Prepared query

Prepared queries provide much better performance than simple queries, but they need to be prepared before use.

#![allow(unused)]
fn main() {
extern crate scylla;
use scylla::Session;
use std::error::Error;
async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
use scylla::prepared_statement::PreparedStatement;

// Prepare the query for later execution
let prepared: PreparedStatement = session
    .prepare("INSERT INTO ks.tab (a) VALUES(?)")
    .await?;

// Run the prepared query with some values, just like a simple query
let to_insert: i32 = 12345;
session.execute(&prepared, (to_insert,)).await?;
Ok(())
}
}

Warning
For token/shard aware load balancing to work properly, all partition key values must be sent as bound values (see performance section)

Warning
Don't use execute to receive large amounts of data.
By default the query is unpaged and might cause heavy load on the cluster. In such cases set a page size and use a paged query instead.

When page size is set, execute will return only the first page of results.

Session::prepare

Session::prepare takes query text and prepares the query on all nodes and shards. If at least one succeds returns success.

Session::execute

Session::execute takes a prepared query and bound values and runs the query. Passing values and the result is the same as in simple query.

Query options

To specify custom options, set them on the PreparedStatement before execution. For example to change the consistency:

#![allow(unused)]
fn main() {
extern crate scylla;
use scylla::Session;
use std::error::Error;
async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
use scylla::prepared_statement::PreparedStatement;
use scylla::statement::Consistency;

// Prepare the query for later execution
let mut prepared: PreparedStatement = session
    .prepare("INSERT INTO ks.tab (a) VALUES(?)")
    .await?;

// Set prepared query consistency to One
// This is the consistency with which this query will be executed
prepared.set_consistency(Consistency::One);

// Run the prepared query with some values, just like a simple query
let to_insert: i32 = 12345;
session.execute(&prepared, (to_insert,)).await?;
Ok(())
}
}

See PreparedStatement API documentation for more options.

Note Prepared statements can be created from Query structs and will inherit from the custom options that the Query was created with. This is especially useful when using CachingSession::execute for example.

Performance

Prepared queries have good performance, much better than simple queries. By default they use shard/token aware load balancing.

Always pass partition key values as bound values. Otherwise the driver can't hash them to compute partition key and they will be sent to the wrong node, which worsens performance.

Let's say we have a table like this:

TABLE ks.prepare_table (
    a int,
    b int,
    c int,
    PRIMARY KEY (a, b)
)

#![allow(unused)]
fn main() {
extern crate scylla;
use scylla::Session;
use std::error::Error;
async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
use scylla::prepared_statement::PreparedStatement;

// WRONG - partition key value is passed in query string
// Load balancing will compute the wrong partition key
let wrong_prepared: PreparedStatement = session
    .prepare("INSERT INTO ks.prepare_table (a, b, c) VALUES(12345, ?, 16)")
    .await?;

session.execute(&wrong_prepared, (54321,)).await?;

// GOOD - partition key values are sent as bound values
// Other values can be sent any way you like, it doesn't matter
let good_prepared: PreparedStatement = session
    .prepare("INSERT INTO ks.prepare_table (a, b, c) VALUES(?, ?, 16)")
    .await?;

session.execute(&good_prepared, (12345, 54321)).await?;

Ok(())
}
}