Re: Microvacuum support for Hash Index

On Sat, Jan 28, 2017 at 8:02 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Fri, Jan 27, 2017 at 5:15 PM, Ashutosh Sharma <ashu(dot)coek88(at)gmail(dot)com> wrote:
>>>
>>> Don't you think we should try to identify the reason of the deadlock
>>> error reported by you up thread [1]? I know that you and Ashutosh are
>>> not able to reproduce it, but still I feel some investigation is
>>> required to find the reason. It is quite possible that the test case
>>> is such that the deadlock is expected in rare cases, if that is the
>>> case then it is okay. I have not spent enough time on that to comment
>>> whether it is a test or code issue.
>>
>> I am finally able to reproduce the issue using the attached script
>> file (deadlock_report). Basically, once I was able to reproduce the
>> issue with hash index I also thought of checking it with a non unique
>> B-Tree index and was able to reproduce it with B-Tree index as well.
>> This certainly tells us that there is nothing wrong at the code level
>> rather there is something wrong with the test script which is causing
>> this deadlock issue. Well, I have ran pgbench with two different
>> configurations and my observations are as follows:
>>
>> 1) With Primary keys i.e. uinque values: I have never encountered
>> deadlock issue with this configuration.
>>
>> 2) With non unique indexes (be it hash or B-Tree): I have seen
>> deadlock many a times with this configuration. Basically when we have
>> non-unique values associated with a column then there is a high
>> probability that multiple records will get updated with a single
>> 'UPDATE' statement and when there are huge number of backends trying
>> to do so there is high chance of getting deadlock which i assume is
>> expected behaviour in database.
>>
>
> I agree with your analysis, surely trying to update multiple rows with
> same values from different backends can lead to deadlock.

Moved that to CF 2017-03.
--
Michael