Add `Query::fetch_owned`

[Yen]

We are using Query::fetch as we have data we want to select that is too big to fit into memory. Unfortunately, it seems the implementation buffers the whole response rather than reusing the memory. I understand the lifetime-related rationale for this as outlined in #24.

In #247 the RowOwned type was added to help in these scenarios. A function similar to Query::fetch but that is constrained to RowOwned such that it can reuse the memory would solve our scenario and allow querying massive or infinite data sets.

(We cannot split the query up at the moment as we use FINAL and rely on the atomic nature of the single query.)

Thanks

[loyd]

Hi,

Unfortunately, it seems the implementation buffers the whole response rather than reusing the memory

It should reuse the memory; there is no "whole response" buffering in the client. Do you have a counterexample?

Query::fetch() returns RowCursor, which receives data from the database in chunks, trying to reuse the buffer for newly incoming data.

Can you show your code? Do you use wait_end_of_query or buffer_size settings?

[Yen]

Hey, unfortunately we can't share the code, but it's something along the lines of:

let cursor = clickhouse_client
    .query("SELECT lots_of_data")
    .fetch::<T>()?;

while let Some(row) = cursor.next().await? {
    // Handle row.
}

What it seems to be is that the moment the cursor object is dropped we get back 100GB+ of memory in our case.

The implementation of Bytes and BytesExt is quite confusing so cannot point out exactly why/how the data is not being reused, but the signature of RowCursor::next implies that the data has to be retained does it not?

pub async fn next(&mut self) -> Result<Option<T::Value<'_>>>

That is if T::Value may hold a reference to '_, then the first item returned from the cursor must be valid for the lifetime of the cursor? We use RowOwned on our types, but the fact that this cursor operates on T: RowRead not T: RowOwned + RowRead makes it impossible to not hold on to all the data for the lifetime of the cursor?