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Thread: query plan

query plan

From
Torsten Förtsch
Date:
Hi,

This is part of a query plan:

 Nested Loop Left Join  (cost=26.32..47078866.36 rows=1344945195 width=626)
   ->  Nested Loop Left Join  (cost=25.74..5312.48 rows=1344945195 width=608)
         ->  Nested Loop Left Join  (cost=6.79..2876.77 rows=102 width=373)
               ->  Nested Loop Left Join  (cost=1.90..1965.51 rows=102 width=361)
               ->  Bitmap Heap Scan on ...  (cost=4.89..8.91 rows=2 width=28)
         ->  Hash Left Join  (cost=18.95..42.61 rows=3 width=243)
               ->  Hash Left Join  (cost=18.94..42.59 rows=3 width=203)
               ->  Hash  (cost=0.00..0.00 rows=1 width=48)
   ->  Memoize  (cost=0.58..4.59 rows=1 width=172)

What I don't understand is this. The left node of the join is expected to return 102 rows. The right node 3. How can this result in >1e9 rows?

The query involved way further down a partitioned table with 2 partitions, one pretty big in the 1e9 rows range, the other practically empty. The big partition had been analyzed before. But the partitioned table and the empty partition never. After analyzing them all was well.

I am just curious to understand how that number is calculated.

This is PG14.

Thanks.

Re: query plan

From
Tom Lane
Date:
=?UTF-8?Q?Torsten_F=C3=B6rtsch?= <tfoertsch123@gmail.com> writes:
> This is part of a query plan:

>  Nested Loop Left Join  (cost=26.32..47078866.36 rows=1344945195 width=626)
>    ->  Nested Loop Left Join  (cost=25.74..5312.48 rows=1344945195
> width=608)
>          ->  Nested Loop Left Join  (cost=6.79..2876.77 rows=102 width=373)
>                ->  Nested Loop Left Join  (cost=1.90..1965.51 rows=102
> width=361)
>                ->  Bitmap Heap Scan on ...  (cost=4.89..8.91 rows=2
> width=28)
>          ->  Hash Left Join  (cost=18.95..42.61 rows=3 width=243)
>                ->  Hash Left Join  (cost=18.94..42.59 rows=3 width=203)
>                ->  Hash  (cost=0.00..0.00 rows=1 width=48)
>    ->  Memoize  (cost=0.58..4.59 rows=1 width=172)

> What I don't understand is this. The left node of the join is expected to
> return 102 rows. The right node 3. How can this result in >1e9 rows?

The rowcount estimate for the join is not derived by multiplying the
rowcount estimates of the two inputs.  Having said that, this looks
pretty inconsistent.  Can you show a test case that acts like that?

            regards, tom lane