Simon Riggs wrote:
On Mon, 2008-06-09 at 10:57 -0400, Tom Lane wrote:
Decibel! <[EMAIL PROTECTED]> writes:
Actually, in the interest of stating the problem and not the
solution, what we need is a way to add FKs that doesn't lock
everything up to perform the key checks.
Ah, finally a useful comment. I think it might be possible to do an
"add FK concurrently" type of command that would take exclusive lock
for just long enough to add the triggers, then scan the tables with just
AccessShareLock to see if the existing rows meet the constraint, and
if so finally mark the constraint "valid". Meanwhile the constraint
would be enforced against newly-added rows by the triggers, so nothing
gets missed. You'd still get a small hiccup in system performance
from the transient exclusive lock, but nothing like as bad as it is
now. Would that solve your problem?
That's good, but it doesn't solve the original user complaint about
needing to re-run many, many large queries to which we already know the
answer.
But we don't know it for dead sure, we only think we do. What if the
data for one or other of the tables is corrupted? We'll end up with data
we believe is consistent but in fact is not, ISTM. If you can somehow
guarantee the integrity of data in both tables then we might be
justified in assuming that the FK constraint will be consistent - that's
why I suggested some sort of checksum mechanism might serve the purpose.
cheers
andrew
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