Fix PostgreSQL qgen

[LinuzJ]

qgen is not working for PostgreSQL due to the following reasons. The output has:

• negative limits.
• the limit clause outside the select statement.

The following is an example of an output (with some comments):

group by
	cntrycode
order by
	cntrycode; <- closes statement before limit
limit -1;  <- negative limit

This makes using the tool quite difficult since manual intervention is needed. There is a PR open to fix this issue: https://github.com/gregrahn/tpch-kit/pull/4