Re: [HACKERS] Custom compression methods

On 3/20/21 11:45 AM, Tomas Vondra wrote:
>
>
> On 3/20/21 11:18 AM, Dilip Kumar wrote:
>> On Sat, Mar 20, 2021 at 3:05 PM Tomas Vondra
>> <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>>>
>>> Hi,
>>>
>>> I think this bit in brin_tuple.c is wrong:
>>>
>>> ...
>>> Form_pg_attribute att = TupleDescAttr(brdesc->bd_tupdesc,
>>> keyno);
>>> Datum cvalue = toast_compress_datum(value,
>>> att->attcompression);
>>>
>>> The problem is that this is looking at the index descriptor (i.e. what
>>> types are indexed) instead of the stored type. For BRIN those may be
>>> only loosely related, which is why the code does this a couple lines above:
>>>
>>> /* We must look at the stored type, not at the index descriptor. */
>>> TypeCacheEntry *atttype
>>> = brdesc->bd_info[keyno]->oi_typcache[datumno];
>>
>> Ok, I was not aware of this.
>>
>
> Yeah, the BRIN internal structure is not obvious, and the fact that all
> the built-in BRIN variants triggers the issue makes it harder to spot.
>
>>> For the built-in BRIN opclasses this happens to work, because e.g.
>>> minmax stores two values of the original type. But it may not work for
>>> other out-of-core opclasses, and it certainly doesn't work for the new
>>> BRIN opclasses (bloom and minmax-multi).
>>
>> Okay
>>
>>> Unfortunately, the only thing we have here is the type OID, so I guess
>>> the only option is using GetDefaultToastCompression(). Perhaps we might
>>> include that into BrinOpcInfo too, in the future.
>>
>> Right, I think for now we can use default compression for this case.
>>
>
> Good. I wonder if we might have "per type" preferred compression in the
> future, which would address this. But for now just using the default
> compression seems fine.
>

Actually, we can be a bit smarter - when the data types match, we can
use the compression method defined for the attribute. That works fine
for all built-in BRIN opclasses, and it seems quite reasonable - if the
user picked a particular compression method for a column, it's likely
because the data compress better with that method. So why not use that
for the BRIN summary, when possible (even though the BRIN indexes tend
to be tiny).

Attached is a patch doing this. Barring objection I'll push that soon,
so that I can push the BRIN index improvements (bloom etc.).

regards

--
Tomas Vondra
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