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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) |
（编辑：李大同）
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