Обсуждение: Performance issues when the number of records are around 10 Million

venu madhav <venutaurus539@gmail.com> wrote:

> When I try to get the last twenty records from the database, it
> takes around 10-15  mins to complete the operation.

Making this a little easier to read (for me, at least) I get this:

select e.cid, timestamp, s.sig_class, s.sig_priority, s.sig_name,
    e.sniff_ip, e.sniff_channel, s.sig_config, e.wifi_addr_1,
    e.wifi_addr_2, e.view_status, bssid
  FROM event e,
       signature s
  WHERE s.sig_id = e.signature
    AND e.timestamp >= '1270449180'
    AND e.timestamp <  '1273473180'
  ORDER BY
    e.cid DESC,
    e.cid DESC
  limit 21
  offset 10539780
;

Why the timestamp range, the order by, the limit, *and* the offset?
On the face of it, that seems a bit confused.  Not to mention that
your ORDER BY has the same column twice.

Perhaps that OFFSET is not needed?  It is telling PostgreSQL that
whatever results are generated based on the rest of the query, read
through and ignore the first ten and a half million.  Since you said
you had about ten million rows, you wanted the last 20, and the
ORDER by is DESCending, you're probably not going to get what you
want.

What, exactly, *is* it you want again?

-Kevin

Venu,

For starters,

1) You have used the e.cid twice in ORDER BY clause.
2) If you want last twenty records in the table matching the criteria of timestamp, why do you need the offset?
3) Do you have indexes on sig_id, signature and timestamp fields?

If you do not get a good response after that, please post the EXPLAIN ANALYZE for the query.

Thanks,

Shrirang Chitnis
Sr. Manager, Applications Development
HOV Services
Office: (866) 808-0935 Ext: 39210
shrirang.chitnis@hovservices.com
www.hovservices.com


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From: pgsql-performance-owner@postgresql.org [mailto:pgsql-performance-owner@postgresql.org] On Behalf Of venu madhav
Sent: Tuesday, May 11, 2010 2:18 PM
To: pgsql-performance@postgresql.org
Subject: [PERFORM] Performance issues when the number of records are around 10 Million

Hi all,
      In my database application, I've a table whose records can reach 10M and insertions can happen at a faster rate
like100 insertions per second in the peak times. I configured postgres to do auto vacuum on hourly basis. I have
frontendGUI application in CGI which displays the data from the database. When I try to get the last twenty records
fromthe database, it takes around 10-15  mins to complete the operation.This is the query which is used: 

select e.cid, timestamp, s.sig_class, s.sig_priority, s.sig_name, e.sniff_ip, e.sniff_channel, s.sig_config,
e.wifi_addr_1,
e.wifi_addr_2, e.view_status, bssid  FROM event e, signature s WHERE s.sig_id = e.signature   AND e.timestamp >=
'1270449180'AND e.timestamp < '1273473180'  ORDER BY e.cid DESC,  e.cid DESC limit 21 offset 10539780; 

Can any one suggest me a better solution to improve the performance.

Please let me know if you've any further queries.


Thank you,
Venu

> * select e.cid, timestamp, s.sig_class, s.sig_priority, s.sig_name,
> e.sniff_ip, e.sniff_channel, s.sig_config, e.wifi_addr_1,
> e.wifi_addr_2, e.view_status, bssid  FROM event e, signature s WHERE
> s.sig_id = e.signature   AND e.timestamp >= '1270449180' AND e.timestamp
> < '1273473180'  ORDER BY e.cid DESC,  e.cid DESC limit 21 offset 10539780;

Anything with an offset that high is going to result in a sequential
scan of most of the table.

--
                                  -- Josh Berkus
                                     PostgreSQL Experts Inc.
                                     http://www.pgexperts.com

On Wed, May 12, 2010 at 1:45 AM, venu madhav <venutaurus539@gmail.com> wrote:
> [Venu] Yes, autovacuum is running every hour. I could see in the log
> messages. All the configurations for autovacuum are disabled except that it
> should run for every hour. This application runs on an embedded box, so
> can't change the parameters as they effect the other applications running on
> it. Can you please explain what do you mean by default parameters.
> autovacuum = on                         # enable autovacuum
> subprocess?
> autovacuum_naptime = 3600               # time between autovacuum runs, in
> secs

The default value for autovacuum_naptime is a minute.  Why would you
want to increase it by a factor of 60?  That seems likely to result in
I/O spikes, table bloat, and generally poor performance.

There are dramatic performance improvements in PostgreSQL 8.3 and 8.4.
 Upgrading would probably help, a lot.

The points already made about LIMIT <some huge value> are also right on target.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company