Re: gin index trouble

On 10/30/2017 10:56 AM, Peter Geoghegan wrote:
> On Mon, Oct 30, 2017 at 9:45 AM, Rob Sargent <robjsargent(at)gmail(dot)com> wrote:
>> Peter, you beat me to the punch. I was just about to say "Having read the
>> referenced message I thought I would add that we never delete from this
>> table." In this particular case it was written to record by record, in a
>> previous execution and at the time of the error it was only being read. (In
>> case you've been following, the failed execution would have added ~1M
>> "segments", each which references an entry in the gin'd table "probandsets"
>> - but like a rookie I'm looking up each probandset(2^16) individually.
>> Re-working that NOW.)
> It's not surprising that only a SELECT statement could see this
> problem. I guess that it's possible that only page deletions used for
> the pending list are involved here.
>
> I'm not sure how reliably you can recreate the problem, but if it
> doesn't take too long then it would be worth seeing what effect
> turning off the FASTUPDATE storage parameter for the GIN index has.
> That could prevent the problem from recurring, and would support my
> theory about what's up here. (It wouldn't fix the corruption, though.)
>
> Of course, what I'd much prefer is a self-contained test case. But if
> you can't manage that, or if reproducing the issue takes hours, then
> this simpler experiment might be worthwhile.
>
My test database machine is:

Not virtual
Intel(R) Xeon(R) CPU E3-1241 v3 @ 3.50GHz (quad core, hyperthreaded)
MemTotal: 16272548 kB
default postgres.conf from yum install postgresql-10*

I've loaded thrice the number of records (190K) into the problem table,
but no sign yet of the problem. But unlike the production
lookup-notfind-insert (anti)pattern, these were all loaded in a single
transaction. I think the following query has to read the gin'd column of
every record:

select array_length(probands,1) as heads,
count(*) as occurs
from tld.probandset
where probands @>
'{65fe3b60-1c86-4b14-a85d-21abdf68f9e2,f0963403-3f3c-426d-a828-b5bfff914bb4}'
group by array_length(probands,1)
order by array_length(probands,1);

heads | occurs
-------+--------
2 | 1
3 | 14
4 | 91
5 | 364
6 | 1001
7 | 2002
8 | 3003
9 | 3432
10 | 3003
11 | 2002
12 | 1001
13 | 364
14 | 91
15 | 14
16 | 1
(15 rows)

Time: 17.125 ms

Happy as a clam.

I'll try a run of the antipattern. I have NOT diddled FASTUPDATE at all.