Parameter Status Messages

LightDB supports server parameters, also called server variables or, internally, Grand Unified Configuration (GUC) variables. These variables are manipulated by the SET command, lightdb.conf, ALTER SYSTEM SET, ALTER USER SET, ALTER DATABASE SET, the set_config(...) SQL-callable function, etc. See the LightDB manual.

For a subset of these variables the server will automatically report changes to the value to the client driver and application. These variables are known internally as GUC_REPORT variables after the name of the flag that enables the functionality.

The server keeps track of all the variable scopes and reports when a variable reverts to a prior value, so the client doesn’t have to guess what the current value is and whether some server-side function could’ve changed it. Whenever the value changes, no matter why or how it changes, the server reports the new effective value in a Parameter Status protocol message to the client. ltjdbc uses many of these reports internally.

As of ltjdbc 42.2.6, it also exposes the parameter status information to user applications via the PGConnection extensions interface.

Methods

Two methods on org.postgresql.PGConnection provide the client interface to reported parameters. Parameter names are case-insensitive and case-preserving.

• Map PGConnection.getParameterStatuses() - return a map of all reported parameters and their values.

• String PGConnection.getParameterStatus() - shorthand to retrieve one value by name, or null if no value has been reported.

See the PGConnection JavaDoc for details.

Example

If you’re working directly with a java.sql.Connection you can

import org.postgresql.PGConnection;

void my_function(Connection conn) {

    System.out.println("My application name is " +
        ((PGConnection)conn).getParameterStatus("application_name"));

}

Other client drivers

The libpq equivalent is the PQparameterStatus(...) API function.