Re: Using each rel as both outer and inner for JOIN_ANTI

On 24/06/2021 12:50, Richard Guo wrote:
> Hi hackers,
> 
> We may have anti-joins in several cases. Sublinks of 'NOT EXISTS' may be
> pulled up as anti-joins. Left joins whose join quals are strict for any
> nullable var that is forced null by higher qual levels will also be
> reduced to anti-joins. So anti-joins are very commonly used in practice.
> 
> Currently when populating anti-join with paths, we do not try to swap
> the outer and inner to get both paths. That may make us miss some
> cheaper paths.
> 
> # insert into foo select i, i from generate_series(1,10)i;
> INSERT 0 10
> 
> # insert into bar select i, i from generate_series(1,5000000)i;
> INSERT 0 5000000
> 
> # explain select * from foo left join bar on foo.a = bar.c where bar.c 
> is null;
>                                 QUERY PLAN
> -------------------------------------------------------------------------
>   Hash Anti Join  (cost=154156.00..173691.19 rows=1 width=16)
>     Hash Cond: (foo.a = bar.c)
>     ->  Seq Scan on foo  (cost=0.00..1.10 rows=10 width=8)
>     ->  Hash  (cost=72124.00..72124.00 rows=5000000 width=8)
>           ->  Seq Scan on bar  (cost=0.00..72124.00 rows=5000000 width=8)
> (5 rows)
> 
> I believe if we use the smaller table 'foo' as inner side for this
> query, we would have a cheaper plan.

How would that work?

- Heikki