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Chapter 6. Arrays

Postgres allows columns of a table to be defined as variable-length multi-dimensional arrays. Arrays of any built-in type or user-defined type can be created. To illustrate their use, we create this table:

CREATE TABLE sal_emp (
    name            text,
    pay_by_quarter  integer[],
    schedule        text[][]
);
The above query will create a table named sal_emp with a text string (name), a one-dimensional array of type integer (pay_by_quarter), which shall represent the employee's salary by quarter, and a two-dimensional array of text (schedule), which represents the employee's weekly schedule.

Now we do some INSERTs; note that when appending to an array, we enclose the values within braces and separate them by commas. If you know C, this is not unlike the syntax for initializing structures.

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"}}');

Now, we can run some queries on sal_emp. 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)
Postgres 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 subscript : upper subscript for one or more array dimensions. 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 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.

An array value can be replaced completely:

UPDATE sal_emp SET pay_by_quarter = '{25000,25000,27000,27000}'
    WHERE name = 'Carol';
or 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';

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.

The syntax for CREATE TABLE allows fixed-length arrays to be defined:

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 base type are all considered to be of the same type, regardless of size or number of dimensions.

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.

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):

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. Although it is not part of the primary PostgreSQL distribution, in the contributions directory, there is an extension to PostgreSQL that defines new functions and operators for iterating over array values. Using this, the above query could be:
SELECT * FROM sal_emp WHERE pay_by_quarter[1:4] *= 10000;
To search the entire array (not just specified columns), you could use:
SELECT * FROM sal_emp WHERE pay_by_quarter *= 10000;
In addition, you could find rows where the array had all values equal to 10 000 with:
SELECT * FROM sal_emp WHERE pay_by_quarter **= 10000;
To install this optional module, look in the contrib/array directory of the PostgreSQL source distribution.

Tip: Arrays are not lists; 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.

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