| From: | Heikki Linnakangas <hlinnaka(at)iki(dot)fi> |
|---|---|
| To: | Alvaro Herrera <alvherre(at)dcc(dot)uchile(dot)cl> |
| Cc: | Simon Riggs <simon(at)2ndquadrant(dot)com>, Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Shared row locking |
| Date: | 2004-12-19 21:12:41 |
| Message-ID: | Pine.OSF.4.61.0412192308580.479956@kosh.hut.fi |
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| Lists: | pgsql-hackers |
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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