Comment by wbolster

not sure about mysql, but in postgresql an update on a table with a foreign key will take 'FOR KEY SHARE' locks on the referenced table, which is a weaker type of lock. updates on the referenced table that do not update (primary) key columns (changing a pk is very uncommon anyway) will suffice with a 'FOR NO KEY UPDATE' lock, which does not get blocked by 'FOR KEY SHARE' locks. in fact, the main reason postgresql has these weaker 'FOR KEY SHARE' and 'FOR NO KEY UPDATE' lock types is for handing of foreign keys.