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Re: reading pg_stat_activity view

From: "Scott Marlowe" <scott(dot)marlowe(at)gmail(dot)com>
To: "Tena Sakai" <tsakai(at)gallo(dot)ucsf(dot)edu>
Cc: pgsql-admin(at)postgresql(dot)org
Subject: Re: reading pg_stat_activity view
Date: 2007-12-13 19:47:56
Message-ID: dcc563d10712131147h320259bfke93af844a85eefa4@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-admin
On Dec 13, 2007 1:33 PM, Tena Sakai <tsakai(at)gallo(dot)ucsf(dot)edu> wrote:
>
> Hi everybody,
>
>  As I look at output from pg_stat_activity view,
>  I see often <idle> under query column and on the
>  same row waiting column is always f.  I gather
>  that this is someone sitting at prompt of psql.
>
>  My question: What does it mean when a query column
>  lists a sql statment and waiting column f?  Would
>  this mean something is in a hung state?

Not what that means at all. :)

It means that the query is not waiting on locks from another query to
be released.  Here's an example you can do yourself to see it happen:

T1: # create table test (a int, b text);
T1: # insert into test values (1,'abc');
T1: # insert into test values (2,'def');
T1: # begin;
T1: # select * from test where a=1 for update;
 a |  b
---+-----
 1 | abc
(1 row)
T2: # update test set b='xyz' where a=1;
(T2 enters a wait state waiting on T1 to commit or rollback)

In another psql session, select * from pg_stat_activity where datname='mydb';
16385 | mydb |   13379 |    16384 | myname | update test set b='xyz'
where a=1;                       | t

Make sense?

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