Re: DSA overflow in hash join

Thank you for review!

On 14/08/2025 10:34 PM, Heikki Linnakangas wrote:
> On 31/07/2025 18:13, Konstantin Knizhnik wrote:
>>
>> On 27/07/2025 8:24 PM, Konstantin Knizhnik wrote:
>>>
>>> I still trying to understand the reason of DSA overflow in hash join.
>>> In addition to two suspicious places where number of buckets is
>>> doubled without chek for overflow (nodeHash.c:1668 and
>>> nodeHash.c:3290),
>>> there is one  more place  where number of batches is multiplied by
>>> `EstimateParallelHashJoinBatch(hashtable)` which is
>>>
>>> sizeof(ParallelHashJoinBatch) + (sizeof(SharedTuplestore)  +
>>> sizeof(SharedTuplestoreParticipant) * participants) * 2
>>>
>>> which is 480 bytes!
>>>
>>> But when we calculate maximal number of batches, we limit it by
>>> macximal number of pointers (8 bytes):
>>>
>>>     max_pointers = hash_table_bytes / sizeof(HashJoinTuple);
>>>     max_pointers = Min(max_pointers, MaxAllocSize /
>>> sizeof(HashJoinTuple));
>>>     /* If max_pointers isn't a power of 2, must round it down to one */
>>>     max_pointers = pg_prevpower2_size_t(max_pointers);
>>>
>>>     /* Also ensure we avoid integer overflow in nbatch and nbuckets */
>>>     /* (this step is redundant given the current value of
>>> MaxAllocSize) */
>>>     max_pointers = Min(max_pointers, INT_MAX / 2 + 1);
>>>
>>>     dbuckets = ceil(ntuples / NTUP_PER_BUCKET);
>>>     dbuckets = Min(dbuckets, max_pointers);
>>>     nbuckets = (int) dbuckets;
>>>
>>>
>>> But as we see, here multiplier is 480 bytes, not 8 bytes.
>>>
>>
>> Below is script to reproduce the problem:
>>
>> CREATE TABLE IF NOT EXISTS t0(c0 FLOAT, PRIMARY KEY(c0)) WITH
>> (parallel_workers=966);
>> CREATE TABLE t2(c0 DECIMAL, c1 int4range ) WITH (parallel_workers=393);
>> CREATE TABLE t4(LIKE t2);
>> CREATE TABLE t5(LIKE t0);
>> INSERT INTO t4(c0) VALUES(0.5934077416223362);
>>
>> set work_mem='10MB';
>> set max_parallel_workers_per_gather=5;
>>
>> explain SELECT SUM(count) FROM (SELECT ALL CAST(FALSE AS INT) as
>> count FROM ONLY t5, ONLY t2 CROSS JOIN ONLY t0 LEFT OUTER JOIN t4* ON
>> (upper(((t2.c1)+(t2.c1))))::BOOLEAN CROSS JOIN (SELECT t4.c0 FROM
>> ONLY t0, t2*, t5*, t4* WHERE (((t2.c1)*(t2.c1))) IN (t4.c1)) AS sub0)
>> as res;
>>
>> SELECT SUM(count) FROM (SELECT ALL CAST(FALSE AS INT) as count FROM
>> ONLY t5, ONLY t2 CROSS JOIN ONLY t0 LEFT OUTER JOIN t4* ON
>> (upper(((t2.c1)+(t2.c1))))::BOOLEAN CROSS JOIN (SELECT t4.c0 FROM
>> ONLY t0, t2*, t5*, t4* WHERE (((t2.c1)*(t2.c1))) IN (t4.c1)) AS sub0)
>> as res;
>
> Great repro, thanks!
>
>> And attached please find patch fixing the issue.
>
> There are a lot of allocations in hash join. For example this in
> ExecParallelHashEnsureBatchAccessors():
>
>     /* Allocate this backend's accessor array. */
>     hashtable->nbatch = pstate->nbatch;
>     hashtable->batches =
>         palloc0_array(ParallelHashJoinBatchAccessor, hashtable->nbatch);
>
> That could also exceed MaxAllocSize, right? I think we need to take
> that into account in calculating the max number of batches, too.
>
Yes, I concentrated on DSA and didn't notice it. Also added it to limit
number of batches.

>
> Is this only a problem for parallel hash joins?
>
> In general, it's really hard to follow the logic of where we enforce
> what limits in ExecChooseHashTableSize(). How can we be sure that we
> accounted for all allocations? Could we have more (static) assertions
> or something, to ensure we've covered everything?

Definitely I can not guarantee that there are no much such places in
Postgres code.
But at least in executor files, only nodeHash takes in account
MaxAllocSize when choosing hash table parameters.
Certainly if some module doesn't take in account `MaxAllocSize`, it
doesn't means that there can not be OOM problems (either in Postgres
memory context or in shared memory). May be OOM problem was just not
considered...

>
> A different approach might be to make ExecChooseHashTableSize() return
> just estimates, and enforce the hard limits later. But that could lead
> to worse decisions: it's good for ExecChooseHashTableSize() to take
> the maximum limits into account.

Yes, I think that number of batches should be correlated with number of
buckets. So it is better to restrict their values from the very
beginning (in ExecChooseHashTableSize) rather then do it right before
allocation.

>
> On the proposed patch:
>
>> @@ -775,6 +777,16 @@ ExecChooseHashTableSize(double ntuples, int
>> tupwidth, bool useskew,
>>      /* If max_pointers isn't a power of 2, must round it down to one */
>>      max_pointers = pg_prevpower2_size_t(max_pointers);
>>
>> +    /*
>> +     * Prevent DSA overflow. This is expanded definition of
>> EstimateParallelHashJoinBatch function
>> +     * used in ExecParallelHashJoinSetUpBatches:
>> +     *     dsa_allocate0(hashtable->area,
>> +     * EstimateParallelHashJoinBatch(hashtable) * nbatch)
>> +     */
>> +    max_batches = MaxAllocSize /
>> (MAXALIGN(sizeof(ParallelHashJoinBatch)) +
>> + MAXALIGN(sts_estimate(max_parallel_workers_per_gather + 1) * 2));
>> +    max_batches = pg_prevpower2_size_t(max_batches);
>> +
>>      /* Also ensure we avoid integer overflow in nbatch and nbuckets */
>>      /* (this step is redundant given the current value of
>> MaxAllocSize) */
>>      max_pointers = Min(max_pointers, INT_MAX / 2 + 1);
>
> Would be nice to not copy the macro here..

Yeh... extr4acting macro looks really ugly.
But I don't know how to avoid it, because macro is using pointer to hash
table which is not allocated yes. But what is worser,
result depends on number of participants which is not known yet at this
stage.
So I have to use upper limit (`max_parallel_workers_per_gather`) instead.

>
> BTW I don't like the name EstimateParallelHashJoinBatch(). It's not
> just an estimate, it's used to allocate memory for an array, and it
> better be accurate. See also NthParallelHashJoinBatch.
>
>> @@ -910,7 +923,8 @@ ExecChooseHashTableSize(double ntuples, int
>> tupwidth, bool useskew,
>>       * this during the initial sizing - once we start building the
>> hash,
>>       * nbucket is fixed.
>>       */
>> -    while (nbatch > 0)
>> +    while (nbatch > 0 &&
>> +           nbuckets * 2 <= max_pointers) /* prevent allocation limit
>> overflow */
>
> Hmm, that doesn't seem quite right. 'max_pointers' is derived from
> work_mem, but the point of this loop is to reduce memory usage, when
> you're already over the work_mem limit. I think we should use a hard
> limit derived only from MaxAllocSize here.
>
Fixed

>>      {
>>          /* how much memory are we using with current nbatch value */
>>          size_t        current_space = hash_table_bytes + (2 * nbatch
>> * BLCKSZ);
>
> Hmm, should we add the memory usage calculated by
> EstimateParallelHashJoinBatch() to this per-batch overhead, too?
>
Should also take in account
`palloc0_array(ParallelHashJoinBatchAccessor, hashtable->nbatch)` - it
is ~80*nbatches.
But may be this multiplier (80) as well multiplier calculated by
EstimateParallelHashJoinBatch (480) can be ignore comparing with BLCKSZ=8kB?
May be it will have not so much influence on choice of optimal ration
between number of buckets and batches?

Attached please find new version of the patch (without change of
current_space calculation).