问题
Given an array, how to find all combinations (subsets) of elements of a certain size in postgresql. For example, given an array [1, 2, 3, 4] all combinations of size 3 would be
[1, 2, 3],
[1, 2, 4],
[1, 3, 4],
[2, 3, 4]
Order is not important in combinations and therefore [1, 2, 3] and [3, 2, 1] are considered the same combination.
Update: The size of the combinations required must be specified at run-time as a parameter, so that the same function/query can be used to find combinations of any size n <= size of the array. The existing solution works only for combinations of size 3 and needs one additional cross join for every increase in the size, which is clearly not practical.
回答1:
The following function produces all the combinations of the requested size as a set of rows with one combination per row:
create or replace function get_combinations(source anyarray, size int) returns setof anyarray as $$
with recursive combinations(combination, indices) as (
select source[i:i], array[i] from generate_subscripts(source, 1) i
union all
select c.combination || source[j], c.indices || j
from combinations c, generate_subscripts(source, 1) j
where j > all(c.indices) and
array_length(c.combination, 1) < size
)
select combination from combinations
where array_length(combination, 1) = size;
$$ language sql;
This function is polymorphic in the array type.
来源：https://stackoverflow.com/questions/26560614/how-to-find-all-combinations-subset-of-any-size-of-an-array-in-postgresql