How to get the dimensionality of an ARRAY column?

I'm working on a project that collects information about your schema from the database directly. I can get the data_type of the column using information_schema.columns, which will tell me if it's an ARRAY or not. I can also get the underlying type (integer, bytea etc) of the ARRAY by querying information_schema.element_types as described here:

https://www.postgresql.org/docs/9.1/static/infoschema-element-types.html

My problem is that I also need to know how many dimensions the array has, whether it is integer[], or integer[][] for example. Does anyone know of a way to do this? Google isn't being very helpful here, hopefully someone more familiar with the Postgres spec can lead me in the right direction.

Solution

For starters, the dimensionality of an array is not reflected in the data type in Postgres. The syntax integer[][] is tolerated, but it's really just integer[] internally.
Read the manual here.

This means that dimensions can vary within the same array type (the same table column).

To get actual dimensions of a particular array value:

SELECT array_dims(my_arr);  -- [1:2][1:3]

Or to just get the number of dimensions:

SELECT array_ndims(my_arr);  -- 2

There are more array functions for similar needs. See table of array functions in the manual.

Use string[][] with ngpsql

If you need to enforce particular dimensions in a column, add a CHECK constraint. To enforce 2-dimensional arrays:

ALTER TABLE tbl ADD CONSTRAINT tbl_arr_col_must_have_2_dims
CHECK (array_ndims(arr_col) = 2);