I have used rhdb-explain from the tag RHDB-3-0 and ran the following query against an empty PostgreSQL 7.4.1 database: select * from pg_tables; This causes a null pointer exception. The output on the console is java.lang.NullPointerException at com.redhat.rhdb.explain.ExplainParserV73.addQual(ExplainParserV73.java:796) at com.redhat.rhdb.explain.ExplainParserV73.buildExplainTree(ExplainParserV73.java:343) at com.redhat.rhdb.explain.ExplainParserV73.explain(ExplainParserV73.java:187) at com.redhat.rhdb.explain.Explain$ExplainThread.run(Explain.java:298) The reason is that the parser code only knows "Hash Join" but PG 7.4 emits a "Hash Left Join" in this case. Attached is a patch that kind of papers over this issue by ignoring "Left" and other possible hash-join attributes. Eventually one might want to show this distinction to the user.