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group by week (ww), plus -S performance

From: Michael Blakeley <mike(at)blakeley(dot)com>
To: pgsql-general(at)postgresql(dot)org
Subject: group by week (ww), plus -S performance
Date: 2000-05-27 21:27:17
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-general
I thought I'd pass along a work-around I came up with for the limits 
in 'ww' support (7.0 final). Maybe this would be a useful example for 
the docs? They're a little lean on date/time examples, IMO. So is the 
new book.

Select a count of records from a table, grouped by the week of the 
record. The table is something like

		(event varchar(128) not null, stamp datetime default now());

And I want the output to use human-readable dates, like

     week    | count
  2000-03-06 |     4
  2000-03-13 |     5
  2000-03-20 |     3

My immediate solution was something like
	SELECT to_date(date_part('year',stamp)||'-'||date_part('week',stamp),
		'yyyy-ww'),count(*) FROM EVENTS GROUP BY to_date;

but to_date() doesn't seem to support 'ww' format for text-to-date 
translation (not documented, AFAIK).

The solution I eventually found was
	SELECT to_date(date_part('year',stamp),'yyyy')-5+7*date_part('week',stamp)
		as week,count(*) FROM EVENTS GROUP BY week ORDER BY week;

I haven't tested the '-5' kludge very extensively. It seems to work 
ok, for the dates I tested in 2000. I'm sure it would run into 
trouble with calendar-change weirdness pre-1900.

I'd also love to hear any suggestions for performance improvements - 
it's cpu-bound on my system, and takes about 70 sec to run with 
86,000 rows.

The query plan is
Aggregate  (cost=9155.76..9584.66 rows=8578 width=20)
   ->  Group  (cost=9155.76..9370.21 rows=85780 width=20)
         ->  Sort  (cost=9155.76..9155.76 rows=85780 width=20)
               ->  Seq Scan on events  (cost=0.00..2126.80 rows=85780 width=20)

The order-by clause doesn't seem to add much overhead - the query 
plan is the same with or without it.

I'm running with "-i -N 64 -B 1024 -o '-F'", and I've tried up to '-S 
8192' without seeing any noticeable improvement. At higher values, 
performance actually went down by almost 50% - something to do with 
shmem segment sizes? This is on Solaris 2.6, and I compiled PG7.0 
using gcc 2.95.

The -S does reduce disk I/O, but I think that's only about 5% of the 
work that's going on (90% user time). An index on 'stamp' doesn't 
seem to help, either.

-- Mike


pgsql-general by date

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