[GENERAL] Why does increasing the precision of a numeric column rewrites thetable?

When increasing the length constraint on a varchar column, Postgres is smart enough to not rewrite the table. 

I expected the same thing to be true when increasing the size of a numeric column.

However this does not seem to be the case:

Consider the following table:
   create table foo    (     some_number numeric(12,2)   );


The following statement returns "immediately", regardless of the number of rows in the table
   alter table foo alter column some_number numeric(15,2);

However, when running (on the original table definition)
   alter table foo alter column some_number numeric(15,3);

it takes quite a while (depending on the number of rows) which indicates a table rewrite is taking place. 

I don't understand why going from numeric(12,2) to numeric(15,3) would require a table rewrite. 

Thomas



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