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.