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8.15. Arrays #

   8.15.1. Declaration of Array Types
   8.15.2. Array Value Input
   8.15.3. Accessing Arrays
   8.15.4. Modifying Arrays
   8.15.5. Searching in Arrays
   8.15.6. Array Input and Output Syntax

   PostgreSQL allows columns of a table to be defined as variable-length
   multidimensional arrays. Arrays of any built-in or user-defined base
   type, enum type, composite type, range type, or domain can be created.

8.15.1. Declaration of Array Types #

   To illustrate the use of array types, we create this table:
CREATE TABLE sal_emp (
    name            text,
    pay_by_quarter  integer[],
    schedule        text[][]
);

   As shown, an array data type is named by appending square brackets ([])
   to the data type name of the array elements. The above command will
   create a table named sal_emp with a column of type text (name), a
   one-dimensional array of type integer (pay_by_quarter), which
   represents the employee's salary by quarter, and a two-dimensional
   array of text (schedule), which represents the employee's weekly
   schedule.

   The syntax for CREATE TABLE allows the exact size of arrays to be
   specified, for example:
CREATE TABLE tictactoe (
    squares   integer[3][3]
);

   However, the current implementation ignores any supplied array size
   limits, i.e., the behavior is the same as for arrays of unspecified
   length.

   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 the array size or number of dimensions in
   CREATE TABLE is simply documentation; it does not affect run-time
   behavior.

   An alternative syntax, which conforms to the SQL standard by using the
   keyword ARRAY, can be used for one-dimensional arrays. pay_by_quarter
   could have been defined as:
    pay_by_quarter  integer ARRAY[4],

   Or, if no array size is to be specified:
    pay_by_quarter  integer ARRAY,

   As before, however, PostgreSQL does not enforce the size restriction in
   any case.

8.15.2. Array Value Input #

   To write an array value as a literal constant, 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.) You can
   put double quotes around any element value, and must do so if it
   contains commas or curly braces. (More details appear below.) Thus, the
   general format of an array constant is the following:
'{ val1 delim val2 delim ... }'

   where delim is the delimiter character for the type, as recorded in its
   pg_type entry. Among the standard data types provided in the PostgreSQL
   distribution, all use a comma (,), except for type box which uses a
   semicolon (;). Each val is either a constant of the array element type,
   or a subarray. An example of an array constant is:
'{{1,2,3},{4,5,6},{7,8,9}}'

   This constant is a two-dimensional, 3-by-3 array consisting of three
   subarrays of integers.

   To set an element of an array constant to NULL, write NULL for the
   element value. (Any upper- or lower-case variant of NULL will do.) If
   you want an actual string value “NULL”, you must put double quotes
   around it.

   (These kinds of array constants are actually only a special case of the
   generic type constants discussed in Section 4.1.2.7. The constant is
   initially treated as a string and passed to the array input conversion
   routine. An explicit type specification might be necessary.)

   Now we can show some INSERT statements:
INSERT INTO sal_emp
    VALUES ('Bill',
    '{10000, 10000, 10000, 10000}',
    '{{"meeting", "lunch"}, {"training", "presentation"}}');

INSERT INTO sal_emp
    VALUES ('Carol',
    '{20000, 25000, 25000, 25000}',
    '{{"breakfast", "consulting"}, {"meeting", "lunch"}}');

   The result of the previous two inserts looks like this:
SELECT * FROM sal_emp;
 name  |      pay_by_quarter       |                 schedule
-------+---------------------------+-------------------------------------------
 Bill  | {10000,10000,10000,10000} | {{meeting,lunch},{training,presentation}}
 Carol | {20000,25000,25000,25000} | {{breakfast,consulting},{meeting,lunch}}
(2 rows)

   Multidimensional arrays must have matching extents for each dimension.
   A mismatch causes an error, for example:
INSERT INTO sal_emp
    VALUES ('Bill',
    '{10000, 10000, 10000, 10000}',
    '{{"meeting", "lunch"}, {"meeting"}}');
ERROR:  malformed array literal: "{{"meeting", "lunch"}, {"meeting"}}"
DETAIL:  Multidimensional arrays must have sub-arrays with matching dimensions.

   The ARRAY constructor syntax can also be used:
INSERT INTO sal_emp
    VALUES ('Bill',
    ARRAY[10000, 10000, 10000, 10000],
    ARRAY[['meeting', 'lunch'], ['training', 'presentation']]);

INSERT INTO sal_emp
    VALUES ('Carol',
    ARRAY[20000, 25000, 25000, 25000],
    ARRAY[['breakfast', 'consulting'], ['meeting', 'lunch']]);

   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 ARRAY
   constructor syntax is discussed in more detail in Section 4.2.12.

8.15.3. Accessing Arrays #

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

 name
-------
 Carol
(1 row)

   The array subscript numbers are written within square brackets. By
   default PostgreSQL uses a one-based numbering convention for arrays,
   that is, an array of n elements starts with array[1] and ends with
   array[n].

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

 pay_by_quarter
----------------
          10000
          25000
(2 rows)

   We can also access arbitrary rectangular slices of an array, or
   subarrays. An array slice is denoted by writing lower-bound:upper-bound
   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:
SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';

        schedule
------------------------
 {{meeting},{training}}
(1 row)

   If any dimension is written as a slice, i.e., contains a colon, then
   all dimensions are treated as slices. Any dimension that has only a
   single number (no colon) is treated as being from 1 to the number
   specified. For example, [2] is treated as [1:2], as in this example:
SELECT schedule[1:2][2] FROM sal_emp WHERE name = 'Bill';

                 schedule
-------------------------------------------
 {{meeting,lunch},{training,presentation}}
(1 row)

   To avoid confusion with the non-slice case, it's best to use slice
   syntax for all dimensions, e.g., [1:2][1:1], not [2][1:1].

   It is possible to omit the lower-bound and/or upper-bound of a slice
   specifier; the missing bound is replaced by the lower or upper limit of
   the array's subscripts. For example:
SELECT schedule[:2][2:] FROM sal_emp WHERE name = 'Bill';

        schedule
------------------------
 {{lunch},{presentation}}
(1 row)

SELECT schedule[:][1:1] FROM sal_emp WHERE name = 'Bill';

        schedule
------------------------
 {{meeting},{training}}
(1 row)

   An array subscript expression will return null if either the array
   itself or any of the subscript expressions are null. Also, null is
   returned if a subscript is outside the array bounds (this case does not
   raise an error). For example, if schedule currently has the dimensions
   [1:3][1:2] then referencing schedule[3][3] yields NULL. Similarly, an
   array reference with the wrong number of subscripts yields a null
   rather than an error.

   An array slice expression likewise yields null if the array itself or
   any of the subscript expressions are null. However, in other cases such
   as selecting an array slice that is completely outside the current
   array bounds, a slice expression yields an empty (zero-dimensional)
   array instead of null. (This does not match non-slice behavior and is
   done for historical reasons.) If the requested slice partially overlaps
   the array bounds, then it is silently reduced to just the overlapping
   region instead of returning null.

   The current dimensions of any array value can be retrieved with the
   array_dims function:
SELECT array_dims(schedule) FROM sal_emp WHERE name = 'Carol';

 array_dims
------------
 [1:2][1:2]
(1 row)

   array_dims produces a text result, which is convenient for people to
   read but perhaps inconvenient for programs. Dimensions can also be
   retrieved with array_upper and array_lower, which return the upper and
   lower bound of a specified array dimension, respectively:
SELECT array_upper(schedule, 1) FROM sal_emp WHERE name = 'Carol';

 array_upper
-------------
           2
(1 row)

   array_length will return the length of a specified array dimension:
SELECT array_length(schedule, 1) FROM sal_emp WHERE name = 'Carol';

 array_length
--------------
            2
(1 row)

   cardinality returns the total number of elements in an array across all
   dimensions. It is effectively the number of rows a call to unnest would
   yield:
SELECT cardinality(schedule) FROM sal_emp WHERE name = 'Carol';

 cardinality
-------------
           4
(1 row)

8.15.4. Modifying Arrays #

   An array value can be replaced completely:
UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}'
    WHERE name = 'Carol';

   or using the ARRAY expression syntax:
UPDATE sal_emp SET pay_by_quarter = ARRAY[25000,25000,27000,27000]
    WHERE name = 'Carol';

   An array can also be updated at a single element:
UPDATE sal_emp SET pay_by_quarter[4] = 15000
    WHERE name = 'Bill';

   or updated in a slice:
UPDATE sal_emp SET pay_by_quarter[1:2] = '{27000,27000}'
    WHERE name = 'Carol';

   The slice syntaxes with omitted lower-bound and/or upper-bound can be
   used too, but only when updating an array value that is not NULL or
   zero-dimensional (otherwise, there is no existing subscript limit to
   substitute).

   A stored array value can be enlarged by assigning to elements not
   already present. Any positions between those previously present and the
   newly assigned elements will be filled with nulls. For example, if
   array myarray currently has 4 elements, it will have six elements after
   an update that assigns to myarray[6]; myarray[5] will contain null.
   Currently, enlargement in this fashion is only allowed for
   one-dimensional arrays, not multidimensional arrays.

   Subscripted assignment allows creation of arrays that do not use
   one-based subscripts. For example one might assign to myarray[-2:7] to
   create an array with subscript values from -2 to 7.

   New array values can also be constructed using the concatenation
   operator, ||:
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)

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

   When a single element is pushed onto either the beginning or end of a
   one-dimensional array, the result is an array with the same lower bound
   subscript as the array operand. For example:
SELECT array_dims(1 || '[0:1]={2,3}'::int[]);
 array_dims
------------
 [0:2]
(1 row)

SELECT array_dims(ARRAY[1,2] || 3);
 array_dims
------------
 [1:3]
(1 row)

   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:
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)

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

   An array can also be constructed by using the functions array_prepend,
   array_append, or array_cat. The first two only support one-dimensional
   arrays, but array_cat supports multidimensional arrays. Some examples:
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}}

   In simple cases, the concatenation operator discussed above is
   preferred over direct use of these functions. However, because the
   concatenation operator is overloaded to serve all three cases, there
   are situations where use of one of the functions is helpful to avoid
   ambiguity. For example consider:
SELECT ARRAY[1, 2] || '{3, 4}';  -- the untyped literal is taken as an array
 ?column?
-----------
 {1,2,3,4}

SELECT ARRAY[1, 2] || '7';                 -- so is this one
ERROR:  malformed array literal: "7"

SELECT ARRAY[1, 2] || NULL;                -- so is an undecorated NULL
 ?column?
----------
 {1,2}
(1 row)

SELECT array_append(ARRAY[1, 2], NULL);    -- this might have been meant
 array_append
--------------
 {1,2,NULL}

   In the examples above, the parser sees an integer array on one side of
   the concatenation operator, and a constant of undetermined type on the
   other. The heuristic it uses to resolve the constant's type is to
   assume it's of the same type as the operator's other input — in this
   case, integer array. So the concatenation operator is presumed to
   represent array_cat, not array_append. When that's the wrong choice, it
   could be fixed by casting the constant to the array's element type; but
   explicit use of array_append might be a preferable solution.

8.15.5. Searching in Arrays #

   To search for a value in an array, each value must be checked. This can
   be done manually, if you know the size of the array. For example:
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;

   However, this quickly becomes tedious for large arrays, and is not
   helpful if the size of the array is unknown. An alternative method is
   described in Section 9.25. The above query could be replaced by:
SELECT * FROM sal_emp WHERE 10000 = ANY (pay_by_quarter);

   In addition, you can find rows where the array has all values equal to
   10000 with:
SELECT * FROM sal_emp WHERE 10000 = ALL (pay_by_quarter);

   Alternatively, the generate_subscripts function can be used. For
   example:
SELECT * FROM
   (SELECT pay_by_quarter,
           generate_subscripts(pay_by_quarter, 1) AS s
      FROM sal_emp) AS foo
 WHERE pay_by_quarter[s] = 10000;

   This function is described in Table 9.70.

   You can also search an array using the && operator, which checks
   whether the left operand overlaps with the right operand. For instance:
SELECT * FROM sal_emp WHERE pay_by_quarter && ARRAY[10000];

   This and other array operators are further described in Section 9.19.
   It can be accelerated by an appropriate index, as described in
   Section 11.2.

   You can also search for specific values in an array using the
   array_position and array_positions functions. The former returns the
   subscript of the first occurrence of a value in an array; the latter
   returns an array with the subscripts of all occurrences of the value in
   the array. For example:
SELECT array_position(ARRAY['sun','mon','tue','wed','thu','fri','sat'], 'mon');
 array_position
----------------
              2
(1 row)

SELECT array_positions(ARRAY[1, 4, 3, 1, 3, 4, 2, 1], 1);
 array_positions
-----------------
 {1,4,8}
(1 row)

Tip

   Arrays are not sets; searching for specific array elements can 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 better for a large number of elements.

8.15.6. Array Input and Output Syntax #

   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 ({ and }) around the
   array value plus delimiter characters between adjacent items. The
   delimiter character is usually a comma (,) but can be something else:
   it is determined by the typdelim setting for the array's element type.
   Among the standard data types provided in the PostgreSQL distribution,
   all use a comma, except for type box, which uses a semicolon (;). 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.

   The array output routine will put double quotes around element values
   if they are empty strings, contain curly braces, delimiter characters,
   double quotes, backslashes, or white space, or match the word NULL.
   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 the presence or absence of quotes.

   By default, the lower bound index value of an array's dimensions is set
   to one. To represent arrays with other lower bounds, the array
   subscript ranges can be specified explicitly before writing the array
   contents. This decoration consists of square brackets ([]) around each
   array dimension's lower and upper bounds, with a colon (:) delimiter
   character in between. The array dimension decoration is followed by an
   equal sign (=). For example:
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)

   The array output routine will include explicit dimensions in its result
   only when there are one or more lower bounds different from one.

   If the value written for an element is NULL (in any case variant), the
   element is taken to be NULL. The presence of any quotes or backslashes
   disables this and allows the literal string value “NULL” to be entered.
   Also, for backward compatibility with pre-8.2 versions of PostgreSQL,
   the array_nulls configuration parameter can be turned off to suppress
   recognition of NULL as a NULL.

   As shown previously, when writing an array value you can use double
   quotes around any individual array element. You must do so if the
   element value would otherwise confuse the array-value parser. For
   example, elements containing curly braces, commas (or the data type's
   delimiter character), double quotes, backslashes, or leading or
   trailing whitespace must be double-quoted. Empty strings and strings
   matching the word NULL must be quoted, too. To put a double quote or
   backslash in a quoted array element value, precede it with a backslash.
   Alternatively, you can avoid quotes and use backslash-escaping to
   protect all data characters that would otherwise be taken as array
   syntax.

   You can add whitespace before a left brace or after a right brace. You
   can also add 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.

Tip

   The ARRAY constructor syntax (see Section 4.2.12) is often easier to
   work with than the array-literal syntax when writing array values in
   SQL commands. In ARRAY, individual element values are written the same
   way they would be written when not members of an array.