Crash with PostgreSQL 7.4

[Peter Eisentraut]

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.
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