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The MQL Preview button allows the user to preview the data that will be returned from the MQL Query. In order for the preview to function properly, your Pentaho BI Server must be running. Prior to release 1.6.M5 preview was not functional. A preliminary fix, requiring manual configuration of JNDI connections, has been made in version 1.6.M5 that corrects this issue. To enable MQL Preview functionality create the following directory <workspace-dir>/.metadata/.For preview to work, the JDBC driver jar file for the database you are trying to use must be in the plugin classpath. At add a new JDBC driver to your plugin classpath, the jar should be dropped into the <eclipse_install_directory>/plugins/org.pentaho.designstudio.editors.actionsequence_x.x.x/third_party/lib directory. You'll then need to edit <eclipse_install_directory>/plugins/org.pentaho.designstudio.editors.actionsequence/simple-jndi. In this director create a jdbc.properties file and define the jndi connection name used in the metadata file. A sample jndi definition is shown below:
SampleData/type=javax.sql.DataSource
SampleData/driver=org.hsqldb.jdbcDriver
SampleData/url=jdbc:hsqldb:hsql://localhost/sampledata
SampleData/user=pentaho_user
SampleData/password=password
You'll also have to make sure that the JDBC driver jar file for the database you are trying to use is on your Java classpath. The easiest way to do this is to drop the jar in the <JAVA_HOME>/lib directory_x.x.x/META-INF/MANIFEST.MF and add your new jar to the bundle classpath.
Once this is done MQL queries can be previewed.Future releases of the design studio will enable the user to define their JNDI connections using the Pentaho preferences dialog (Refer to JIRA case #PDS-277).