Re: zabbix on postgresql - very slow delete of events

Hi.

Just a short message, confirming that after we've altered the tables to have matching
types, deletes now take 1 ms, instead of 5 sec. Indexes are being used now.

Thanks for assistance.

Regards
Kristian

ps. would be nice with some warnings or indications in analyze output when this happens.





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Kristian Ejvind
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From: Maxim Boguk <[email protected]>
Date: Wednesday, 24 July 2019 at 19:17
To: Kristian Ejvind <[email protected]>
Cc: "[email protected]" <[email protected]>
Subject: Re: zabbix on postgresql - very slow delete of events



On Wed, Jul 24, 2019 at 6:12 PM Kristian Ejvind <[email protected]<mailto:[email protected]>> wrote:
Hi Maxim

Thanks for your advice, and let me start with your second email, which I'll copy here:

=====
Hi Kristian,

After comparing structure of zabbix tables with same in my zabbix installation I found one very weird difference.
Why type of events.eventid had been changed from default bigint to numeric?

I suspect that the difference between events.eventid (numeric) type and event_recovery.*_eventid (bigint) types might lead to inability of use index during foreign key checks.
Anyway it will be clearly visible on the pg_stat_xact_user_tables results (I now expect to see 3 sequential scan on event_recovery and may be on some other tables as well).

Kind Regards,
Maxim
=====

Well spotted! On closer examination it seems that data types are wrong in several places. I suspect that this comes
from the time when our Zabbix ran on a MySQL database, which was converted over to PostgreSQL a few years
ago. I agree this discrepancy is suspicious and I will continue to examine it.

Regarding your ideas in the email below, I can say that 1) is not valid, disk latency is in the range of a few ms.
This is the output from your recommended query, which seems to verify your suspicions.

zabbix_34=# begin; delete from zabbix.events where eventid = 7123123; select * from pg_stat_xact_user_tables where seq_scan>0 or idx_scan>0 order by seq_scan+idx_scan desc; rollback;
Time: 0.113 ms
Time: 4798.189 ms (00:04.798)
relid  | schemaname |    relname     | seq_scan | seq_tup_read | idx_scan | idx_tup_fetch | n_tup_ins | n_tup_upd | n_tup_del | n_tup_hot_upd
--------+------------+----------------+----------+--------------+----------+---------------+-----------+-----------+-----------+---------------
  41940 | zabbix     | event_recovery |        3 |     35495224 |        0 |             0 |         0 |         0 |         1 |             0
  41675 | zabbix     | alerts         |        1 |       544966 |        1 |             0 |         0 |         0 |         0 |             0
  42573 | zabbix     | problem        |        2 |        13896 |        0 |             0 |         0 |         0 |         0 |             0
  41943 | zabbix     | event_tag      |        1 |        22004 |        0 |             0 |         0 |         0 |         0 |             0
  41649 | zabbix     | acknowledges   |        1 |           47 |        0 |             0 |         0 |         0 |         0 |             0
  41951 | zabbix     | events         |        0 |            0 |        1 |             1 |         0 |         0 |         1 |             0
260215 | zabbix     | event_suppress |        1 |            0 |        0 |             0 |         0 |         0 |         0 |             0

Hi Kristian,

This result definitely proves that indexes not used during foreign key checks (see that non-zero seq_scan counters for linked tables).
Only possible reason (IMHO) that wrong usage numeric in place of bigint.
I recommend change types of events.eventid (and any other similar fields) to bigint.
It should resolve your performance issues with deletes on events table (as additional bonus - bigint a lot faster and compact type than numeric).

--
Maxim Boguk
Senior Postgresql DBA
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