On 7 August 2015 at 12:34, Thom Brown <thom@xxxxxxxxx> wrote:
On 30 July 2015 at 13:35, Rowan Collins <rowan.collins@xxxxxxxxx> wrote:Hi,
When working with partition sets, we're seeing occasional errors of "could not find inherited attribute..." in Select queries. This is apparently caused when an "ALTER TABLE ... NO INHERIT" runs concurrently with another transaction selecting from the relevant child table.
I found an old bug report filed against 8.3 back in 2008 [1] I can still reproduce the test case in that report on 9.1.11 and 9.3.5, and it seems to match what we're seeing in production.
Tom Lane said at the time that a lock would cause more problems than it solved [2], but when I add an explicit lock statement ("LOCK TABLE ONLY p_tbl, c1_tbl IN ACCESS EXCLUSIVE MODE;") between "BEGIN" and "ALTER TABLE", I get the behaviour I would expect - the SELECT blocks until the transaction is committed, then returns rows from the remaining child table.
So what I want to understand is what the risk of adding this lock are - under what circumstances would I expect to see dead locks if I manually added this lock to my partition maintenance functions?I'm not clear on the problems such a change would present either, but I'm probably overlooking the relevant scenario.
Has anyone got insight as to what's wrong with exclusively locking a parent table to disinherit a child table?
Thom
