Change conversion for difftime to INTERVAL, not TIME

[hannes]

This PR changes the DuckDB representation of difftime R objects from TIME to INTERVAL. The reasoning here is that R's difftime can represent way larger time differences than single days, which is what TIME is for. This goes wrong if the difftime is longer than one day and one tries for example casting the value to a string.

Here is an example that demonstrates the problem

con <- DBI::dbConnect(duckdb::duckdb())
duckdb::duckdb_register(con, "difftime", data.frame(difftime=structure(710, units = "days", class = "difftime")))
print(DBI::dbGetQuery(con, 'select difftime, difftime::varchar difftime_string from difftime'))

Currently, this returns random garbage for the string

       difftime difftime_string
1 61344000 secs  \xd2\034:00:00

and triggers an assertion when built in debug mode.

With this PR, it returns

       difftime difftime_string
1 61344000 secs        710 days

[krlmlr]

Thanks, looks good. Let me sleep over the implications.

I wonder what we should return to R land if the database computes a TIME value. The following seems to work, although I don't understand why. OTOH, looks like we can't create fractional intervals from SQL?

con <- DBI::dbConnect(duckdb::duckdb())
print(DBI::dbGetQuery(con, "select TIME '01:02:03' as t"))
#>           t
#> 1 3723 secs
print(DBI::dbGetQuery(con, "select INTERVAL 1 HOUR as t"))
#>           t
#> 1 3600 secs
print(DBI::dbGetQuery(con, "select INTERVAL 1.2 HOUR as t"))
#> Error: {"exception_type":"Parser","exception_message":"syntax error at or near \"1.2\"","position":"16","error_subtype":"SYNTAX_ERROR"}

^{Created on 2024-04-30 with reprex v2.1.0}

[hannes]

we've always returned difftime for TIME because AFAIK R does not have another suitable time type.

[krlmlr]

Looks good, just one question.

[krlmlr]

This is consistent with the old implementation, but when converting datetimes I think we're rounding. Should we change this as part of the change here and further below?

Suggested change

	return Interval::FromMicro(int64_t(val * Interval::MICROS_PER_SEC));
	return Interval::FromMicro(int64_t(round(val * Interval::MICROS_PER_SEC)));

[krlmlr]

How do I construct a literal interval of 1.5 hours?

I'll try to formulate issues upstream.

[krlmlr]

No more objections after reading the docs. Tests are failing locally, need to investigate.

[krlmlr]

Thanks, part of the CRAN release just submitted.