Hilary Forbes wrote:

I need to set the value of a field in table A to a value in table B depending 
on the existence of the record in table B.  So what I have done is

UPDATE tableA set a.val1=b.somefield FROM tableA a, tableB b WHERE 

Check the EXPLAIN carefully, are you sure the tableA in "UPDATE" is the same as that in your "FROM" clause. If so, why are you SETting a.val1?

If not, you've probably got an unconstrained join.
  Richard Huxton
  Archonet Ltd

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