数组也是我们sql中经常使用的类型。像unnest这个函数可以把数组转换成一个子查询，可以方便地实现一些功能。利用好数组有时候会提高系统性能。
比如减少JDBC执行次数，使用数组后能够使得查询条件使用索引等。
Table 9-40. Array Operators
| Operator | Description | Example | Result |
|---|---|---|---|
| = | equal | ARRAY[1.1,2.1,3.1]::int[] = ARRAY[1,2,3] | t |
| <> | not equal | ARRAY[1,3] <> ARRAY[1,4] | t |
| < | less than | ARRAY[1,3] < ARRAY[1,4] | t |
| > | greater than | ARRAY[1,4,3] > ARRAY[1,4] | t |
| <= | less than or equal | ARRAY[1,3] <= ARRAY[1,3] | t |
| >= | greater than or equal | ARRAY[1,3] >= ARRAY[1,3] | t |
| @> | contains | ARRAY[1,3] @> ARRAY[3,1] | t |
| <@ | is contained by | ARRAY[2,7] <@ ARRAY[1,7,6] | t |
| && | overlap (have elements in common) | ARRAY[1,3] && ARRAY[2,1] | t |
| || | array-to-array concatenation | ARRAY[1,3] || ARRAY[4,5,6] | {1,3,6} |
| || | array-to-array concatenation | ARRAY[1,3] || ARRAY[[4,6],[7,8,9]] | {{1,3},{4,6},{7,9}} |
| || | element-to-array concatenation | 3 || ARRAY[4,6] | {3,6} |
| || | array-to-element concatenation | ARRAY[4,6] || 7 | {4,6,7} |
Table 9-41. Array Functions
| Function | Return Type | Description | Example | Result |
|---|---|---|---|---|
array_append(anyarray,anyelement) |
anyarray | append an element to the end of an array | array_append(ARRAY[1,2],3) | {1,3} |
array_cat(anyarray,anyarray) |
anyarray | concatenate two arrays | array_cat(ARRAY[1,3],ARRAY[4,5]) | {1,5} |
array_ndims(anyarray) |
int | returns the number of dimensions of the array | array_ndims(ARRAY[[1,[4,6]]) | 2 |
array_dims(anyarray) |
text | returns a text representation of array's dimensions | array_dims(ARRAY[[1,6]]) | [1:2][1:3] |
array_fill(anyelement,int[],[,int[]]) |
anyarray | returns an array initialized with supplied value and dimensions,optionally with lower bounds other than 1 | array_fill(7,ARRAY[3],ARRAY[2]) | [2:4]={7,7} |
array_length(anyarray,int) |
int | returns the length of the requested array dimension | array_length(array[1,1) | 3 |
array_lower(anyarray,int) |
int | returns lower bound of the requested array dimension | array_lower('[0:2]={1,3}'::int[],1) | 0 |
array_prepend(anyelement,anyarray) |
anyarray | append an element to the beginning of an array | array_prepend(1,ARRAY[2,3]) | {1,3} |
array_to_string(anyarray,text [,text]) |
text | concatenates array elements using supplied delimiter and optional null string | array_to_string(ARRAY[1,NULL,5],','*') | 1,*,5 |
array_upper(anyarray,int) |
int | returns upper bound of the requested array dimension | array_upper(ARRAY[1,7],1) | 4 |
string_to_array(text,text]) |
text[] | splits string into array elements using supplied delimiter and optional null string | string_to_array('xx~^~yy~^~zz','~^~','yy') | {xx,zz} |
unnest(anyarray) |
setof anyelement | expand an array to a set of rows | unnest(ARRAY[1,2]) | 1 2(2 rows) |
