Re: Sort operation displays more tuples than it contains its subnode

Yes, I got it. Thank you very much for the explanation.

On 23.05.2024 00:17, Tom Lane wrote:
> "a.rybakina" <a(dot)rybakina(at)postgrespro(dot)ru> writes:
>> I faced the issue, when the sorting node in the actual information
>> shows a larger number of tuples than it actually is. And I can not
>> understand why?
> If I'm reading this correctly, the sort node you're worrying about
> feeds the inner side of a merge join. Merge join will rewind its
> inner side to the start of the current group of equal-keyed tuples
> whenever it sees that the next outer tuple must also be joined to
> that group. Since what EXPLAIN is counting is the number of tuples
> returned from the node, that causes it to double-count those tuples.
> The more duplicate-keyed tuples on the outer side, the bigger the
> effect.
>
> You can see the same thing happening at the Materialize a little
> further up, which is feeding the inside of the other merge join.

--
Regards,
Alena Rybakina
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company