postgres/doc/src/sgml/array.sgml

<!-- $Header: /cvsroot/pgsql/doc/src/sgml/array.sgml,v 1.31 2003/08/31 17:32:18 petere Exp $ -->

<sect1 id="arrays">
 <title>Arrays</title>

 <indexterm>
  <primary>array</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>

 <para>
  The syntax for <command>CREATE TABLE</command> allows the exact size of
  arrays to be specified, for example:

<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.  So, declaring number of dimensions or sizes in
  <command>CREATE TABLE</command> is simply documentation, it does not
  affect runtime behavior.
 </para>

 <para>
  An alternative, SQL99-standard syntax may be used for one-dimensional arrays.
  <structfield>pay_by_quarter</structfield> could have been defined as:
<programlisting>
    pay_by_quarter  integer ARRAY[4],
</programlisting>
  This syntax requires an integer constant to denote the array size.
  As before, however, <productname>PostgreSQL</> does not enforce the
  size restriction.
 </para>
 </sect2>

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

 <para>
  Now we can show some <command>INSERT</command> statements.  To write an array
  value as a literal constant, we enclose the element values within curly
  braces and separate them by commas.  (If you know C, this is not unlike the
  C syntax for initializing structures.)  We may put double quotes around any
  element value, and must do so if it contains commas or curly braces.
  (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 <literal>ARRAY</literal> 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. A mismatch causes an error report, rather than
  silently discarding values as in the previous case.
  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 the array elements are ordinary SQL constants or
  expressions; for instance, string literals are single quoted, instead of
  double quoted as they would be in an array literal.  The <literal>ARRAY</>
  expression syntax is discussed in more detail in <xref
  linkend="sql-syntax-array-constructors">.
 </para>
 </sect2>

 <sect2>
  <title>Accessing Arrays</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, as in this example:
<programlisting>
SELECT schedule[1:2][2] FROM sal_emp WHERE name = 'Bill';
         schedule
---------------------------
 {{meeting,lunch},{"",""}}
(1 row)
</programlisting>
 </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.  Dimensions can also be retrieved with
  <function>array_upper</function> and <function>array_lower</function>,
  which return the upper and lower bound of a
  specified array dimension, respectively.

<programlisting>
SELECT array_upper(schedule, 1) FROM sal_emp WHERE name = 'Carol';

 array_upper
-------------
           2
(1 row)
</programlisting>
 </para>
 </sect2>

 <sect2>
  <title>Modifying Arrays</title>

 <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 <literal>ARRAY</literal> expression syntax:

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

  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>

 </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 righthand 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 lefthand 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 lefthand operand's outer
  dimension. The result is an array comprising every element of the lefthand
  operand followed by every element of the righthand 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.
   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 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 white space 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 or ignorable white space.
  </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>

 <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>

 <tip>
  <para>
   The <literal>ARRAY</> constructor syntax 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>