PostgreSQL two dimensional array intersection - sql

Question

is it possible with postgres tools to intersect two-dimensional arrays?
For instance I have:
id arr
1 {{1,2}, {3,4}, {4,5}, {4,7}}
2 {{4,2}, {7,4}, {8,5}, {9,7}}
...
And I want to get all records that have {4,5} in theirs array, record id=1 here.
If I do something like:
select * from table where arr && '{4,5}'
I'll get both of those example records. It finds all records where are 4 and 5 are anywhere in the array - as if it was looking in fully extracted arrays, like {1,2,3,4,4,5,4,7}.

Answer 1

I thought of using functions from the extension intarray, but:
Many of these operations are only sensible for one-dimensional arrays.
Although they will accept input arrays of more dimensions, the data is
treated as though it were a linear array in storage order.
My other ideas:
Quick and dirty
WITH x(id, arr) AS (VALUES
(1, '{{1,2}, {3,4}, {4,5}, {4,7}}'::int[])
,(2, '{{4,2}, {7,4}, {8,5}, {9,7}}')
)
SELECT *
FROM x
WHERE arr::text LIKE '%{4,5}%';
The simple trick is to transform the array to its text representation and check with LIKE.
With array-functions
WITH x(id, arr) AS (
VALUES
(1, '{{1,2}, {3,4}, {4,5}, {4,7}}'::int[])
,(2, '{{4,2}, {7,4}, {8,5}, {9,7}}')
)
,y AS (
SELECT id, arr, generate_subscripts(arr, 1) AS i
FROM x
)
SELECT id, arr
FROM y
WHERE arr[i:i] = '{{4,5}}'::int[];
Or, the same in different notation, building on your example:
SELECT id, arr
FROM (
SELECT id, arr, generate_subscripts(arr, 1) AS i
FROM tbl
) x
WHERE arr[i:i] = '{{4,5}}'::int[];
This generates the array subscripts for the first dimension. With the help of this set-returning function we check each array-slice. Note the double brackets on the right hand side of the expression to create a two-dimensional array that matches.
If {4,5} can be in arr multiple times, add GROUP BY or a DISTINCT clause to avoid duplicate rows.