Pressing Ctrl+C during UPDATE query in Postgres, was the query partially run? I accidentally launched unwanted UPDATE query inside psql (9.4) and killed it with CTRL+C a few seconds later. Were there any records modified, or it's all or nothing?
A statement is always atomic. That is true for all DML statements and nearly all DDL statements. 

So either everything succeeds or nothing. If you interrupt it halfway through, everything that was changed until then will be rolled back.