londiste compare improvements
londiste compare improvements

* Calculate common column names, to allow different order
  or different subset.
* Use different hash function on 8.3 <> something else comparision
  as hashtext() changed in 8.4.
* Always calculate final hash as bigint.

overdrive3000/skytools,markokr/skytools,tarvip/skytools,markokr/skytools,tarvip/skytools,overdrive3000/skytools,overdrive3000/skytools,markokr/skytools,tarvip/skytools