Re: Bug? ExecChooseHashTableSize() got assertion failed with crazy number of rows

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Kevin Grittner <kgrittn(at)ymail(dot)com> writes:
>> Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com> wrote:
>>> we may need a couple of overhaul around HashJoin to support large
>>> size of data, not only nbuckets around 0x80000000.

>> Perhaps, but this is a clear bug, introduced to the 9.5 code, with
>> an obvious fix; so I've pushed the change from 1 to 1L on that left
>> shift.

> I don't think it's anywhere near as clear as you think. The preceding
> lines should have limited nbuckets to be no more than INT_MAX/2, so how
> could an overflow occur there?

There is a 1 shifted for nbuckets that I didn't touch because it
could not. The limits on nbuckets are not applied at the point of
the bug.

> (The result of 1<<mylog() should be
> at most 0x40000000 AFAICS.) If overflow is possible, how will s/1/1L/
> make it better on machines where int and long are the same size?

In such cases there is not a bug -- at least not with my_log
returning less than 63. In those cases the change will make no
difference.

> And on machines where long is wider than int, you've still got a bug
> if the result of the left shift somehow overflowed an int, because
> it's going to be assigned to nbuckets which is an int.

You seem to have missed reading the line in between:

| lbuckets = 1L << my_log2(hash_table_bytes / bucket_size);
| lbuckets = Min(lbuckets, max_pointers);
| nbuckets = (int) lbuckets;

There is a Min() and a cast to sort that out.

> So I think the "1" coding was just fine in context.

I disagree.

> If there is an overflow in ExecChooseHashTableSize, it's
> somewhere else.

I do believe that there are other places we need to place limits,
to avoid attempts to allocate more than 1GB, but I think patches to
address that should be separate from fixing this particular bug.

--
Kevin Grittner
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