Re: count(*) of zero rows returns 1

Gurjeet Singh escribió:

> Interesting to note that SELECT * FROM table_with_zero_cols does not
> complain of anything.
>
> postgres=# select * from test1;
> --
> (0 rows)
>
> This I believe result of the fact that we allow user to drop all columns of
> a table.
>
> On a side note, Postgres allows me to do this (which I don't think is a bug
> or useless): I inserted some rows into a table, and then dropped the
> columns. The resulting table has no columns, but live rows.
>
> postgres=# select * from test_0_col_table ;
> --
> (200000 rows)

Yeah.

alvherre=# create table foo ();
CREATE TABLE
alvherre=# insert into foo default values;
INSERT 0 1
alvherre=# insert into foo default values;
INSERT 0 1
alvherre=# insert into foo default values;
INSERT 0 1
alvherre=# insert into foo default values;
INSERT 0 1
alvherre=# insert into foo select * from foo;
INSERT 0 4
alvherre=# insert into foo select * from foo;
INSERT 0 8
alvherre=# insert into foo select * from foo;
INSERT 0 16
alvherre=# insert into foo select * from foo;
INSERT 0 32
alvherre=# insert into foo select * from foo;
INSERT 0 64
alvherre=# select count(*) from foo;
count
-------
128
(1 fila)

alvherre=# select * from foo;
--
(128 filas)

If you examine the ctid system column you can even see that those empty
rows consume some storage space.

--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services