Re: Wrong rows count in EXPLAIN

[Пантюшин Александр Иванович]

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 
Отправлено: 27 апреля 2022 г. 13:08:22
Кому: Пантюшин Александр Иванович
Копия: pgsql-hack...@postgresql.org; Тарасов Георгий Витальевич
Тема: Re: Wrong rows count in EXPLAIN

Hi!

> 26 апр. 2022 г., в 13:45, Пантюшин Александр Иванович 
>  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.

Wrong rows count in EXPLAIN

[Пантюшин Александр Иванович]

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?