django/django — pull request #2068 by intgr (2013-12-12 19:38:43+00:00)
Title: Clarify table rewrites in migration docs.

Small nitpicks. All column-related ALTER TABLE commands take an
exclusive table lock in PostgreSQL. The difference is that adding a
column without default doesn't cause a table rewrite, so the lock is
held only for a short time.

The time taken is more accurately proportional to table size, not row
count.