# 9.18. Array Functions and Operators

Table 9-45 shows the operators available for array types.

Table 9-45. Array Operators

OperatorDescriptionExampleResult
= equalARRAY[1.1,2.1,3.1]::int[] = ARRAY[1,2,3]t
<> not equalARRAY[1,2,3] <> ARRAY[1,2,4]t
< less thanARRAY[1,2,3] < ARRAY[1,2,4]t
> greater thanARRAY[1,4,3] > ARRAY[1,2,4]t
<= less than or equalARRAY[1,2,3] <= ARRAY[1,2,3]t
>= greater than or equalARRAY[1,4,3] >= ARRAY[1,4,3]t
@> containsARRAY[1,4,3] @> ARRAY[3,1,3]t
<@ is contained byARRAY[2,2,7] <@ ARRAY[1,7,4,2,6]t
&& overlap (have elements in common)ARRAY[1,4,3] && ARRAY[2,1]t
|| array-to-array concatenationARRAY[1,2,3] || ARRAY[4,5,6]{1,2,3,4,5,6}
|| array-to-array concatenationARRAY[1,2,3] || ARRAY[[4,5,6],[7,8,9]]{{1,2,3},{4,5,6},{7,8,9}}
|| element-to-array concatenation3 || ARRAY[4,5,6]{3,4,5,6}
|| array-to-element concatenationARRAY[4,5,6] || 7{4,5,6,7}

The array ordering operators (<, >=, etc) compare the array contents element-by-element, using the default B-tree comparison function for the element data type, and sort based on the first difference. In multidimensional arrays the elements are visited in row-major order (last subscript varies most rapidly). If the contents of two arrays are equal but the dimensionality is different, the first difference in the dimensionality information determines the sort order. (This is a change from versions of PostgreSQL prior to 8.2: older versions would claim that two arrays with the same contents were equal, even if the number of dimensions or subscript ranges were different.)

The array containment operators (<@ and @>) consider one array to be contained in another one if each of its elements appears in the other one. Duplicates are not treated specially, thus ARRAY[1] and ARRAY[1,1] are each considered to contain the other.

See Section 8.15 for more details about array operator behavior. See Section 11.2 for more details about which operators support indexed operations.

Table 9-46 shows the functions available for use with array types. See Section 8.15 for more information and examples of the use of these functions.

Table 9-46. Array Functions

FunctionReturn TypeDescriptionExampleResult
`array_append`(anyarray, anyelement) anyarrayappend an element to the end of an arrayarray_append(ARRAY[1,2], 3){1,2,3}
`array_cat`(anyarray, anyarray) anyarrayconcatenate two arraysarray_cat(ARRAY[1,2,3], ARRAY[4,5]){1,2,3,4,5}
`array_ndims`(anyarray) intreturns the number of dimensions of the arrayarray_ndims(ARRAY[[1,2,3], [4,5,6]])2
`array_dims`(anyarray) textreturns a text representation of array's dimensionsarray_dims(ARRAY[[1,2,3], [4,5,6]])[1:2][1:3]
`array_fill`(anyelement, int[] [, int[]]) anyarrayreturns an array initialized with supplied value and dimensions, optionally with lower bounds other than 1array_fill(7, ARRAY[3], ARRAY[2])[2:4]={7,7,7}
`array_length`(anyarray, int) intreturns the length of the requested array dimensionarray_length(array[1,2,3], 1)3
`array_lower`(anyarray, int) intreturns lower bound of the requested array dimensionarray_lower('[0:2]={1,2,3}'::int[], 1)0
`array_prepend`(anyelement, anyarray) anyarrayappend an element to the beginning of an arrayarray_prepend(1, ARRAY[2,3]){1,2,3}
`array_remove`(anyarray, anyelement) anyarrayremove all elements equal to the given value from the array (array must be one-dimensional)array_remove(ARRAY[1,2,3,2], 2){1,3}
`array_replace`(anyarray, anyelement, anyelement) anyarrayreplace each array element equal to the given value with a new valuearray_replace(ARRAY[1,2,5,4], 5, 3){1,2,3,4}
`array_to_string`(anyarray, text [, text]) textconcatenates array elements using supplied delimiter and optional null stringarray_to_string(ARRAY[1, 2, 3, NULL, 5], ',', '*')1,2,3,*,5
`array_upper`(anyarray, int) intreturns upper bound of the requested array dimensionarray_upper(ARRAY[1,8,3,7], 1)4
`cardinality`(anyarray) intreturns the total number of elements in the array, or 0 if the array is emptycardinality(ARRAY[[1,2],[3,4]])4
`string_to_array`(text, text [, text]) text[]splits string into array elements using supplied delimiter and optional null stringstring_to_array('xx~^~yy~^~zz', '~^~', 'yy'){xx,NULL,zz}
`unnest`(anyarray) setof anyelementexpand an array to a set of rowsunnest(ARRAY[1,2])
```1
2```
(2 rows)
`unnest`(anyarray, anyarray [, ...]) setof anyelement, anyelement [, ...]expand multiple arrays (possibly of different types) to a set of rows. This is only allowed in the FROM clause; see Section 7.2.1.4unnest(ARRAY[1,2],ARRAY['foo','bar','baz'])
```1    foo
2    bar
NULL baz```
(3 rows)

In `string_to_array`, if the delimiter parameter is NULL, each character in the input string will become a separate element in the resulting array. If the delimiter is an empty string, then the entire input string is returned as a one-element array. Otherwise the input string is split at each occurrence of the delimiter string.

In `string_to_array`, if the null-string parameter is omitted or NULL, none of the substrings of the input will be replaced by NULL. In `array_to_string`, if the null-string parameter is omitted or NULL, any null elements in the array are simply skipped and not represented in the output string.

Note: There are two differences in the behavior of `string_to_array` from pre-9.1 versions of PostgreSQL. First, it will return an empty (zero-element) array rather than NULL when the input string is of zero length. Second, if the delimiter string is NULL, the function splits the input into individual characters, rather than returning NULL as before.

See also Section 9.20 about the aggregate function `array_agg` for use with arrays.