On 12/18/2013 02:59 AM, Josh Berkus wrote:
On 12/17/2013 01:42 PM, Kevin Grittner wrote:
It works fine as long as you set default_transaction_isolation =
'serializable' and never override that.  :-)  Of course, it sure
would be nice to have a way to prohibit overrides, but that's
another issue.

Otherwise it is hard to see how to make it work in a general way
without a mutually exclusive lock mode on the table for the
duration of any transaction which modifies the table.

Serializable or not, *what* do we lock for assertions?  It's not rows.
Tables?  Which tables?  What if the assertion is an interpreted language
function?  Does the SSI reference counter really take care of all of this?

Here's another idea that doesn't involve SSI:

At COMMIT, take a new snapshot and check that the assertion still passes with that snapshot. Now, there's a race condition, if another transaction is committing at the same time, and performs the same check concurrently. That race condition can be eliminated by holding an exclusive lock while running the assertion, until commit, effectively allowing the assertion to be checked by only one transaction at a time.

I think that would work, and would be simple, although it wouldn't scale too well.

- Heikki

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