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Re: count(*) of zero rows returns 1

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Gurjeet Singh <singh(dot)gurjeet(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, David Johnston <polobo(at)yahoo(dot)com>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: count(*) of zero rows returns 1
Date: 2013-01-15 04:03:33
Message-ID: 20130115040333.GI5106@alvh.no-ip.org (view raw or flat)
Thread:
Lists: pgsql-hackers
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


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