Why does `array_ndims(array[])` produce NULL?

Question

I'm trying to test for an empty array. I understand the reasoning behind array_length returing NULL since it's a 0-dimensional array, but why is array_ndims(array[]::int array) not 0, but rather NULL?

postgresql-archive.org/… please accept @Lukasz answer - as it the right one (at least today) and indeed is not documented (at least today) properly — Vao Tsun
– Vao Tsun, Commented Dec 29, 2017 at 19:09

Łukasz Kamiński · Accepted Answer · 2017-12-29 13:16:44Z

1

Looks like it will never return 0:

/* Sanity check: does it look like an array at all? */
    if (AARR_NDIM(v) <= 0 || AARR_NDIM(v) > MAXDIM)
        PG_RETURN_NULL();

You could write your own C function for this, simply change <= 0 into < 0. As for reasoning behind this, I can't find anything.

Source is in: https://www.postgresql.org/ftp/source/v10.1/ (same thing for 9.6.2, with is what I checked initially)

File: ./src/backend/utils/adt/arrayfuncs.c

Look for simple, 10 line function: array_ndims

edited Dec 29, 2017 at 13:16

answered Dec 29, 2017 at 12:56

Łukasz Kamiński

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3 Comments

Vao Tsun Over a year ago

please put a link to codebase. I also would expect to see the number of empty array to be zero. Maybe smbd just wanted to avoid checking if empty array is array?..

Łukasz Kamiński Over a year ago

@VaoTsun I suppose counting dimensions again (function doesn't hold it in variable) and returning them is a bit more time consuming than just returning NULL and someone writing this function figured no one needed to check 0 dimension arrays.

Vao Tsun Over a year ago

sorry, I meant smth like github.com/postgres/postgres/blob/master/src/backend/utils/adt/… :)

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