Re: foreign key locks, 2nd attempt

On 15.01.2012 06:49, Alvaro Herrera wrote:
> --- 164,178 ----
> #define HEAP_HASVARWIDTH 0x0002 /* has variable-width attribute(s) */
> #define HEAP_HASEXTERNAL 0x0004 /* has external stored attribute(s) */
> #define HEAP_HASOID 0x0008 /* has an object-id field */
> ! #define HEAP_XMAX_KEYSHR_LOCK 0x0010 /* xmax is a key-shared locker */
> #define HEAP_COMBOCID 0x0020 /* t_cid is a combo cid */
> #define HEAP_XMAX_EXCL_LOCK 0x0040 /* xmax is exclusive locker */
> ! #define HEAP_XMAX_IS_NOT_UPDATE 0x0080 /* xmax, if valid, is only a locker.
> ! * Note this is not set unless
> ! * XMAX_IS_MULTI is also set. */
> !
> ! #define HEAP_LOCK_BITS (HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_IS_NOT_UPDATE | \
> ! HEAP_XMAX_KEYSHR_LOCK)
> #define HEAP_XMIN_COMMITTED 0x0100 /* t_xmin committed */
> #define HEAP_XMIN_INVALID 0x0200 /* t_xmin invalid/aborted */
> #define HEAP_XMAX_COMMITTED 0x0400 /* t_xmax committed */

HEAP_XMAX_IS_NOT_UPDATE is a pretty opaque name for that. From the name,
I'd say that a DELETE would set that, but the comment says it has to do
with locking. After going through all the combinations in my mind, I
think I finally understood it: HEAP_XMAX_IS_NOT_UPDATE is set if xmax is
a multi-xact, that represent only locking xids, no updates. How about
calling it "HEAP_XMAX_LOCK_ONLY", and setting it whenever whether or not
xmax is a multi-xid?

> - I haven't done anything about exposing FOR KEY UPDATE to the SQL
> level. There clearly isn't consensus about exposing this; in fact
> there isn't consensus on exposing FOR KEY SHARE, but I haven't
> changed that from the previous patch, either.

I think it would be useful to expose it. Not that anyone should be using
them in an application (or would it be useful?), but I feel it could
make testing significantly easier.

> - pg_upgrade bits are missing.

I guess we'll need to rewrite pg_multixact contents in pg_upgrade. Is
the page format backwards-compatible?

Why are you renaming HeapTupleHeaderGetXmax() into
HeapTupleHeaderGetRawXmax()? Any current callers of
HeapTupleHeaderGetXmax() should already check that HEAP_XMAX_IS_MULTI is
not set.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com