ALTER TABLE command with no default value taking forever?

I’m attempting to add a NOT NULL constraint to a table I have (so I can use it as part of a primary key, so I can create a spatial index…) but it is taking a really long time – 1 hour 31 minutes and counting.

I had read this post where the answer seemed to be that it was taking so long because the person was adding a default value but i’m not doing that:

ALTER TABLE unacastVisitRaw ALTER COLUMN timestamp varchar(64) NOT NULL 

The table is rather large (290 million records) but it still seems like a long time for just adding a constraint and not changing any values.

Is there anything I can do to speed up this operation? Why would it be taking so long if I’m not updating the existing records?

Is there is any way either in Obj c or swift, to alter the background so nicely that one could feel the person must be in the image

I just want to know is it alter the background image so nicely that one could feel the person must be in the picture..if yes then please let me know any tutorial regarding this..or any kind of help. both obj c or swift language code will be helpfull..thanks in advance