Re: pgsql: Add parallel-aware hash joins.

On 2017-12-21 01:29:40 -0800, Andres Freund wrote:
> On 2017-12-21 08:49:46 +0000, Andres Freund wrote:
> > Add parallel-aware hash joins.
>
> There's to relatively mundane failures:
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=tern&dt=2017-12-21%2008%3A48%3A12
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=termite&dt=2017-12-21%2008%3A50%3A08
>
> but also one that's a lot more interesting:
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=capybara&dt=2017-12-21%2008%3A50%3A08
>
> which shows an assert failure:
>
> #2 0x00000000008687d1 in ExceptionalCondition (conditionName=conditionName(at)entry=0xa76a98 "!(!accessor->sts->participants[i].writing)", errorType=errorType(at)entry=0x8b2c49 "FailedAssertion", fileName=fileName(at)entry=0xa76991 "sharedtuplestore.c", lineNumber=lineNumber(at)entry=273) at assert.c:54
> #3 0x000000000089883e in sts_begin_parallel_scan (accessor=0xfaf780) at sharedtuplestore.c:273
> #4 0x0000000000634de4 in ExecParallelHashRepartitionRest (hashtable=0xfaec18) at nodeHash.c:1369
> #5 ExecParallelHashIncreaseNumBatches (hashtable=0xfaec18) at nodeHash.c:1198
> #6 0x000000000063546b in ExecParallelHashTupleAlloc (hashtable=hashtable(at)entry=0xfaec18, size=40, shared=shared(at)entry=0x7ffee26a8868) at nodeHash.c:2778
> #7 0x00000000006357c8 in ExecParallelHashTableInsert (hashtable=hashtable(at)entry=0xfaec18, slot=slot(at)entry=0xfa76f8, hashvalue=<optimized out>) at nodeHash.c:1696
> #8 0x0000000000635b5f in MultiExecParallelHash (node=0xf7ebc8) at nodeHash.c:288
> #9 MultiExecHash (node=node(at)entry=0xf7ebc8) at nodeHash.c:112
>
> which seems to suggest that something in the state machine logic is
> borked. ExecParallelHashIncreaseNumBatches() should've ensured that
> everyone has called sts_end_write()...

Thomas, I wonder if the problem is that PHJ_GROW_BATCHES_ELECTING
updates, via ExecParallelHashJoinSetUpBatches(), HashJoinTable->nbatch,
while other backends also access ->nbatch in
ExecParallelHashCloseBatchAccessors(). Both happens after waiting for
the WAIT_EVENT_HASH_GROW_BATCHES_ELECTING phase.

That'd lead to ExecParallelHashCloseBatchAccessors() likely not finish
writing all batches (because nbatch < nbatch_old), which seems like it'd
explain this?

Greetings,

Andres Freund