Re: BUG #16886: string equivalence predicate causes system slowdown

Hi,

On Tue, Feb 23, 2021 at 5:26 AM PG Bug reporting form <
noreply(at)postgresql(dot)org> wrote:

>
> Actual Behavior
> We executed both queries on the TPC-H benchmark of scale factor 5: the
> first
> query takes over 23 seconds, while the second query only takes 1.4 seconds.
> We think the time difference results from different plans selected.
> Specifically, in the first (slow) query, the optimizer does not push the
> predicate through the LEFT JOIN operator.
>

Well, in the first (slow) query, the qual "nation.n_name IS NOT distinct
FROM 'ALGERIA'" is not considered as strict. So it cannot be used to
reduce the left outer join here. Meanwhile, since it is a qual above the
outer join and references nullable Var 'nation.n_name', it cannot be
pushed down below the outer join, otherwise we would have unexpected
null-extended rows from the outer join.

Thanks
Richard