Re: Why is LockClassinfoForUpdate()'s mark4update a good idea?

Hiroshi Inoue <Inoue@tpf.co.jp> writes:
> I like neither unexpected errors nor doing the wrong
> thing by handling tuples which aren't guaranteed to
> be up-to-date. After mark4update, the tuple is 
> guaranteed to be up-to-date and heap_update won't
> fail even though some commands etc neglect to lock
> the correspoding relation. Isn't it proper to guard
> myself as much as possible ?

If one piece of the system "guards itself" and others do not, what have
you gained?  Not much.  What I want is a consistently applied coding
rule that protects all commands; and the simpler that coding rule is,
the more likely it is to be consistently applied.  I do not think that
adding mark4update improves matters when seen in this light.  The code
to do it is bulky and error-prone, and I have no confidence that it will
be done right everywhere.

In fact, at the moment I'm not convinced that it's done right anywhere.
The uses of mark4update for system-catalog updates are all demonstrably
broken right now, and the ones in the executor make use of a hugely
complex and probably buggy qualification re-evaluation mechanism.  What
is the equivalent of qual re-evaluation for a system catalog tuple,
anyway?
        regards, tom lane