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Prepared queries provide much better performance than simple queries, but they need to be prepared before use.
Benefits that prepared statements have to offer:
Type safety - thanks to metadata provided by the server, the driver can verify bound values’ types before serialization. This way, we can be always sure that the Rust type provided by the user is compatible (and if not, the error is returned) with the destined native type. The same applies for deserialization.
Performance - when executing a simple query with non-empty values list, the driver prepares the statement before execution. The reason for this is to provide type safety for simple queries. However, this implies 2 round trips per simple query execution. On the other hand, the cost of prepared statement’s execution is only 1 round trip.
Improved load-balancing - using the statement metadata, the driver can compute a set of destined replicas for current statement execution. These replicas will be preferred when choosing the node (and shard) to send the request to. For more insight on this, see performance section.
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_unpaged(&prepared, (to_insert,)).await?;
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 useexecuteto 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,
executewill 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 succeeds 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.
To specify custom options, set them on the PreparedStatement before execution.
For example to change the consistency:
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_unpaged(&prepared, (to_insert,)).await?;
See PreparedStatement API documentation for more options.
Note Prepared statements can be created from
Querystructs and will inherit from the custom options that theQuerywas created with. This is especially useful when usingCachingSession::executefor example.
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)
)
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_unpaged(&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_unpaged(&good_prepared, (12345, 54321)).await?;
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