Re: foreign key locks, 2nd attempt

On Tue, Mar 06, 2012 at 04:39:32PM -0300, Alvaro Herrera wrote:
> Here's a first attempt at a README illustrating this. I intend this to
> be placed in src/backend/access/heap/README.tuplock; the first three
> paragraphs are stolen from the comment in heap_lock_tuple, so I'd remove
> those from there, directing people to this new file instead. Is there
> something that you think should be covered more extensively (or at all)
> here?
...
>
> When there is a single locker in a tuple, we can just store the locking info
> in the tuple itself. We do this by storing the locker's Xid in XMAX, and
> setting hint bits specifying the locking strength. There is one exception
> here: since hint bit space is limited, we do not provide a separate hint bit
> for SELECT FOR SHARE, so we have to use the extended info in a MultiXact in
> that case. (The other cases, SELECT FOR UPDATE and SELECT FOR KEY SHARE, are
> presumably more commonly used due to being the standards-mandated locking
> mechanism, or heavily used by the RI code, so we want to provide fast paths
> for those.)

Are those tuple bits actually "hint" bits? They seem quite a bit more
powerful than a "hint".

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +