Re: Enable partitionwise join for partition keys wrapped by RelabelType

On Mon, Dec 15, 2025 at 10:46 PM Matheus Alcantara
<matheusssilv97(at)gmail(dot)com> wrote:
>
> Hi,
>
> The function exprs_known_equal() is used by the planner to determine if
> two expressions are semantically equivalent, often by checking if they
> belong to the same Equivalence Class (EC).
>

hi.

src/include/nodes/primnodes.h CollateExpr comments:
/*----------
* CollateExpr - COLLATE
*
* The planner replaces CollateExpr with RelabelType during expression
* preprocessing, so execution never sees a CollateExpr.
*----------
*/

examine_variable handling RelabelType (transformed from CollateExpr) is wrong, i
think. Roughly speaking it will reduce "t2.c collate case_insensitive" to
"t2.c". see [1].

If examine_variable does not strip the RelabelType(CollateExpr) node, then
estimate_num_groups->add_unique_group_var may need to deal with RelabelType.

The estimate_num_groups function should account for collation settings. "GROUP
BY a" and "GROUP BY a COLLATE case_insensitive" may result in different row
estimates and should be handled distinctly. Therefore exprs_known_equal within
add_unique_group_var must ensure collation is compared before assuming equality.

In this context, while strippping RelabelType, we should ensure
exprCollation(RelabelType->arg) is the same as the RelabelType->resultcollid.

However, it does not affect partitionwise join, because commit [2] already fixed
the collation issue. so we don't have to worry about
have_partkey_equi_join->exprs_known_equal code path for the RelabelType node.

so i change exprs_known_equal to:

+ /* Remove any relabel decorations if collation match */
+ if (IsA(expr, RelabelType))
+ {
+ RelabelType *relabel = castNode(RelabelType, expr);
+ Expr *rexpr = (Expr *) relabel;
+
+ while (rexpr && IsA(rexpr, RelabelType))
+ rexpr = ((RelabelType *) rexpr)->arg;
+
+ if (exprCollation((Node *) rexpr) == relabel->resultcollid)
+ expr = rexpr;
+ }

[1] https://postgr.es/m/CACJufxGLCiyhM+P0gxesg2x--PTrMY3PszqSqOq_H4QS_oq3Jg@mail.gmail.com
[2] https://git.postgresql.org/cgit/postgresql.git/commit/?id=075acdd93388c080c0fb0aca5723144ad7a56dac

--
jian
https://www.enterprisedb.com