Array mega-patch.

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

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<sect1 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.
 </para>

 <sect2>
  <title>Declaration of Array Types</title>

 <para>
  To illustrate the use of array types, 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 command will create a table named
  <structname>sal_emp</structname> with a column of type
  <type>text</type> (<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>
 </sect2>

 <sect2>
  <title>Array Value Input</title>

 <para>
  Now we can show some <command>INSERT</command> statements.  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.  (More details appear below.)

<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>
  A limitation of the present array implementation is that individual
  elements of an array cannot be SQL null values.  The entire array can be set
  to null, but you can't have an array with some elements null and some
  not.
 </para>
 <para>
  This can lead to surprising results. For example, the result of the
  previous two inserts looks like this:
<programlisting>
SELECT * FROM sal_emp;
 name  |      pay_by_quarter       |      schedule
-------+---------------------------+--------------------
 Bill  | {10000,10000,10000,10000} | {{meeting},{""}}
 Carol | {20000,25000,25000,25000} | {{talk},{meeting}}
(2 rows)
</programlisting>
  Because the <literal>[2][2]</literal> element of
  <structfield>schedule</structfield> is missing in each of the
  <command>INSERT</command> statements, the <literal>[1][2]</literal>
  element is discarded.
 </para>

 <note>
  <para>
   Fixing this is on the to-do list.
  </para>
 </note>

 <para>
  The <command>ARRAY</command> expression syntax may also be used:
<programlisting>
INSERT INTO sal_emp
    VALUES ('Bill',
    ARRAY[10000, 10000, 10000, 10000],
    ARRAY[['meeting', 'lunch'], ['','']]);

INSERT INTO sal_emp
    VALUES ('Carol',
    ARRAY[20000, 25000, 25000, 25000],
    ARRAY[['talk', 'consult'], ['meeting', '']]);
SELECT * FROM sal_emp;
 name  |      pay_by_quarter       |           schedule
-------+---------------------------+-------------------------------
 Bill  | {10000,10000,10000,10000} | {{meeting,lunch},{"",""}}
 Carol | {20000,25000,25000,25000} | {{talk,consult},{meeting,""}}
(2 rows)
</programlisting>
  Note that with this syntax, multidimensional arrays must have matching
  extents for each dimension. This eliminates the missing-array-elements
  problem above. For example:
<programlisting>
INSERT INTO sal_emp
    VALUES ('Carol',
    ARRAY[20000, 25000, 25000, 25000],
    ARRAY[['talk', 'consult'], ['meeting']]);
ERROR:  Multidimensional arrays must have array expressions with matching dimensions
</programlisting>
  Also notice that string literals are single quoted instead of double quoted.
 </para>

 <note>
  <para>
   The examples in the rest of this section are based on the
   <command>ARRAY</command> expression syntax <command>INSERT</command>s.
  </para>
 </note>

 </sect2>

 <sect2>
  <title>Array Value References</title>

 <para>
  Now, we can run some queries on the table.
  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.
  By default <productname>PostgreSQL</productname> uses the
  one-based 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-bound</replaceable>:<replaceable>upper-bound</replaceable></literal>
  for one or more array dimensions.  For example, 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 always taken to
  represent an array slice if any of the subscripts are written in the form
  <literal><replaceable>lower</replaceable>:<replaceable>upper</replaceable></literal>.
  A lower bound of 1 is assumed for any subscript where only one value
  is specified; another example follows:
<programlisting>
SELECT schedule[1:2][2] FROM sal_emp WHERE name = 'Bill';
         schedule
---------------------------
 {{meeting,lunch},{"",""}}
(1 row)
</programlisting>
 </para>

 <para>
  Additionally, we can also access a single arbitrary array element of 
  a one-dimensional array with the <function>array_subscript</function>
  function:
<programlisting>
SELECT array_subscript(pay_by_quarter, 2) FROM sal_emp WHERE name = 'Bill';
 array_subscript
-----------------
           10000
(1 row)
</programlisting>
 </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 using the <command>ARRAY</command> expression syntax:

<programlisting>
UPDATE sal_emp SET pay_by_quarter = ARRAY[25000,25000,27000,27000]
    WHERE name = 'Carol';
</programlisting>

  <note>
   <para>
    Anywhere you can use the <quote>curly braces</quote> array syntax,
    you can also use the <command>ARRAY</command> expression syntax. The
    remainder of this section will illustrate only one or the other, but
    not both.
   </para>
  </note>

  An array may also be 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>

  A one-dimensional array may also be updated with the
  <function>array_assign</function> function:

<programlisting>
UPDATE sal_emp SET pay_by_quarter = array_assign(pay_by_quarter, 4, 15000)
    WHERE name = 'Bill';
</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 <literal>array[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>array[-2:7]</> to
  create an array with subscript values running from -2 to 7.
 </para>

 <para>
  An array can also be enlarged by using the concatenation operator,
  <command>||</command>.
<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>

  The concatenation operator allows a single element to be pushed on to the
  beginning or end of a one-dimensional array. It also allows two
  <replaceable>N</>-dimensional arrays, or an <replaceable>N</>-dimensional
  and an <replaceable>N+1</>-dimensional array. In the former case, the two
  <replaceable>N</>-dimension arrays become outer elements of an
  <replaceable>N+1</>-dimensional array. In the latter, the
  <replaceable>N</>-dimensional array is added as either the first or last
  outer element of the <replaceable>N+1</>-dimensional array.

  The array is extended in the direction of the push. Hence, by pushing
  onto the beginning of an array with a one-based subscript, a zero-based
  subscript array is created:

<programlisting>
SELECT array_dims(t.f) FROM (SELECT 1 || ARRAY[2,3] AS f) AS t;
 array_dims
------------
 [0:2]
(1 row)
</programlisting>
 </para>

 <para>
  An array can also be enlarged 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>

 <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>
  An alternative syntax for one-dimensional arrays may be used.
  <structfield>pay_by_quarter</structfield> could have been defined as:
<programlisting>
    pay_by_quarter  integer ARRAY[4],
</programlisting>
  This syntax may <emphasis>only</emphasis> be used with the integer
  constant to denote the array size.
 </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.  So, declaring number of dimensions or sizes in
  <command>CREATE TABLE</command> is simply documentation, it does not
  affect runtime behavior.
 </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.  <function>array_upper</function> and <function>
  array_lower</function> return the upper/lower bound of the
  given array dimension, respectively.
 </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 unknown. Although it is not built
  into <productname>PostgreSQL</productname>,
  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 slices), 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>
 </sect2>

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

  <para>
   The external 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.
   You may write whitespace before a left brace, after a right
   brace, or before any individual item string.  Whitespace after an item
   is not ignored, however: after skipping leading whitespace, everything
   up to the next right brace or delimiter is taken as the item value.
  </para>

  <para>
   As illustrated earlier in this chapter, arrays may also be represented
   using the <command>ARRAY</command> expression syntax. This 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 the keyword
   <command>ARRAY</command> and square brackets (<literal>[</> and
   <literal>]</>) around the array values, plus delimiter characters between
   adjacent items. The delimiter character is always a comma (<literal>,</>).
   When representing multidimensional arrays, the keyword
   <command>ARRAY</command> is only necessary for the outer level. For example,
   <literal>'{{"hello world", "happy birthday"}}'</literal> could be written as:
<programlisting>
SELECT ARRAY[['hello world', 'happy birthday']];
               array
------------------------------------
 {{"hello world","happy birthday"}}
(1 row)
</programlisting>
  or it also could be written as:
<programlisting>
SELECT ARRAY[ARRAY['hello world', 'happy birthday']];
               array
------------------------------------
 {{"hello world","happy birthday"}}
(1 row)
</programlisting>
  </para>

  <para>
   A final method to represent an array, is through an
   <command>ARRAY</command> sub-select expression. For example:
<programlisting>
SELECT ARRAY(SELECT oid FROM pg_proc WHERE proname LIKE 'bytea%');
                          ?column?
-------------------------------------------------------------
 {2011,1954,1948,1952,1951,1244,1950,2005,1949,1953,2006,31}
(1 row)
</programlisting>
  The sub-select may <emphasis>only</emphasis> return a single column. The
  resulting one-dimensional array will have an element for each row in the
  sub-select result, with an element type matching that of the sub-select's
  target column.
  </para>

  <para>
   Arrays may be cast from one type to another in similar fashion to other
   data types:

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

SELECT CAST(ARRAY[1,2,3] AS float8[]);
  array
---------
 {1,2,3}
(1 row)
</programlisting>

  </para>

 </sect2>

 <sect2>
  <title>Quoting Array Elements</title>

  <para>
   As shown above, 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 white space must be double-quoted.  To put a double
   quote or backslash in an 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 or ignorable white space.
  </para>

 <note>
  <para>
   The discussion in the preceding paragraph with respect to double quoting does
   not pertain to the <command>ARRAY</command> expression syntax. In that case,
   each element is quoted exactly as any other literal value of the element type.
  </para>
 </note>

  <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>

 <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.)
  </para>
 </note>
 </sect2>

</sect1>