Date: Wed, 5 Feb 2003 13:51:37 -0500
From: george young <gry@ll.mit.edu>
To: pgsql-sql@postgresql.org
Subject: design question: status table+log table, indexes, triggers
Message-Id: <20030205135137.4abc14b5.gry@ll.mit.edu>
Reply-To: gry@ll.mit.edu
Organization: MIT Lincoln Laboratory
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[postgresql-7.2, pgsql, linux]
Here's a schema-design problem I've hit a few times -- it seems
like there should be a better way:

I have a machine table (140 rows), currently very static:
  machine(machine_name text NOT NULL, machine_id smallint NOT NULL,
          area text NOT NULL, text text NOT NULL);

and a machine_log table (8400 rows), appended to ~4 times/hour:

  machine_log(machine_name text,date timestamp, status text, usr text,
              comment text);

This schema seemed logical at the outset, but the most common query is:
select m.machine_name, m.text, ml.status, ml.date 
   from machine m, machine_log ml 
   where m.machine_name=ml.machine_name and ml.date=(select max(date)from
machine_log where machine_name=ml.machine_name);

This takes ~25 seconds which is way too long for interactive status check.
The max(date) subselect kills me -- I've tried various indexes but to no avail.
It looks like I need to put status and date_changed columns into the
machine table, even though that info is implicit in the machine_log table.  

Is there some other schema that I'm just not thinking of which neatly
stores some static info about each of a list of things as well as log
info of status changes of those things?

Or is there some index I could create on machine_log that would do the
above query fast?

Finally, I've never used triggers or rules -- what's the best way to
maintain the status and date_changed columns in "machine" automatically
when "machine_log" is appended to?


Thanks,
	George
-- 
 I cannot think why the whole bed of the ocean is
 not one solid mass of oysters, so prolific they seem. Ah,
 I am wandering! Strange how the brain controls the brain!
	-- Sherlock Holmes in "The Dying Detective"