PL/Python adding support for multi-dimensional arrays

Hi

Current implementation of PL/Python does not allow the use of
multi-dimensional arrays, for both input and output parameters. This forces
end users to introduce workarounds like casting arrays to text before
passing them to the functions and parsing them after, which is an
error-prone approach

This patch adds support for multi-dimensional arrays as both input and
output parameters for PL/Python functions. The number of dimensions
supported is limited by Postgres MAXDIM macrovariable, by default equal to
6. Both input and output multi-dimensional arrays should have fixed
dimension sizes, i.e. 2-d arrays should represent MxN matrix, 3-d arrays
represent MxNxK cube, etc.

This patch does not support multi-dimensional arrays of composite types, as
composite types in Python might be represented as iterators and there is no
obvious way to find out when the nested array stops and composite type
structure starts. For example, if we have a composite type of (int, text),
we can try to return "[ [ [1,'a'], [2,'b'] ], [ [3,'c'], [4,'d'] ] ]", and
it is hard to find out that the first two lists are lists, and the third
one represents structure. Things are getting even more complex when you
have arrays as members of composite type. This is why I think this
limitation is reasonable.

Given the function:

CREATE FUNCTION test_type_conversion_array_int4(x int4[]) RETURNS int4[] AS
$$
plpy.info(x, type(x))
return x
$$ LANGUAGE plpythonu;

Before patch:

# SELECT * FROM test_type_conversion_array_int4(ARRAY[[1,2,3],[4,5,6]]);
ERROR: cannot convert multidimensional array to Python list
DETAIL: PL/Python only supports one-dimensional arrays.
CONTEXT: PL/Python function "test_type_conversion_array_int4"

After patch:

# SELECT * FROM test_type_conversion_array_int4(ARRAY[[1,2,3],[4,5,6]]);
INFO: ([[1, 2, 3], [4, 5, 6]], <type 'list'>)
test_type_conversion_array_int4
---------------------------------
{{1,2,3},{4,5,6}}
(1 row)

--
Best regards,
Alexey Grishchenko