memo：PostgreSQL的数组函数

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)