replace_text optimization (StringInfo to varlena)

Hi,

replace_text() in varlena.c builds the result in a StringInfo buffer,
and finishes by copying it into a freshly allocated varlena structure
with cstring_to_text_with_len(), in the same memory context.

It looks like that copy step could be avoided by preprending the
varlena header to the StringInfo to begin with, and return the buffer
as a text*, as in the attached patch.

On large strings, the time saved can be significant. For instance
I'm seeing a ~20% decrease in total execution time on a test with
lengths in the 2-3 MB range, like this:

select sum(length(
replace(repeat('abcdefghijklmnopqrstuvwxyz', i*10), 'abc', 'ABC')
))
from generate_series(10000,12000) as i;

Also, at a glance, there are a few other functions with similar
StringInfo-to-varlena copies that seem avoidable:
concat_internal(), text_format(), replace_text_regexp().

Are there reasons not to do this? Otherwise, should it be considered
in in a more principled way, such as adding to the StringInfo API
functions like void InitStringInfoForVarlena() and
text *StringInfoAsVarlena()?

Best regards,
--
Daniel Vérité
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