I need some help to understand why a transaction wiht a row delete aquire a AccessExclusiveLock and a row update a ExclusiveLock.
UPDATE can use the weaker lock type if it's not modifying any column that is part of a unique index. This is to allow concurrency with foreign-key checks that might wish to grab a read-only (shared) lock on such a tuple.
A DELETE, or an UPDATE that is modifying key columns, has to conflict with foreign-key checks.
regards, tom lane
Tom,
thank you very much for your straight to the point answer!
