Re: Add limit option to copy function

=?UTF-8?B?6rmA64yA7Zi4?= <daiho1(dot)kim(at)samsung(dot)com> writes:
> I suggest adding a limit option to the copy function that limits count of input/output.
> I think this will be useful for testing with sample data.

I'm quite skeptical of the value of this. On the output side, you
can already do it with

COPY (SELECT ... LIMIT n) TO wherever;

Moreover, that approach allows you to include an ORDER BY, which is
generally good practice in any query that includes LIMIT, in case
you'd like deterministic results.

On the input side, it's true that you'd have to resort to some
outside features (perhaps applying "head" to the input file, or
some such), or else copy the data into a temp table and post-process.
But that's true for most ways that you might want to adjust or
filter the input data; why should this one be different?

We don't consider that COPY is a general-purpose ETL engine, and
have resisted addition of features to it in the past because
they'd slow down the primary use-case. That objection applies
here too. Yeah, it's (probably) not a big slowdown ... but it's
hard to justify any cost at all for a feature that is outside
the design scope of COPY.

regards, tom lane