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Re: Accessing database statistics

From: "Joshua b(dot) Jore" <josh(at)greentechnologist(dot)org>
To: "Henshall, Stuart - WCP" <SHenshall(at)westcountrypublications(dot)co(dot)uk>
Cc: "'Duncan Adams (DNS)'" <duncan(dot)adams(at)vcontractor(dot)co(dot)za>,"'Tony Griffiths(RA)'" <griffitt(at)cs(dot)man(dot)ac(dot)uk>,<pgsql-novice(at)postgresql(dot)org>
Subject: Re: Accessing database statistics
Date: 2002-05-30 14:10:57
Message-ID: Pine.BSO.4.44.0205300905430.8388-100000@kitten.greentechnologist.org (view raw or flat)
Thread:
Lists: pgsql-novice
Even faster than a trigger (you avoid the overhead of calling a function)
you can use rules to manage this. I do something similar for versioning:

-- tracking rows here
CREATE TABLE OrgPeople (
....
);

CREATE TABLE TableCount (
	TableName NAME NOT NULL,
	-- use INT8 if that's needed
	TableCount INTEGER NOT NULL
);
INSERT INTO TableCount (TableName,TableCount) VALUES
('OrgPeople'::name,0::integer);

CREATE RULE OrgPeopleIns AS ON INSERT TO OrgPeople DO
	UPDATE TableCount SET Count = Count + 1 WHERE TableName =
'orgpeople'::name;
CREATE RULE OrgPeopleDel AS ON DELETE TO OrgPeople DO
	UPDATE TableCount SET Count = Count - 1 WHERE TableName =
'orgpeople'::name;

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On Thu, 30 May 2002, Henshall, Stuart - WCP wrote:

> count(*) does a scan of the appropriate table.
> If you really need a fast count you could try having a trigger update a row
> in another
> table to +1 every time a row is inserted and -1 every time a row deleted.
> However this
> could lead to uneeded contention. It is also worth considering the case of a
> yet to be
> commited transaction that has inserted/deleted having a different count to a
> one that
> does not. The triggers should handle this correctly, but not tried it
> myself.
> hth,
> - Stuart
>
> > -----Original Message-----
> > From: Duncan Adams (DNS) [mailto:duncan(dot)adams(at)vcontractor(dot)co(dot)za]
> > Sent: 30 May 2002 13:37
> > To: 'Tony Griffiths(RA)'; pgsql-novice(at)postgresql(dot)org
> > Subject: Re: [NOVICE] Accessing database statistics
> >
> >
> > o yes i forgot u could try to see
> >
> > EXPLAIN select count(*) from <table>;
> >
> > but i still think tom is u'r best bet for this.
> >
> > Ok, this all depends on how postgresql does this query. If it (behind
> > the scenes) does a call to a system table which holds a field for the
> > count of each table then fine - nice and efficient. However
> > if this does
> > a scan of the appropriate table and counts the number of tuples then
> > returns this figure, then this is a really expensive operation, and I
> > need this to be fast.
> >
> > Tony
> >
> > Duncan Adams (DNS) wrote:
> >
> > >for the first part u might try
> > >
> > >select count(*) from <table>;
> > >
> > >-----Original Message-----
> > >From: Tony Griffiths(RA) [mailto:griffitt(at)cs(dot)man(dot)ac(dot)uk]
> > >Sent: Thursday, May 30, 2002 2:01 PM
> > >To: pgsql-novice(at)postgresql(dot)org
> > >Subject: [NOVICE] Accessing database statistics
> > >
> > >
> > >Hi,
> > >I need to write a query that returns the number of rows
> > currently stored
> > >in a table. I presume that I issue a query against the
> > system tables,
> > >but don't know which one(s) to do this against. So a couple
> > of questions:
> > >
> > >1) With specific reference to my problem, how do I do this?
> > >2) More generally, is there any where that gives detailed
> > descriptions
> > >of the system tables?
> > >
> > >Many thanks,
> > >
> > >
> > >Tony
> > >
> > >
> > >---------------------------(end of
> > broadcast)---------------------------
> > >TIP 6: Have you searched our list archives?
> > >
> > >http://archives.postgresql.org
> > >
> >
> > ---------------------------(end of
> > broadcast)---------------------------
> > TIP 6: Have you searched our list archives?
> >
> > http://archives.postgresql.org
> >
>


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Subject: Re: Accessing database statistics
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