Re: TRIM_ARRAY - Mailing list pgsql-hackers

From Vik Fearing
Subject Re: TRIM_ARRAY
Date
Msg-id bf054104-8c21-672a-f694-ed38fdec5cbc@postgresfriends.org
Whole thread Raw
In response to Re: TRIM_ARRAY  (Vik Fearing <vik@postgresfriends.org>)
List pgsql-hackers
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@postgresfriends.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



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