Re: BUG #17502: View based on window functions returns wrong results when queried

On Mon, 30 May 2022 at 06:58, Daniel Farkaš <daniel(dot)farkas(at)datoris(dot)com> wrote:
> SELECT metric_name FROM analytics_view;
> metric_name|
> -----------+
> metric_1 |
> metric_1 |
> metric_1 |
> metric_1 |

This is certainly a bug. Thanks for reporting it.

The problem seems to be down to the fact that
remove_unused_subquery_outputs() does not check if the to-be-removed
target entry references WindowClauses which contain set-returning
functions.

We only seem to check if the target entry itself is an SRF, per:

/*
* If it contains a set-returning function, we can't remove it since
* that could change the number of rows returned by the subquery.
*/
if (subquery->hasTargetSRFs &&
expression_returns_set(texpr))
continue;

This ensures queries such as the following don't have SRF columns removed:

postgres=# explain verbose select a from (select
a,generate_series(1,2) as b from t) t;
QUERY PLAN
----------------------------------------------------------------------------
Subquery Scan on t (cost=0.00..131.13 rows=5100 width=4)
Output: t.a
-> ProjectSet (cost=0.00..80.13 rows=5100 width=8)
Output: t_1.a, generate_series(1, 2)
-> Seq Scan on public.t t_1 (cost=0.00..35.50 rows=2550 width=4)
Output: t_1.a
(6 rows)

I'm a little bit uncertain if the correct fix is to have
expression_returns_set() look deeper into WindowFuncs to check if the
WindowClause that the function belongs to has any SRFs in the
PARTITION BY / ORDER BY clause. Unfortunately, doing that means
having to pass the PlannerInfo to expression_returns_set(). I don't
quite see how that could be made to work in the back branches.

The other fix would be to make remove_unused_subquery_outputs() pull
out all WindowFuncs from the texpr and check if any of the
WindowClauses have SRFs in the PARTITION BY / ORDER BY clause.

I'll need to look a bit deeper into the usages of
expression_returns_set() to know which of the fixes is correct. There
might be some other bugs lurking due to expression_returns_set() not
checking WindowClauses for SRFs.

David