From: Vik Fearing <vik(at)postgresfriends(dot)org>
To: Isaac Morland <isaac(dot)morland(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: TRIM_ARRAY
Date: 2021-02-16 22:38:30
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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.

V2 attached.
Vik Fearing

Attachment Content-Type Size
0001-implement-trim_array.v2.patch text/x-patch 4.0 KB

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