Re: TRIM_ARRAY

On 2/16/21 11:38 PM, Vik Fearing wrote:
> On 2/16/21 7:32 PM, Isaac Morland wrote:
>> On Tue, 16 Feb 2021 at 12:54, Vik Fearing <vik(at)postgresfriends(dot)org> wrote:
>>
>>> The SQL standard defines a function called TRIM_ARRAY that surprisingly
>>> has syntax that looks like a function! So I implemented it using a thin
>>> wrapper around our array slice syntax. It is literally just ($1)[1:$2].
>>>
>>> An interesting case that I decided to handle by explaining it in the
>>> docs is that this won't give you the first n elements if your lower
>>> bound is not 1. My justification for this is 1) non-standard lower
>>> bounds are so rare in the wild that 2) people using them can just not
>>> use this function. The alternative is to go through the unnest dance
>>> (or write it in C) which defeats inlining.
>>>
>>
>> I don't recall ever seeing non-default lower bounds, so I actually think
>> it's OK to just rule out that scenario, but why not something like this:
>>
>> ($1)[:array_lower ($1, 1) + $2 - 1]
>
> I'm kind of embarrassed that I didn't think about doing that; it is a
> much better solution. You lose the non-standard bounds but I don't
> think there is any way besides C to keep the lower bound regardless of
> how you trim it.

I've made a bit of a mess out of this, but I partly blame the standard
which is very unclear. It actually describes trimming the right n
elements instead of the left n like I've done here. I'll be back later
with a better patch that does what it's actually supposed to.
--
Vik Fearing