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Using the statement cache
The statement cache is enabled by default. You can use it to avoid extra
compilation overhead:
- Your application can use PreparedStatements instead of Statements.
PreparedStatements are JDBC objects that you prepare (compile)
once and execute multiple times. See the figure below. If your application
executes statements that are almost but not exactly alike, use PreparedStatements,
which can contain dynamic or IN parameters. Instead of using the literals
for changing parameters, use question marks (?) as placeholders for such parameters.
Provide the values when you execute the statement.
Derby supports the ParameterMetaData interface,
new in JDBC 3.0. This interface describes the number, type, and properties
of prepared statement parameters. See the Derby Developer's Guide for
more information. Figure 1. A connection need only
compile a PreparedStatement once. Subsequent executions
can use the same statement execution plan even if the parameter values are
different. (PreparedStatements are not shared across connections.)
- Even if your statement uses Statements instead of PreparedStatements, Derby can reuse the statement
execution plan for the statements from the statement cache. Statements from
any connection can share the same statement execution plan, avoiding compilation,
by using the single-statement cache. The statement cache maintains statement
plans across connections. It does not, however, maintain them across reboots.
See the figure below.
When, in the same database, an application submits
an SQL Statement that exactly matches one already in the cache, Derby grabs the statement from
the cache, even if the Statement has already been closed by the application.
To
match exactly with a statement already in the cache, the SQL Statement must
meet the following requirements:
- The text must match exactly
- The current schema must match
- The Unicode flag that the statement was compiled under must match the
current connection's flag
Remember: If your application executes statements that are almost
but not exactly alike, it is more efficient to use PreparedStatements with
dynamic or IN parameters.
Figure 2. A database can reuse a statement
execution plan when the SQL text matches a prior statement exactly. (PreparedStatements are much more efficient.)
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