Re: (PATCH) Adding CORRESPONDING to Set Operations - Mailing list pgsql-hackers

> This explain plan doesn't look right to me:
>
> test=# explain select a,b,c from one intersect corresponding by (a,c)
> select a,b,c from two;
>                                   QUERY PLAN
> ---------------------------------------------------------------------------------
>  HashSetOp Intersect  (cost=0.00..117.00 rows=200 width=8)
>   ->  Append  (cost=0.00..97.60 rows=3880 width=8)
>         ->  Subquery Scan on "*SELECT* 3"  (cost=0.00..48.80 rows=1940 width=8)
>               ->  Seq Scan on one  (cost=0.00..29.40 rows=1940 width=8)
>         ->  Subquery Scan on "*SELECT* 4"  (cost=0.00..48.80 rows=1940 width=8)
>               ->  Seq Scan on two  (cost=0.00..29.40 rows=1940 width=8)
> (6 rows)

In the current implementation,

select a,b,c from one intersect corresponding by (a,c) select a,b,c from two;

is translated to equivalent

select a, c from (select a,b,c from one)
intersect
select a, c from (select a,b,c from two);

Methinks that's the reason for this explain output.

Corresponding is currently implemented in the parse/analyze phase. If
it were to be implemented in the planning phase, explain output would
likely be as you expect it to be.