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Index / Performance issues

From: Lucas Adamski <ladamski(at)breakwatersecurity(dot)com>
To: "Postgresql Performance Mailing list (E-mail)" <pgsql-performance(at)postgresql(dot)org>
Subject: Index / Performance issues
Date: 2003-03-08 00:15:42
Message-ID: 3D6F4C287380B1419D7427E111FA3207C68863@OLYMPIA.breakwater.net (view raw or flat)
Thread:
Lists: pgsql-performance
Hi all,
 
I've been using pgsql heavily for about 2 years now, and I keep running into
some index-related wierdness that's rather puzzling.  This is for release
7.2.1, so if a more recent release has solved these, great!  Never the less:
 
I have a table with about 170,000 rows, each of them a network event.  I
also have a serial 8 primary key set up, with a corresponding (unique) btree
index.  The primary key is basically sequential, being incremented
dynamically at insert time.  The problems I've had revolve around selecting
an individual entry, or trying to figure out the current maximum ID in the
table.  In both cases, the results are rather counter-intuitive.  Example
below, with my comments in bold.
I've had this problem using functions such as max(), etc.  For example:

Obvious way, using max():

# explain analyze select max(my_e_id) from my_events;
Aggregate  (cost=68132.85..68132.85 rows=1 width=8) (actual
time=16103.03..16103.03 rows=1 loops=1)
  ->  Seq Scan on my_events  (cost=0.00..67699.28 rows=173428 width=8)
(actual time=0.09..15932.27 rows=173480 loops=1)
Total runtime: 16103.11 msec

Obtuse way, using ORDER BY DESC/LIMIT

# explain analyze select my_e_id from sn_events ORDER BY my_e_id DESC LIMIT
1;
Limit  (cost=0.00..1.48 rows=1 width=8) (actual time=36.02..36.03 rows=1
loops=1)
  ->  Index Scan Backward using my_events_pkey on my_events
(cost=0.00..256931.94 rows=173428 width=8) (actual time=36.02..36.02 rows=2
loops=
1)
Total runtime: 36.09 msec

In this case, the obtuse way is faster... 446 times faster, in fact.  I'd
understand if this was a corner cases, but this has been the situation with
ever PGSQL db I've built.

Here's another example, just trying to pick out a single random entry out of
a 170,000.  
First, the simple approach (status quo):<?xml:namespace prefix = o ns =
"urn:schemas-microsoft-com:office:office" />

# explain analyze select * from my_events WHERE my_e_id = 10800000;

Seq Scan on my_events  (cost=0.00..68132.85 rows=1 width=771) (actual
time=15916.75..16337.31 rows=1 loops=1)
Total runtime: 16337.42 msec

Pretty darned slow.. (16 secs in fact, ouch).  So now lets try our idea with
limiting the query by order it in reverse order, and limiting to 1 result
(even though the limit is unnecessary, but performance is identical without
it)

# explain analyze select * from my_events WHERE my_e_id = 10800000 ORDER BY
my_e_id DESC LIMIT 1;
Limit  (cost=68132.86..68132.86 rows=1 width=771) (actual
time=16442.42..16442.43 rows=1 loops=1)
  ->  Sort  (cost=68132.86..68132.86 rows=1 width=771) (actual
time=16442.42..16442.42 rows=1 loops=1)
        ->  Seq Scan on my_events  (cost=0.00..68132.85 rows=1 width=771)
(actual time=16009.50..16441.91 rows=1 loops=1)
Total runtime: 16442.70 msec

Well, that's not any better... over a few runs, sometimes this was even
slower that the status quo.  Well, at this point there was only one thing
left to try... put in a <= in place of =, and see if it made a difference.

# explain analyze select * from my_events WHERE my_e_id <= 10800000 ORDER BY
my_e_id DESC LIMIT 1;
Limit  (cost=0.00..5.52 rows=1 width=771) (actual time=474.40..474.42 rows=1
loops=1)
  ->  Index Scan Backward using my_events_pkey on my_events
(cost=0.00..257365.51 rows=46663 width=771) (actual time=474.39..474.41
rows=2 loo
ps=1)
Total runtime: 474.55 msec

Oddly enough, it did... note the "Index Scan Backward"... finally!  So for
whatever reason, the DB decides not to use an index scan unless there's a
greater or less than comparison operator in conjunction with an ORDER
BY/LIMIT.  Now it takes half a second, instead of 16.

# explain analyze select * from my_events WHERE my_e_id >= 10800000 ORDER BY
my_e_id LIMIT 1;
Limit  (cost=0.00..2.03 rows=1 width=771) (actual time=1379.74..1379.76
rows=1 loops=1)
  ->  Index Scan using my_events_pkey on my_events  (cost=0.00..257365.51
rows=126765 width=771) (actual time=1379.73..1379.75 rows=2 loops=1)
Total runtime: 1380.10 msec

Just for fun, run it in regular order (front to back, versus back to front,
looking for >=).  Sure enough, still far better than the scan... 1.4 seconds
vs 16.  So even the worst case index scan is still far better than the
default approach.  Note that I tried using "set enable_seqscan=off", and it
STILL insisted on scanning the table, but even slower this time.

Am I missing something really obvious?  Is there a proven way to
consistantly encourage it to use indexes for these sorts of (rather obvious)
queries?

Several runs of the above resulted in some variations in run time, but the
corresponding orders of difference performance stayed pretty consistant.
I'm just confused as to why I have to go through such convoluted methods to
force it to use the index when its obviously a FAR more efficient route to
go regardless of which order it scans it in (forwards or backwards).  Any
thoughts are appreciated.  Thanks!

  Lucas.

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Subject: Re: Index / Performance issues
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