On 28th of July, Simon Riggs committed patch which:
Log Message: ----------- Reduce lock levels of CREATE TRIGGER and some ALTER TABLE, CREATE RULE actions. Avoid hard-coding lockmode used for many altering DDL commands, allowing easier future changes of lock levels. Implementation of initial analysis on DDL sub-commands, so that many lock levels are now at ShareUpdateExclusiveLock or ShareRowExclusiveLock, allowing certain DDL not to block reads/writes. First of number of planned changes in this area; additional docs required when full project complete.