Since we insert a new entry into the index for every update that's being
made into the table, we inevitably make a unique check against the older
version of the newly inserted row, even when the values are not updated. Of
course i am talking about non-HOT updates. (We will not go to the index for
a) The page which contains the index entry is Exclusively locked
b) We go ahead and visit the heap page for its HeapTupleSatisfiesDirty.
If we have the information of the old tuple(its tuple-id) after a heap
update, during the index insert, we can avoid the uniqueness check for this
tuple,as we know for sure that tuple won't satisfy the visibility criteria.
If the table has 'n' unique indexes it avoids 'n' heap tuple lookups, also
increasing the concurrency in the btree, as the write lock duration is
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