Thread

Re: Fix HAVING-to-WHERE pushdown with nondeterministic collations

Richard Guo <guofenglinux@gmail.com> — 2026-05-01T02:47:20Z

On Thu, Apr 30, 2026 at 12:08 PM Richard Guo <guofenglinux@gmail.com> wrote:
> I was about to push the v2 patch, but I just can't shake off the
> concern Wenhui Qiu raised about the repeated subtree scan.  I still
> don't have a concrete real-world case where a query has a large enough
> HAVING clause for it to matter, but let's just be paranoid.
>
> I think we can fix it easily.  The current walker calls
> pull_var_clause() at every collation-aware node, which re-walks the
> subtree.  The fix is to flip it inside out: walk top-down, push
> inputcollids onto a LIFO stack, and at each GROUP Var check against
> the stack.  This way, we only need to walk the expression tree once.
> Attached v3 does this.
>
> v3 also fixes the RowCompareExpr case.  Unlike the node types covered
> by exprInputCollation(), RowCompareExpr carries per-column
> inputcollids[] rather than a single inputcollid, so we need to descend
> into each (largs[i], rargs[i]) pair with the matching collation pushed
> onto the stack.  Without this, a HAVING clause like:
>
>   HAVING ROW(x, 1) < ROW('ABC' COLLATE case_sensitive, 1)
>
> over a case_insensitive group would give wrong results.

I've committed this and back-patched it to v18.  I was not
back-patching further because pre-v18 branches would need a very
different and more complex fix due to the lack of the RTE_GROUP
mechanism.  I think it's too risky, and doesn't seem justified given
the absence of field reports.

- Richard