TODO request: multi-dimensional arrays in PL/pythonU

On Tue, 2013-08-13 at 14:30 -0700, Josh Berkus wrote:

Currently PL/python has 1 dimension hardcoded for returning arrays:

create or replace function nparr ()
returns float[][]
language plpythonu
as $f$
from numpy import array
x = ((1.0,2.0),(3.0,4.0),(5.0,6.0),)
return x
$f$;

There is no way to know how many dimensions the function expects to get
back. (float[][] doesn't actually mean anything.) So when converting
the return value back to SQL, you'd have to guess, is the first element
convertible to float (how do you know?), if not, does it support the
sequence protocol, if yes, so let's try to construct a multidimensional
array. What if the first element is a float but the second is not?

It would be useful to have a solution for that, but it would need to be
more principled than what I just wrote.

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On Wed, Aug 14, 2013 at 9:34 PM, Peter Eisentraut <peter_e@gmx.net> wrote:

On Tue, 2013-08-13 at 14:30 -0700, Josh Berkus wrote:

Currently PL/python has 1 dimension hardcoded for returning arrays:

create or replace function nparr ()
returns float[][]
language plpythonu
as $f$
from numpy import array
x = ((1.0,2.0),(3.0,4.0),(5.0,6.0),)
return x
$f$;

There is no way to know how many dimensions the function expects to get
back. (float[][] doesn't actually mean anything.) So when converting
the return value back to SQL, you'd have to guess, is the first element
convertible to float (how do you know?), if not, does it support the
sequence protocol, if yes, so let's try to construct a multidimensional
array. What if the first element is a float but the second is not?

It would be useful to have a solution for that, but it would need to be
more principled than what I just wrote.

ndarray has a shape attribute. Perhaps they could be supported if they
follow the ndarray-like protocol? (ie: have a shape attribute)

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There is no way to know how many dimensions the function expects to get
back. (float[][] doesn't actually mean anything.) So when converting
the return value back to SQL, you'd have to guess, is the first element
convertible to float (how do you know?), if not, does it support the
sequence protocol, if yes, so let's try to construct a multidimensional
array. What if the first element is a float but the second is not?

It would be useful to have a solution for that, but it would need to be
more principled than what I just wrote.

Well, PL/R is able to return multi-dim arrays. So we have some code
precedent for this. Mind you, there's fewer checks required for PL/R,
because like Postgres it requires each dimension of the array to have
identical length and all items to be the same type.

Given that, it might be easier to support this first for numpy, which
also has the same restrictions.

--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com

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