Re: Wrong rows count in EXPLAIN

Hi,
>Which Postgres version do you use?
I checked this on PG 11
postgres=# select version();
version
-------------------------------------------------------------------------------------------------------------------------------------------------
PostgreSQL 11.5 on x86_64-w64-mingw32, compiled by x86_64-w64-mingw32-gcc.exe (x86_64-win32-seh-rev0, Built by MinGW-W64 project) 8.1.0, 64-bit
(1 row)

and on PG 13
postgres=# select version();
version
-----------------------------------------------------------------------------------------------------
PostgreSQL 13.5 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0, 64-bit
(1 row)

Both versions shows same strange rows counting in EXPLAINE

________________________________
От: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
Отправлено: 27 апреля 2022 г. 13:08:22
Кому: Пантюшин Александр Иванович
Копия: pgsql-hackers(at)postgresql(dot)org; Тарасов Георгий Витальевич
Тема: Re: Wrong rows count in EXPLAIN

Hi!

> 26 апр. 2022 г., в 13:45, Пантюшин Александр Иванович <AI(dot)Pantyushin(at)gaz-is(dot)ru> написал(а):
>
> When I create a new table, and then I evaluate the execution of the SELECT query, I see a strange rows count in EXPLAIN
> CREATE TABLE test1(f INTEGER PRIMARY KEY NOT NULL);
> ANALYZE test1;
> EXPLAIN SELECT * FROM test1;
> QUERY PLAN
> ---------------------------------------------------------
> Seq Scan on test1 (cost=0.00..35.50 rows=2550 width=4)
> (1 row)
>
> Table is empty but rows=2550. Seem like it was calculated from some default values.
> Is this normal behavior or a bug? Can it lead to a poor choice of the plan of a query in general?

Which Postgres version do you use?

I observe:
postgres=# CREATE TABLE test1(f INTEGER PRIMARY KEY NOT NULL);
CREATE TABLE
postgres=# ANALYZE test1;
ANALYZE
postgres=# EXPLAIN SELECT * FROM test1;
QUERY PLAN
-----------------------------------------------------------------------------
Index Only Scan using test1_pkey on test1 (cost=0.12..8.14 rows=1 width=4)
(1 row)

postgres=# select version();
version
----------------------------------------------------------------------------------------------------------------------
PostgreSQL 15devel on x86_64-apple-darwin19.6.0, compiled by Apple clang version 11.0.3 (clang-1103.0.32.62), 64-bit
(1 row)

Without "ANALYZE test1;" table_block_relation_estimate_size() assumes relation size is 10 blocks.

Best regards, Andrey Borodin.