Re: proposal: searching in array function - array_position - Mailing list pgsql-hackers

Hi

here is a proof concept of array_offset function

possible question:

* used comparation "=" or "IS NOT DISTINCT FROM"

In this initial proof concept I used "IS NOT DISTINCT FROM" operator - but my opinion is not strong in this question. Both has some advantages and disadvantages.

Regards

Pavel

2015-01-16 19:12 GMT+01:00 Pavel Stehule <pavel.stehule@gmail.com>:

2015-01-16 18:37 GMT+01:00 Jim Nasby <Jim.Nasby@bluetreble.com>:

On 1/16/15 11:16 AM, Pavel Stehule wrote:

2015-01-16 17:57 GMT+01:00 Jim Nasby <Jim.Nasby@bluetreble.com <mailto:Jim.Nasby@bluetreble.com>>:

    On 1/16/15 3:39 AM, Pavel Stehule wrote:

        I am proposing a simple function, that returns a position of element in array.


    Yes please!

        FUNCTION array_position(anyarray, anyelement) RETURNS int


    That won't work on a multi-dimensional array. Ideally it needs to accept a slice or an element and return the specifier for the slice.


It is question, what is a result - probably, there can be a multidimensional variant, where result will be a array

array_position([1,2,3],2) --> 2
array_position([[1,2],[2,3],[3,4]], [2,3]) --> 2 /* 2nd parameter should to have N-1 dimension of first parameter */

The problem with that is you can't actually use '2' to get [2,3] back:

select (array[[1,2,3],[4,5,6],[7,8,9]])[1] IS NULL;
 ?column?
----------
 t
(1 row)

yes, but when you are searching a array in array you can use a full slice selection:

postgres=# select (ARRAY[[1,2],[4,5]])[1][1:2]; -- [1:2] should be a constant every time in this case -- so it should not be returned
  array 
---------
 {{1,2}}
(1 row)

 

I think the bigger problem here is we need something better than slices for handling subsets of arrays. Even if the function returned [2:2] it's still going to behave differently than it will in the non-array case because you won't be getting the expected number of dimensions back. :(

you cannot to return a slice and I don't propose it, although we can return a range type or array of range type - but still we cannot to use range for a arrays.

array_position_md([1,2,3],2) --> [2]
array_position_md([[1,2],[2,3],[3,4]], 2) --> [2,1]

another question is how to solve more than one occurrence on one value - probably two sets of functions - first returns first occurrence of value, second returns set of occurrence

Gee, if only way had some way to return multiple elements of something... ;P

In other words, I think all of these should actually return an array of positions. I think it's OK for someone that only cares about the first instance to just do [1].

there can be two functions - "position" - returns first and "positions" returns all as a array

 

--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com