Issue type:

[ ] bug report
[ ] feature request
[X] documentation issue

Database system/driver:

[ ] cordova
[ ] mongodb
[ ] mssql
[ ] mysql / mariadb
[ ] oracle
[X] postgres
[ ] cockroachdb
[ ] sqlite
[ ] sqljs
[ ] react-native
[ ] expo

TypeORM version:

[X] latest
[ ] @next
[ ] 0.x.x (or put your version here)

Steps to reproduce or a small repository showing the problem:

Hey folks, how u guys doing?

Well, I wondering why Typeorm create ALTER TABLE script everytime a run migration generate.
Here's an example:

I have a prop call createdAt.

  @Column('timestamp', { nullable: false, default: () => 'CURRENT_TIMESTAMP' })
  createdAt: Date;

When I run the first migration the result is:
CREATE TABLE "name" ("createdAt" TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP)

When I change any thing, or even generate a migration without changes, I get this:
ALTER TABLE "name" ALTER COLUMN "createdAt" SET DEFAULT CURRENT_TIMESTAMP

Why I receive this ALTER TABLE, and how a configure to not create this script.