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8.10. Arrays

PostgreSQL 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.

8.10.1. 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 does not enforce the array size limits --- the behavior is the same as for arrays of unspecified length.

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 CREATE TABLE is simply documentation, it does not affect runtime behavior.

An alternative, SQL99-standard syntax may be used for one-dimensional arrays. pay_by_quarter could have been defined as:

    pay_by_quarter  integer ARRAY[4],

This syntax requires an integer constant to denote the array size. As before, however, PostgreSQL does not enforce the size restriction.

8.10.2. 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 may 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. (For all built-in types, this is the comma character ",".) 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.

(These kinds of array constants are actually only a special case of the generic type constants discussed in Section 4.1.2.4. 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"}, {}}');

INSERT INTO sal_emp
    VALUES ('Carol',
    '{20000, 25000, 25000, 25000}',
    '{{"talk", "consult"}, {"meeting"}}');

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.

This can lead to surprising results. For example, 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},{""}}
 Carol | {20000,25000,25000,25000} | {{talk},{meeting}}
(2 rows)

Because the [2][2] element of schedule is missing in each of the INSERT statements, the [1][2] element is discarded.

Note: Fixing this is on the to-do list.

The ARRAY expression syntax may also be used:

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)

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:

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

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 ARRAY expression syntax is discussed in more detail in Section 4.2.10.

8.10.3. Accessing Arrays

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:

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 the 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},{""}}
(1 row)

We could also have written

SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';

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 lower:upper. A lower bound of 1 is assumed for any subscript where only one value is specified, as in this example:

SELECT schedule[1:2][2] FROM sal_emp WHERE name = 'Bill';
         schedule
---------------------------
 {{meeting,lunch},{"",""}}
(1 row)

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][1:1]
(1 row)

array_dims produces a text result, which is convenient for people to read but perhaps not so convenient 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)

8.10.4. 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 may 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';

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

Array slice 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 running from -2 to 7.

New array values can also be constructed by 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 on to 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 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:

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)

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 on to 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
------------
 [0:2][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. 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:

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}}

8.10.5. Searching in Arrays

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:

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 uncertain. An alternative method is described in Section 9.17. The above query could be replaced by:

SELECT * FROM sal_emp WHERE 10000 = ANY (pay_by_quarter);

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

SELECT * FROM sal_emp WHERE 10000 = ALL (pay_by_quarter);

Tip: 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.

8.10.6. 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, type box uses a semicolon (;) 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.

As shown previously, when writing an array value you may write 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 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.

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 PostgreSQL releases.)

Note: 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 text array value containing a backslash and a double quote, you'd need to write

INSERT ... VALUES ('{"\\\\","\\""}');

The string-literal processor removes one level of backslashes, so that what arrives at the array-value parser looks like {"\\","\""}. In turn, the strings fed to the text data type's input routine become \ and " respectively. (If we were working with a data type whose input routine also treated backslashes specially, bytea for example, we might need as many as eight backslashes in the command to get one backslash into the stored array element.)

Tip: The ARRAY constructor syntax 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.

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