On Sun, 19 Dec 2004, Alvaro Herrera wrote:
On Sun, Dec 19, 2004 at 09:52:01AM +0000, Simon Riggs wrote:
Simon,
In similar circumstances, DB2 uses these techniques:
- when locktable X % full, then escalate locks to full table locks: both locktable memory and threshold% are instance parameters
This is not useful at all, because the objective of this exercise is to downgrade locks, from exclusive row locking (SELECT ... FOR UPDATE) to shared row locking. Keep in mind that this is for foreign key locking, which is one area where deadlocks are frequently encountered because we use too strong a lock.
Actually it might help in some scenarios. Remember, we're not talking about upgrading shared locks to exclusive locks. We're only talking about locking more rows than necessary (all rows).
I believe DB2 does the escalation also for perfomance. Getting a full table lock is cheaper than individually locking every row.
- Heikki
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