Re: [HACKERS] [POC] hash partitioning - Mailing list pgsql-hackers

On Tue, May 16, 2017 at 1:02 PM, Ashutosh Bapat
<ashutosh.bapat@enterprisedb.com> wrote:
 [...]
>>>
>>> +        if (key->strategy == PARTITION_STRATEGY_HASH)
>>> +        {
>>> +            ndatums = nparts;
>>> +            hbounds = (PartitionHashBound **) palloc(nparts *
>>> +
>>> sizeof(PartitionHashBound *));
>>> +            i = 0;
>>> +            foreach (cell, boundspecs)
>>> +            {
>>> +                PartitionBoundSpec *spec = lfirst(cell);
>>> +
>>> [ clipped ]
>>> +                hbounds[i]->index = i;
>>> +                i++;
>>> +            }
>>> For list and range partitioned table we order the bounds so that two
>>> partitioned tables have them in the same order irrespective of order in which
>>> they are specified by the user or hence stored in the catalogs. The partitions
>>> then get indexes according the order in which their bounds appear in ordered
>>> arrays of bounds. Thus any two partitioned tables with same partition
>>> specification always have same PartitionBoundInfoData. This helps in
>>> partition-wise join to match partition bounds of two given tables.  Above code
>>> assigns the indexes to the partitions as they appear in the catalogs. This
>>> means that two partitioned tables with same partition specification but
>>> different order for partition bound specification will have different
>>> PartitionBoundInfoData represenation.
>>>
>>> If we do that, probably partition_bounds_equal() would reduce to just matching
>>> indexes and the last element of datums array i.e. the greatest modulus datum.
>>> If ordered datums array of two partitioned table do not match exactly, the
>>> mismatch can be because missing datums or different datums. If it's a missing
>>> datum it will change the greatest modulus or have corresponding entry in
>>> indexes array as -1. If the entry differs it will cause mismatching indexes in
>>> the index arrays.
>>>
>> Make sense, will fix this.
>
> I don't see this being addressed in the patches attached in the reply to Dilip.
>

Fixed in the attached version.

>>
>>>
>>> +        if (key->partattrs[i] != 0)
>>> +        {
>>> +            keyCol = (Node *) makeVar(1,
>>> +                                      key->partattrs[i],
>>> +                                      key->parttypid[i],
>>> +                                      key->parttypmod[i],
>>> +                                      key->parttypcoll[i],
>>> +                                      0);
>>> +
>>> +            /* Form hash_fn(value) expression */
>>> +            keyCol = (Node *) makeFuncExpr(key->partsupfunc[i].fn_oid,
>>> +                                    get_fn_expr_rettype(&key->partsupfunc[i]),
>>> +                                    list_make1(keyCol),
>>> +                                    InvalidOid,
>>> +                                    InvalidOid,
>>> +                                    COERCE_EXPLICIT_CALL);
>>> +        }
>>> +        else
>>> +        {
>>> +            keyCol = (Node *) copyObject(lfirst(partexprs_item));
>>> +            partexprs_item = lnext(partexprs_item);
>>> +        }
>>> I think we should add FuncExpr for column Vars as well as expressions.
>>>
>> Okay, will fix this.
>
> Here, please add a check similar to get_quals_for_range()
> 1840             if (partexprs_item == NULL)
> 1841                 elog(ERROR, "wrong number of partition key expressions");
>
>

Fixed in the attached version.

>>
>>> I think we need more comments for compute_hash_value(), mix_hash_value() and
>>> satisfies_hash_partition() as to what each of them accepts and what it
>>> computes.
>>>
>>> +        /* key's hash values start from third argument of function. */
>>> +        if (!PG_ARGISNULL(i + 2))
>>> +        {
>>> +            values[i] = PG_GETARG_DATUM(i + 2);
>>> +            isnull[i] = false;
>>> +        }
>>> +        else
>>> +            isnull[i] = true;
>>> You could write this as
>>> isnull[i] = PG_ARGISNULL(i + 2);
>>> if (isnull[i])
>>>     values[i] = PG_GETARG_DATUM(i + 2);
>>>
>> Okay.
>
> If we have used this technique somewhere else in PG code, please
> mention that function/place.
>         /*
>          * Rotate hash left 1 bit before mixing in the next column.  This
>          * prevents equal values in different keys from cancelling each other.
>          */
>

Fixed in the attached version.

>
>>
>>> +                    foreach (lc, $5)
>>> +                    {
>>> +                        DefElem    *opt = (DefElem *) lfirst(lc);
>>> A search on WITH in gram.y shows that we do not handle WITH options in gram.y.
>>> Usually they are handled at the transformation stage. Why is this an exception?
>>> If you do that, we can have all the error handling in
>>> transformPartitionBound().
>>>
>> If so, ForValues need to return list for hash and PartitionBoundSpec
>> for other two; wouldn't  that break code consistency? And such
>> validation is not new in gram.y see xmltable_column_el.
>
> Thanks for pointing that out. Ok, then may be leave it in gram.y. But
> may be we should move the error handling in transform function.
>

IMO, let it be there for readability.  It will be easier to understand
why do we have set -1 for modulus and remainder.

>
>>
>>> +DATA(insert OID = 5028 ( satisfies_hash_partition PGNSP PGUID 12 1 0
>>> 2276 0 f f f f f f i s 3 0 16 "23 23 2276" _null_ _null_ _null_ _null_
>>> _null_ satisfies_hash_partition _null_ _null_ _null_ ));
>>> Why is third argument to this function ANY? Shouldn't it be INT4ARRAY (variadic
>>> INT4)?
>>>
>> Will use INT4ARRAY in next patch, but I am little sceptical of it.  we
>> need an unsigned int32, but unfortunately there is not variadic uint32
>> support.  How about INT8ARRAY?
>
> Hmm, I think as long as the binary representation of given unsigned
> integer doesn't change in the function call, we could cast an INT32
> datums into unsigned int32, so spending extra 4 bytes per partition
> key doesn't look like worth the effort.
>
> A related question is, all hash functions have return type as
> "integer" but internally they return uint32. Why not to do the same
> for this function as well?

I see. IIUC, there is no harm to use INT4ARRAY,  thanks for explanation.

Regards,
Amul

-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers