Re: max() versus order/limit (WAS: High update activity, PostgreSQL vs BigDBMS)

Did anybody get a chance to look at this? Is it expected behavior?
Everyone seemed so incredulous, I hoped maybe this exposed a bug
that would be fixed in a near release.

-----Original Message-----
From: Adam Rich [mailto:adam(dot)r(at)sbcglobal(dot)net]
Sent: Sunday, January 07, 2007 11:53 PM
To: 'Joshua D. Drake'; 'Tom Lane'
Cc: 'Craig A. James'; 'PostgreSQL Performance'
Subject: RE: [PERFORM] High update activity, PostgreSQL vs BigDBMS

Here's another, more drastic example... Here the order by / limit
version
runs in less than 1/7000 the time of the MAX() version.

select max(item_id)
from events e, receipts r, receipt_items ri
where e.event_id=r.event_id and r.receipt_id=ri.receipt_id

Aggregate (cost=10850.84..10850.85 rows=1 width=4) (actual
time=816.382..816.383 rows=1 loops=1)
-> Hash Join (cost=2072.12..10503.30 rows=139019 width=4) (actual
time=155.177..675.870 rows=147383 loops=1)
Hash Cond: (ri.receipt_id = r.receipt_id)
-> Seq Scan on receipt_items ri (cost=0.00..4097.56
rows=168196 width=8) (actual time=0.009..176.894 rows=168196 loops=1)
-> Hash (cost=2010.69..2010.69 rows=24571 width=4) (actual
time=155.146..155.146 rows=24571 loops=1)
-> Hash Join (cost=506.84..2010.69 rows=24571 width=4)
(actual time=34.803..126.452 rows=24571 loops=1)
Hash Cond: (r.event_id = e.event_id)
-> Seq Scan on receipts r (cost=0.00..663.58
rows=29728 width=8) (actual time=0.006..30.870 rows=29728 loops=1)
-> Hash (cost=469.73..469.73 rows=14843 width=4)
(actual time=34.780..34.780 rows=14843 loops=1)
-> Seq Scan on events e (cost=0.00..469.73
rows=14843 width=4) (actual time=0.007..17.603 rows=14843 loops=1)
Total runtime: 816.645 ms

select item_id
from events e, receipts r, receipt_items ri
where e.event_id=r.event_id and r.receipt_id=ri.receipt_id
order by item_id desc limit 1

Limit (cost=0.00..0.16 rows=1 width=4) (actual time=0.047..0.048 rows=1
loops=1)
-> Nested Loop (cost=0.00..22131.43 rows=139019 width=4) (actual
time=0.044..0.044 rows=1 loops=1)
-> Nested Loop (cost=0.00..12987.42 rows=168196 width=8)
(actual time=0.032..0.032 rows=1 loops=1)
-> Index Scan Backward using receipt_items_pkey on
receipt_items ri (cost=0.00..6885.50 rows=168196 width=8) (actual
time=0.016..0.016 rows=1 loops=1)
-> Index Scan using receipts_pkey on receipts r
(cost=0.00..0.02 rows=1 width=8) (actual time=0.010..0.010 rows=1
loops=1)
Index Cond: (r.receipt_id = ri.receipt_id)
-> Index Scan using events_pkey on events e (cost=0.00..0.04
rows=1 width=4) (actual time=0.009..0.009 rows=1 loops=1)
Index Cond: (e.event_id = r.event_id)
Total runtime: 0.112 ms

-----Original Message-----
From: pgsql-performance-owner(at)postgresql(dot)org
[mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Joshua D.
Drake
Sent: Sunday, January 07, 2007 9:10 PM
To: Adam Rich
Cc: 'Craig A. James'; 'Guy Rouillier'; 'PostgreSQL Performance'
Subject: Re: [PERFORM] High update activity, PostgreSQL vs BigDBMS

On Sun, 2007-01-07 at 20:26 -0600, Adam Rich wrote:
> I'm using 8.2 and using order by & limit is still faster than MAX()
> even though MAX() now seems to rewrite to an almost identical plan
> internally.

Gonna need you to back that up :) Can we get an explain analyze?

> Count(*) still seems to use a full table scan rather than an index
scan.
>

There is a TODO out there to help this. Don't know if it will get done.

Joshua D. Drake

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