Prepared statements

Use prepared statements for queries that are executed multiple times in your application:

PreparedStatement prepared = session.prepare(
  "insert into product (sku, description) values (?, ?)");

BoundStatement bound = prepared.bind("234827", "Mouse");
session.execute(bound);

When you prepare the statement, Cassandra parses the query string, caches the result and returns a unique identifier (the PreparedStatement object keeps an internal reference to that identifier):

Text Diagram

When you bind and execute a prepared statement, the driver only sends the identifier, which allows Cassandra to skip the parsing phase:

Text Diagram

Beyond saving a bit of parsing overhead on the server, prepared statements have other advantages; the PREPARED response also contains useful metadata about the CQL query:

  • information about the result set that will be produced when the statement gets executed. The driver caches this, so that the server doesn’t need to include it with every response. This saves a bit of bandwidth, and the resources it would take to decode it every time.
  • the CQL types of the bound variables. This allows bound statements’ set methods to perform better checks, and fail fast (without a server round-trip) if the types are wrong.
  • which bound variables are part of the partition key. This allows bound statements to automatically compute their routing key.
  • more optimizations might get added in the future. For example, CASSANDRA-10813 suggests adding an “idempotent” flag to the response.

If you have a unique query that is executed only once, a simple statement will be more efficient. But note that this should be pretty rare: most client applications typically repeat the same queries over and over, and a parameterized version can be extracted and prepared.

Preparing

Session.prepare() accepts either a plain query string, or a SimpleStatement object. If you use a SimpleStatement, its execution parameters will propagate to bound statements:

SimpleStatement simpleStatement =
    SimpleStatement.builder("SELECT * FROM product WHERE sku = ?")
        .setConsistencyLevel(DefaultConsistencyLevel.QUORUM)
        .build();
PreparedStatement preparedStatement = session.prepare(simpleStatement);
BoundStatement boundStatement = preparedStatement.bind();
assert boundStatement.getConsistencyLevel() == DefaultConsistencyLevel.QUORUM;

For more details, including the complete list of attributes that are copied, refer to API docs.

The driver caches prepared statements: if you call prepare() multiple times with the same query string (or a SimpleStatement with the same execution parameters), you will get the same PreparedStatement instance:

PreparedStatement ps1 = session.prepare("SELECT * FROM product WHERE sku = ?");
// The second call hits the cache, nothing is sent to the server:
PreparedStatement ps2 = session.prepare("SELECT * FROM product WHERE sku = ?");
assert ps1 == ps2;

We still recommend avoiding repeated calls to prepare(); if that’s not possible (e.g. if query strings are generated dynamically), there will just be a small performance overhead to check the cache on every call.

Note that caching is based on:

  • the query string exactly as you provided it: the driver does not perform any kind of trimming or sanitizing.
  • all other execution parameters: for example, preparing two statements with identical query strings but different consistency levels will yield two distinct prepared statements (that each produce bound statements with their respective consistency level).

The size of the cache is exposed as a session-level metric cql-prepared-cache-size. The cache uses weak values eviction, so this represents the number of PreparedStatement instances that your application has created, and is still holding a reference to.

Parameters and binding

The prepared query string will usually contain placeholders, which can be either anonymous or named:

ps1 = session.prepare("insert into product (sku, description) values (?, ?)");
ps2 = session.prepare("insert into product (sku, description) values (:s, :d)");

To turn the statement into its executable form, you need to bind it in order to create a BoundStatement. As shown previously, there is a shorthand to provide the parameters in the same call:

BoundStatement bound = ps1.bind("324378", "LCD screen");

You can also bind first, then use setters, which is slightly more explicit. Bound statements are immutable, so each method returns a new instance; make sure you don’t accidentally discard the result:

// Positional setters:
BoundStatement bound = ps1.bind()
  .setString(0, "324378")
  .setString(1, "LCD screen");

// Named setters:
BoundStatement bound = ps2.bind()
  .setString("s", "324378")
  .setString("d", "LCD screen");

Finally, you can use a builder to avoid creating intermediary instances, especially if you have a lot of methods to call: 

BoundStatement bound =
  ps1
      .boundStatementBuilder()
      .setString(0, "324378")
      .setString(1, "LCD screen")
      .setExecutionProfileName("oltp")
      .setQueryTimestamp(123456789L)
      .build();

You can use named setters even if the query uses anonymous parameters; Cassandra names the parameters after the column they apply to:

BoundStatement bound = ps1.bind()
  .setString("sku", "324378")
  .setString("description", "LCD screen");

This can be ambiguous if the query uses the same column multiple times, like in select * from sales where sku = ? and date > ? and date < ?. In these situations, use positional setters or named parameters.

Unset values

With native protocol V3, all variables must be bound. With native protocol V4 or above, variables can be left unset, in which case they will be ignored (no tombstones will be generated). If you’re reusing a bound statement, you can use the unset method to unset variables that were previously set:

BoundStatement bound = ps1.bind()
  .setString("sku", "324378")
  .setString("description", "LCD screen");

// Positional:
bound = bound.unset("description");

// Named:
bound = bound.unset(1);

A bound statement also has getters to retrieve the values. Note that this has a small performance overhead, since values are stored in their serialized form.

Since bound statements are immutable, they are safe to reuse across threads and asynchronous executions.

How the driver prepares

Cassandra does not replicate prepared statements across the cluster. It is the driver’s responsibility to ensure that each node’s cache is up to date. It uses a number of strategies to achieve this:

  1. When a statement is initially prepared, it is first sent to a single node in the cluster (this avoids hitting all nodes in case the query string is wrong). Once that node replies successfully, the driver re-prepares on all remaining nodes:

    Text Diagram

    The prepared statement identifier is deterministic (it’s a hash of the query string), so it is the same for all nodes.

  2. if a node crashes, it might lose all of its prepared statements (this depends on the version: since Cassandra 3.10, prepared statements are stored in a table, and the node is able to reprepare on its own when it restarts). So the driver keeps a client-side cache; anytime a node is marked back up, the driver re-prepares all statements on it;

  3. finally, if the driver tries to execute a statement and finds out that the coordinator doesn’t know about it, it will re-prepare the statement on the fly (this is transparent for the client, but will cost two extra roundtrips):

    Text Diagram

You can customize these strategies through the configuration:

  • datastax-java-driver.advanced.prepared-statements.prepare-on-all-nodes controls whether statements are initially re-prepared on other hosts (step 1 above);
  • datastax-java-driver.advanced.prepared-statements.reprepare-on-up controls how statements are re-prepared on a node that comes back up (step 2 above).

Read the reference configuration for a detailed description of each of those options.

Prepared statements and schema changes

With Cassandra 3 and below, avoid preparing SELECT * queries; the driver does not handle schema changes that would affect the results of a prepared statement. Therefore SELECT * queries can create issues, for example:

  • table foo contains columns b and c.
  • the driver prepares SELECT * FROM foo. It gets a reply indicating that executing this statement will return columns b and c, and caches that metadata locally (for performance reasons: this avoids sending it with each response later).
  • someone alters table foo to add a new column a.
  • the next time the driver executes the prepared statement, it gets a response that now contains columns a, b and c. However, it’s still using its stale copy of the metadata, so it decodes a thinking it’s b. In the best case scenario, a and b have different types and decoding fails; in the worst case, they have compatible types and the client gets corrupt data.

To avoid this, do not create prepared statements for SELECT * queries if you plan on making schema changes involving adding or dropping columns. Instead, always list all columns of interest in your statement, i.e.: SELECT b, c FROM foo.

With Cassandra 4 and native protocol v5, this issue is fixed (CASSANDRA-10786): the server detects that the driver is operating on stale metadata and sends the new version with the response; the driver updates its local cache transparently, and the client can observe the new columns in the result set.