Re: BUG #18568: BUG: Result wrong when do group by on partition table!

On Wed, Oct 23, 2024 at 6:43 PM Tender Wang <tndrwang(at)gmail(dot)com> wrote:
>
> I tried the patch I provided in [1], and the regression test cases all passed.
>

////////////////////////
ComputePartitionAttrs code snippet
ELSE
{
/* Expression */
Node *expr = pelem->expr;
char partattname[16];
Assert(expr != NULL);
atttype = exprType(expr);
attcollation = exprCollation(expr);
}
/*
* Apply collation override if any
*/
if (pelem->collation)
attcollation = get_collation_oid(pelem->collation, false);
partcollation[attn] = attcollation;
////////////////////////

create table coll_pruning_multi (a text) partition by range (substr(a,
1) collate "POSIX", substr(a, 1) collate "C");
PartitionElem->expr only cover "substr(a,1)".
PartitionElem->collation is for explicitly COLLATION clauses.
you can also see
https://github.com/postgres/postgres/blob/master/src/backend/parser/gram.y#L4556

From the above "collation override" comments, we can say
exprCollation(PartitionElem->expr)
does not always equal PartitionElem->collation
PartitionElem->collation is the true collation OID.

so you change in but didn't cover the ELSE branch.

else
{
if (lc == NULL)
elog(ERROR, "wrong number of partition key expressions");
/* Re-stamp the expression with given varno. */
partexpr = (Expr *) copyObject(lfirst(lc));
ChangeVarNodes((Node *) partexpr, 1, varno, 0);
lc = lnext(partkey->partexprs, lc);
}

as you mentioned partkey->partcollation is correct collation for PartitionKey.
but the ELSE branch, we cannot do
else
{
if (lc == NULL)
elog(ERROR, "wrong number of partition key expressions");
/* Re-stamp the expression with given varno. */
partexpr = (Expr *) copyObject(lfirst(lc));
ChangeVarNodes((Node *) partexpr, 1, varno, 0);
exprSetCollation(Node *partexpr, Oid collation)
lc = lnext(partkey->partexprs, lc);
}

because in struct inPartitionElem, collation and expr is seperated.
that means after set_baserel_partition_key_exprs
we still cannot be sure that RelOptInfo->partexprs have the correct
PartitionKey collation information.

I doubt [1] your change will solve all the problems.
[1] https://postgr.es/m/CAHewXNnKLrZYG4iqaYw=uB3XWRrYRZHo7VtcMsbUEbdbajQg2Q@mail.gmail.com