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.von...@enterprisedb.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 EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
>From 7f3b29dae25b08f28cd84dbbeb069d0c7759fafc Mon Sep 17 00:00:00 2001 From: Tomas Vondra <tomas.von...@postgresql.org> Date: Sat, 20 Mar 2021 10:24:00 +0100 Subject: [PATCH] Use valid compression method in brin_form_tuple When compressing the BRIN summary, we can't simply use the compression method from the indexed attribute. The summary may use a different data type, e.g. fixed-length attribute may have varlena summary, leading to compression failures. For the built-in BRIN opclasses this happens to work, because the summary uses the same data type as the attribute. When the data types match, we can inherit use the compression method specified for the attribute (it's copied into the index descriptor). Otherwise we don't have much choice and have to use the default one. Author: Tomas Vondra Discussion: https://postgr.es/m/e0367f27-392c-321a-7411-a58e1a7e4817%40enterprisedb.com --- src/backend/access/brin/brin_tuple.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/src/backend/access/brin/brin_tuple.c b/src/backend/access/brin/brin_tuple.c index 0ab5712c71..279a7e5970 100644 --- a/src/backend/access/brin/brin_tuple.c +++ b/src/backend/access/brin/brin_tuple.c @@ -213,10 +213,20 @@ brin_form_tuple(BrinDesc *brdesc, BlockNumber blkno, BrinMemTuple *tuple, (atttype->typstorage == TYPSTORAGE_EXTENDED || atttype->typstorage == TYPSTORAGE_MAIN)) { + Datum cvalue; + char compression = GetDefaultToastCompression(); Form_pg_attribute att = TupleDescAttr(brdesc->bd_tupdesc, keyno); - Datum cvalue = toast_compress_datum(value, - att->attcompression); + + /* + * If the BRIN summary and indexed attribute use the same data + * type, we can the same compression method. Otherwise we have + * to use the default method. + */ + if (att->atttypid == atttype->type_id) + compression = att->attcompression; + + cvalue = toast_compress_datum(value, compression); if (DatumGetPointer(cvalue) != NULL) { -- 2.30.2
