When you modify existing column with DEFAULT value, then modification to add a DEFAULT value takes effect only from subsequent insertions to the table.
Let’s have a look at EMP table below. All DEFAULT values are set to NULL at this point
Let’s modify EMP .COMMISION_PCT DEFAULT value to 0.777
Now, the EMP .COMMISION_PCT DEFAULT value was set to 0.777, see below
Let insert one row into EMP table to verify DEFAULT value triggered
As we can see, a new row was inserted with DEFAULT value of 0.777 (was rounded to 0.78)
ATTENTION: Keep in mind that adding a new column with DEFAULT value to a table, will act differently. It will store DEFAULT value in the dictionary while inserting DEFAULT value at every NULL field. Modifying existing column DEFAULT value won’t trigger immediate changes to NULL values in this column, instead, DEFAULT values will be used ONLY from subsequent insertions to the table. This difference in behavior worth to be remembered.