oracle decreasing vs increasing column varchar2 performance
Is it because while decreasing oracle check each column size if it can be decreased? However increasing it, no need to do some checking?


Correct. 

When you decrease the length, Oracle needs to check the values in all rows in the table if any violates the new (reduced) length - it effectively reads all rows in the table.

This isn't necessary when you increase the length - no existing value can exceed that.