Re: sql/json query function JsonBehavior default expression's collation may differ from returning type's collation

On Wed, Oct 8, 2025 at 12:10 PM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
>
> After sleeping on this, I realized the right fix is not to strip
> CollateExpr in transformJsonBehavior(), but to stop recursing on
> JsonBehavior.expr in exprSetCollation(). With this patch, the parser
> now ensures that the JsonBehavior.expr has the correct collation, so
> the recursion isn’t needed. There is now an Assert in its place that
> checks that JsonBehavior.expr already has the target collation.
>

hi, Amit.
thanks for pushing it.
https://git.postgresql.org/cgit/postgresql.git/commit/?id=ef5e60a9d352a97791af632e0d26a572bc88e921

it took me a little time to understand why
``
exprSetCollation
case T_JsonBehavior:
Assert(((JsonBehavior *) expr)->expr == NULL ||
exprCollation(((JsonBehavior *) expr)->expr) == collation);
``
works, after looking at assign_collations_walker.

it will recursively set JsonBehavior->expr collation first then call
exprSetCollation for JsonBehavior itself.

I will double-check other JSON constructs with RETURNING clauses for potential
collation issues in the future.