Re: wrong query result with jit_above_cost= 0

> On 26 June 2018 at 22:56, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2018-06-26 21:55:07 +0100, Andrew Gierth wrote:
>> >>>>> "Dmitry" == Dmitry Dolgov <9erthalion6(at)gmail(dot)com> writes:
>>
>> Dmitry> Yep, my bad, forgot to turn it on. Now I see what's the
>> Dmitry> problem, one of the null fields is screwed up, will try to
>> Dmitry> figure out why is that.
>>
>> The handling of nulls in grouping set results is a bit icky, see
>> prepare_projection_slot in nodeAgg.c. The comment there lists a number
>> of assumptions which may or may not hold true under JIT which might give
>> a starting point to look for problems. (Unfortunately I'm not currently
>> in a position to test on a JIT build)
>
> I probably just screwed up a bit of code generation. I can't see any of
> the more fundamental assumptions being changed by the way JITing is
> done.

So far I found out that in agg_retrieve_hash_table, when there is a scan for
TupleHashEntryData, that contains AggStatePerGroup structure in the field
"additional", it's possible to get some garbage data (or at least transValue is
lost). It happens when we do:

ReScanExprContext(aggstate->aggcontexts[i]);

in agg_retrieve_direct before that. Apparently, the reason is that in the jit
code there is a store operation for curaggcontext into aggcontext:

v_aggcontext = l_ptr_const(op->d.agg_trans.aggcontext,
l_ptr(StructExprContext));

/* set aggstate globals */
LLVMBuildStore(b, v_aggcontext, v_curaggcontext);

I haven't found anything similar in the original code or in the other branches
for aggregation logic. I can't say that I fully understand the idea behind it,
but at least it was suspicious for me. When I removed this operation, the
problem has disappeared.