The existing methods which take slices of parameters work well when
directly passing a temporary slice (e.g. `c.query(s, &[&15, &"hi"])`,
but becomes limiting in other contexts like when programmatically
building up a query. We now offer methods which take iterators, which
are significantly more flexible for these kinds of use cases.
Because of the weird object safety of `ToSql`, we can't be generic over
`Iterator<Item = impl ToSql>`, but instead have to specifically work
with `Iterator<Item = &dyn ToSql>`. This may require a `.map()` or two
but should still work fine.
Closes#265
Our codec implementation originally just parsed single messages out of
the stream buffer. However, if a query returns a bunch of rows, we're
spending a ton of time shipping those individual messages from the
connection back to the Query stream. Instead, collect blocks of unparsed
messages that are as large as possible and send those back.
This cuts the processing time of the following query in half, from ~10
seconds to ~5:
`SELECT s.n, 'name' || s.n FROM generate_series(0, 9999999) AS s(n)`
At this point, almost all of the remainder of the time is spent parsing
the rows.
cc #450
copy_in data was previously copied ~3 times - once into the copy_in
buffer, once more to frame it into a CopyData frame, and once to write
that into the stream.
Our Codec is now a bit more interesting. Rather than just writing out
pre-encoded data, we can also send along unencoded CopyData so they can
be framed directly into the stream output buffer. In the future we can
extend this to e.g. avoid allocating for simple commands like Sync.
This also allows us to directly pass large copy_in input directly
through without rebuffering it.
