Interesting tight loop

In production today (8.1.4), I ran into a backend process that  
wouldn't cancel right away -- minutes went by.

It was in

[0] TransactionIdIsCurrentTransactionId
[1] HeapTupleSatisfiesSnapshot
...

But the interesting thing is that there were 4.6 million elements in  
the s->childXids list.  Which is why it took so damn long.  I can't  
quite figure out how I induced this state.  It is an OLAP server with  
about 10-20 connection that run "long" queries (from 5 seconds to 24  
hours).

If this is a common possible state, it seems that perhaps a hash of  
the childXids would be more appropriate.  Does the order of the  
childXids chained off the current transaction state matter?  Most of  
the placed I could find that reference it seem to just attempt to  
find an Xid in there.  O(1) sounds a lot better than O(n)  :-D

Best regards,

Theo

// Theo Schlossnagle
// CTO -- http://www.omniti.com/~jesus/
// OmniTI Computer Consulting, Inc. -- http://www.omniti.com/