Query result¶

Session::query_unpaged, Session::query_single_page, Session::execute_unpaged and Session::execute_single_page return a QueryResult with rows represented as Option<Vec<Row>>.

Note
Using unpaged queries for SELECTs is discouraged in general. Query results may be so big that it is not preferable to fetch them all at once. Even with small results, if there are a lot of tombstones, then there can be similar bad consequences. However, query_unpaged will return all results in one, possibly giant, piece (unless a timeout occurs due to high load incurred by the cluster). This:

• increases latency,

• has large memory footprint,

• puts high load on the cluster,

• is more likely to time out (because big work takes more time than little work, and returning one large piece of data is more work than returning one chunk of data).

To sum up, for SELECTs (especially those that may return a lot of data) prefer paged queries, e.g. with Session::query_iter() (see Paged queries).

// Parse row as a single column containing an int value
let result = session
    .query_unpaged("SELECT a from ks.tab", &[])
    .await?
    .into_rows_result()?;

for row in result.rows::<(i32,)>()? {
    let (int_value,): (i32,) = row?;
}

// first_row gets the first row and parses it as the given type
let first_int_val: (i32,) = session
    .query_unpaged("SELECT a from ks.tab", &[])
    .await?
    .into_rows_result()?
    .first_row::<(i32,)>()?;

// result_not_rows fails when the response is rows
session.query_unpaged("INSERT INTO ks.tab (a) VALUES (0)", &[]).await?.result_not_rows()?;

// Parse row as two columns containing an int and text which might be null
let rows_result = session
    .query_unpaged("SELECT a, b from ks.tab", &[])
    .await?
    .into_rows_result()?;

for row in rows_result.rows::<(i32, Option<&str>)>()? {
    let (int_value, str_or_null): (i32, Option<&str>) = row?;
}

use scylla::DeserializeRow;
use scylla::deserialize::row::DeserializeRow;

#[derive(DeserializeRow)]
struct MyRow {
    age: i32,
    name: Option<String>,
}

// Parse row as two columns containing an int and text which might be null
let result_rows = session
    .query_unpaged("SELECT a, b from ks.tab", &[])
    .await?
    .into_rows_result()?;

for row in result_rows.rows::<MyRow>()? {
    let my_row: MyRow = row?;
}