Re: proposal: searching in array function - array_position

On 1/16/15 11:16 AM, Pavel Stehule wrote:
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> 2015-01-16 17:57 GMT+01:00 Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com <mailto:Jim(dot)Nasby(at)bluetreble(dot)com>>:
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> On 1/16/15 3:39 AM, Pavel Stehule wrote:
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> I am proposing a simple function, that returns a position of element in array.
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>
> Yes please!
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> FUNCTION array_position(anyarray, anyelement) RETURNS int
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> 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.
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> It is question, what is a result - probably, there can be a multidimensional variant, where result will be a array
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> 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)

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. :(

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