Hi,
If you attempt to create an index based on function that is not IMMUTABLE you will get an exception "ERROR: functions in index predicate must be marked IMMUTABLE". However, if you created the index when the function was IMMUTABLE, but later on you updated the function and mistakenly removed the IMMUTABLE key, you will not get any error to alert you that there is an index based on this function and it should remain IMMUTABLE.
I suggest triggering error message also when updating a function that is used by index if it is no longer IMMUTABLE
Avi
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