
    '{20000, 25000, 25000, 25000}',
    '{{"talk", "consult"}, {"meeting"}}');
</programlisting>
 </para>

 <para>
  Now, we can run some queries on <structname>sal_emp</structname>.
  First, we show how to access a single element of an array at a time.
  This query retrieves the names of the employees whose pay changed in
  the second quarter:
     
<programlisting>
SELECT name FROM sal_emp WHERE pay_by_quarter[1] &lt;&gt; pay_by_quarter[2];

 name
-------
 Carol
(1 row)
</programlisting>

  The array subscript numbers are written within square brackets.
  <productname>PostgreSQL</productname> uses the
  <quote>one-based</quote> numbering convention for arrays, that is,
  an array of <replaceable>n</> elements starts with <literal>array[1]</literal> and
  ends with <literal>array[<replaceable>n</>]</literal>.
 </para>

 <para>
  This query retrieves the third quarter pay of all employees:
     
<programlisting>
SELECT pay_by_quarter[3] FROM sal_emp;

 pay_by_quarter
----------------
          10000
          25000
(2 rows)
</programlisting>
 </para>

 <para>
  We can also access arbitrary rectangular slices of an array, or
  subarrays.  An array slice is denoted by writing
  <literal><replaceable>lower subscript</replaceable> :
  <replaceable>upper subscript</replaceable></literal> for one or more
  array dimensions.  This query retrieves the first item on Bill's
  schedule for the first two days of the week:
     
<programlisting>
SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';

      schedule
--------------------
 {{"meeting"},{""}}
(1 row)
</programlisting>

  We could also have written

<programlisting>
SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';
</programlisting>

  with the same result.  An array subscripting operation is taken to
  represent an array slice if any of the subscripts are written in the
  form <replaceable>lower</replaceable> <literal>:</literal>
  <replaceable>upper</replaceable>.  A lower bound of 1 is assumed for
  any subscript where only one value is specified.
 </para>

 <para>
  An array value can be replaced completely:

<programlisting>
UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}'
    WHERE name = 'Carol';
</programlisting>

  or updated at a single element:

<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>
  An array 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 an array
  value currently has 4 elements, it will have five elements after an
  update that assigns to array[5].  Currently, enlargement in this
  fashion is only allowed for one-dimensional arrays, not
  multidimensional arrays.
 </para>

 <para>
  The syntax for <command>CREATE TABLE</command> allows fixed-length
  arrays to be defined:

<programlisting>
CREATE TABLE tictactoe (
    squares   integer[3][3]
);
</programlisting>

  However, the current implementation does not enforce the array size
  limits --- the behavior is the same as for arrays of unspecified
  length.
 </para>

 <para>
  Actually, the current implementation does not enforce the declared
  number of dimensions either.  Arrays of a particular element type are
  all considered to be of the same type, regardless of size or number
  of dimensions.
 </para>

 <para>
  The current dimensions of any array value can be retrieved with the
  <function>array_dims</function> function:

<programlisting>
SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol';

 array_dims
------------
 [1:2][1:1]
(1 row)
</programlisting>

  <function>array_dims</function> produces a <type>text</type> result,
  which is convenient for people to read but perhaps not so convenient
  for programs.
 </para>

 <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):

<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 unknown. Although it is not part
  of the primary <productname>PostgreSQL</productname> distribution,
  there is an extension available that defines new functions and
  operators for iterating over array values. Using this, the above
  query could be:

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

  To search the entire array (not just specified columns), you could
  use:

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

  In addition, you could find rows where the array had all values
  equal to 10 000 with:

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

  To install this optional module, look in the
  <filename>contrib/array</filename> directory of the
  <productname>PostgreSQL</productname> source distribution.
 </para>

 <tip>
  <para>
   Arrays are not sets; using arrays in the manner described in the
   previous paragraph is often a sign of database misdesign.  The
   array field should generally be split off into a separate table.
   Tables can obviously be searched easily.
  </para>
 </tip>

 <note>
  <para>
   A limitation of the present array implementation is that individual
   elements of an array cannot be SQL NULLs.  The entire array can be set
   to NULL, but you can't have an array with some elements NULL and some
   not.  Fixing this is on the to-do list.
  </para>
 </note>

 <formalpara>
  <title>Quoting array elements.</title>
  <para>
   As shown above, when writing an array literal 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, double quotes, backslashes, or white space must be
   double-quoted.  To put a double quote or backslash in an array element
   value, precede it with a backslash.
  </para>
 </formalpara>

 <tip>
  <para>
   Remember that what you write in an SQL query 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 query to get one backslash into the stored array element.)
  </para>
 </tip>

</chapter>