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Thread: What does count(*) count?

What does count(*) count?

From
Együd Csaba
Date:
Hi all,
I tried to dump out a single table and just for a verification I counted the
number of 'INERT INTO' rows.
I found that count(*) results less rows than grep.
*******************

Csaba@compaq ~
$ pg_dump -d -t t_stockchanges alumil6 > sc.dump

*******************

alumil6=# select count(*) from t_stockchanges ;
 count
-------
  9816
(1 row)

*******************

Csaba@compaq ~
$ cat sc.dump | grep 'INSERT INTO' | wc -l
   4908

*******************
What can be the reason of it?

Thx,

-- Csaba

Re: What does count(*) count?

From
Együd Csaba
Date:
Hi,
I've got it, but I don't know the reason of it.
In the database table the records have duplicated and I've two rows for
every primary key - with different OIDs.
Is there any safe method to get rid of only one of each duplicated row?
Very strange however...

Thank you,
-- Csaba




> -----Original Message-----
> From: Reid Thompson [mailto:Reid.Thompson@ateb.com]
> Sent: 2003. december 31. 15:26
> To: csegyud@vnet.hu
> Subject: RE: [GENERAL] What does count(*) count?
>
>
> scan the file visually and see if anything stands out.
>
> reid
>
>
> > -----Original Message-----
> > From: Együd Csaba [mailto:csegyud@vnet.hu]
> > Sent: Wednesday, December 31, 2003 8:07 AM
> > To: Reid Thompson
> > Subject: RE: [GENERAL] What does count(*) count?
> >
> >
> > Hi Reid,
> > I don't realy understand what you mean. Sholud I count all
> > the 'INSERT' rows by hand?
> >
> > -- Csaba
> >
> >
> > > -----Original Message-----
> > > From: Reid Thompson [mailto:Reid.Thompson@ateb.com]
> > > Sent: 2003. december 31. 13:46
> > > To: csegyud@vnet.hu
> > > Subject: RE: [GENERAL] What does count(*) count?
> > >
> > >
> > > vi sc.dump and see.
> > >
> > > reid
> > >
> > >
> > > > -----Original Message-----
> > > > From: Együd Csaba [mailto:csegyud@vnet.hu]
> > > > Sent: Wednesday, December 31, 2003 3:58 AM
> > > > To: Pgsql-General@Postgresql.Org (E-mail)
> > > > Subject: [GENERAL] What does count(*) count?
> > > >
> > > >
> > > > Hi all,
> > > > I tried to dump out a single table and just for a
> > > > verification I counted the number of 'INERT INTO' rows. I
> > > > found that count(*) results less rows than grep.
> > > > *******************
> > > >
> > > > Csaba@compaq ~
> > > > $ pg_dump -d -t t_stockchanges alumil6 > sc.dump
> > > >
> > > > *******************
> > > >
> > > > alumil6=# select count(*) from t_stockchanges ;
> > > >  count
> > > > -------
> > > >   9816
> > > > (1 row)
> > > >
> > > > *******************
> > > >
> > > > Csaba@compaq ~
> > > > $ cat sc.dump | grep 'INSERT INTO' | wc -l
> > > >    4908
> > > >
> > > > *******************
> > > > What can be the reason of it?
> > > >
> > > > Thx,
> > > >
> > > > -- Csaba
> > > >
> > > >
> > > > ---------------------------(end of
> > > > broadcast)---------------------------
> > > > TIP 9: the planner will ignore your desire to choose an index
> > > > scan if your
> > > >       joining column's datatypes do not match
> > > >
> > > -- Incoming mail is certified Virus Free.
> > > Checked by AVG Anti-Virus (http://www.grisoft.com).
> > > Version: 7.0.209 / Virus Database: 261 - Release Date:
> 2003. 12. 26.
> > >
> >
> -- Incoming mail is certified Virus Free.
> Checked by AVG Anti-Virus (http://www.grisoft.com).
> Version: 7.0.209 / Virus Database: 261 - Release Date: 2003. 12. 26.
>

Re: What does count(*) count?

From
Tom Lane
Date:
=?iso-8859-2?Q?Egy=FCd_Csaba?= <csegyud@vnet.hu> writes:
> I found that count(*) results less rows than grep.

That's a tad hard to believe.  I'm suspecting pilot error of some sort.

Perhaps you have more than one table named t_stockchanges (in different
databases or schemas) and you weren't looking at the same one in both
cases?

Another possibility is that t_stockchanges has child table(s).  Your
SELECT would count rows in the child tables, but I don't think that
pg_dump -t would dump them.

            regards, tom lane

Re: What does count(*) count?

From
Karsten Hilbert
Date:
> In the database table the records have duplicated and I've two rows for
> every primary key - with different OIDs.
> Is there any safe method to get rid of only one of each duplicated row?

select distinct on (oid) * from your_table into working_table

check working_table

truncate your_table

select from working_table into your_table
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Re: What does count(*) count?

From
Tom Lane
Date:
=?iso-8859-1?Q?Egy=FCd_Csaba?= <csegyud@vnet.hu> writes:
> I've got it, but I don't know the reason of it.
> In the database table the records have duplicated and I've two rows for
> every primary key - with different OIDs.

Sure sounds like a child table to me.

> Is there any safe method to get rid of only one of each duplicated row?

Drop the child table.

            regards, tom lane

Re: What does count(*) count?

From
Bruce Momjian
Date:
Tom Lane wrote:
> =?iso-8859-1?Q?Egy=FCd_Csaba?= <csegyud@vnet.hu> writes:
> > I've got it, but I don't know the reason of it.
> > In the database table the records have duplicated and I've two rows for
> > every primary key - with different OIDs.
>
> Sure sounds like a child table to me.
>
> > Is there any safe method to get rid of only one of each duplicated row?
>
> Drop the child table.

This does illustrate an unusual case where a unique index does not
guarantee unique rows from a table:

    test=> CREATE TABLE t1 (x INT);
    CREATE TABLE
    test=> CREATE UNIQUE INDEX ii ON t1(x);
    CREATE INDEX
    test=> CREATE TABLE t2 (y INT) inherits (t1);
    CREATE TABLE
    test=> INSERT INTO t1 VALUES (1);
    INSERT 17389 1
    test=> INSERT INTO t2 VALUES (1,2);
    INSERT 17390 1
    test=> SELECT * FROM t1;
     x
    ---
     1
     1
    (2 rows)

--
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

Re: What does count(*) count?

From
Együd Csaba
Date:
Hi Tom,

> Another possibility is that t_stockchanges has child table(s).  Your
> SELECT would count rows in the child tables, but I don't think that

That's the case. I tried to copy the content of t_stockchanges table into a
temp table.
Being very lazy:) I created the temp table using create table... inhetit
from ... command instead of creating it independently. I haven't read the
manual carefuly enough regarding inherit clause.

> pg_dump -t would dump them.
No, pg_dump doesn't dump them - this was what I found strange.

I suppose this behavior disappears if I drop both table and reload the
t_stockchanges from the dump.

Thank you All.

Bye,
-- Csaba