Обсуждение: postgres reorders expressions when inlining

Hi hackers,

I recently ran into some behavior with inlining that seemed strange to me. I managed to break it down into a small
reproducibleexample that I've added. I originally ran into it in more complicated queries that involved inlining of SQL
functionsinstead of subqueries, but it reproduces with subqueries too so that's the example I've added. Tested on v15. 

create table t1 as select a, b from generate_series(0, 5) a, generate_series(0, 10) b;
select * from (
   select a, b, c
   from t1
   left join lateral
   (
      select 1 / b as c
   ) c on true
   where b <> 0
) as o
where o.c is not null

The SELECT query leads to a division by zero. Plan:

Seq Scan on t1
  Filter: (((1 / b) IS NOT NULL) AND (b <> 0))

Postgres seems to decide that it's smart to execute the "o.c is not null" check before checking if b is 0. However,
thisobviously leads to division by 0. In many other cases (for example not using left join lateral but instead putting
thecalculation straight into the SELECT of the subquery), Postgres does the right thing and checks for 0 first. Also,
forcefullydisabling inlining (by adding "offset 0" to the subquery for example) makes the query execute correctly. 

Is it expected that Postgres reorders these expressions? I'd think that, even though Postgres decides to inline a
query,it should at least evaluate the expressions of the inner-part before the outer part to avoid the division by
zero.

-Floris

Is it expected that Postgres reorders these expressions? I'd think that, even though Postgres decides to inline a query, it should at least evaluate the expressions of the inner-part before the outer part to avoid the division by zero.

Floris Van Nee <florisvannee@Optiver.com> writes:
> I recently ran into some behavior with inlining that seemed strange to me. I managed to break it down into a small
reproducibleexample that I've added. I originally ran into it in more complicated queries that involved inlining of SQL
functionsinstead of subqueries, but it reproduces with subqueries too so that's the example I've added. Tested on v15. 

> create table t1 as select a, b from generate_series(0, 5) a, generate_series(0, 10) b;
> select * from (
>    select a, b, c
>    from t1
>    left join lateral
>    (
>       select 1 / b as c
>    ) c on true
>    where b <> 0
> ) as o
> where o.c is not null

> The SELECT query leads to a division by zero. Plan:

This is not a bug.  We make no ordering guarantees about WHERE clause
execution; if we did, it would cripple the planner's ability to
optimize.

            regards, tom lane