Re: wrong query result with jit_above_cost= 0

On 2018-06-26 22:09:10 +0200, Dmitry Dolgov wrote:
> > On 26 June 2018 at 20:23, Andres Freund <andres@anarazel.de> wrote:
> > Hi,
> >
> > On 2018-06-26 23:50:32 +0530, Rushabh Lathia wrote:
> >> I found the below query which returns the wrong output
> >> when jit_above_cost= 0 is set.
> >>
> >> Steps to reproduce:
> >>
> >> CREATE TABLE emp (
> >>     epno           NUMERIC(4),
> >>     ename           VARCHAR(10),
> >>     job             VARCHAR(9),
> >>     mgr             NUMERIC(4),
> >>     hiredate        DATE,
> >>     sal             NUMERIC(7,2),
> >>     comm            NUMERIC(7,2),
> >>     deptno          NUMERIC(2)
> >> );
> >>
> >> INSERT INTO emp VALUES (7369,'SMITH','CLERK',7902,'17-DEC-80',800,NULL,20);
> >> INSERT INTO emp VALUES (7499,'ALLEN','SALESMAN',7698,'20-FEB-81',1600,300,30);
> >>
> >> set jit_above_cost= 0;
> >>
> >> select max(epno) from emp group by rollup((deptno,epno)) order by 1 asc;
> >>
> >> without the ROLLUP, I don't see any problem with results.
> >
> > Interesting.  I've opened an open item referencing this.
> 
> Hi,
> 
> Just out of curiosity, what exactly is wrong with the output of this query? I
> see the same results with jit_above_cost = 0 and with the default value:
> 
> =# show jit_above_cost;
>  jit_above_cost
> ----------------
>  100000
> (1 row)
> 
> =# select max(epno) from emp group by rollup((deptno,epno)) order by 1 asc;
>  max
> ------
>  7369
>  7499
>  7499
> (3 rows)
> 
> =# set jit_above_cost = 0;
> SET
> =# select max(epno) from emp group by rollup((deptno,epno)) order by 1 asc;
>  max
> ------
>  7369
>  7499
>  7499
> (3 rows)
> 
> And as far as I understand it's totally correct, since we do rollup by just two
> values and have one more row as a total (with NULLs):
> 
> =# select max(epno), deptno, epno
>    from emp group by rollup((deptno,epno)) order by 1 asc;
> 
>  max  | deptno | epno
> ------+--------+------
>  7369 |     20 | 7369
>  7499 |   NULL | NULL
>  7499 |     30 | 7499
> (3 rows)

I've not reproduced the problem yet (I'm deep in a review / edit of
another patchset). Could it be that you've not compiled with JIT
support and thus don't see the problem Rushab was complaining about?
SELECT pg_jit_available();

Greetings,

Andres Freund