Sunday, August 30, 2020

New statistics data in Pgpool-II

 

Upcoming Pgpool-II 4.2 will provide new statistics data such as number of INSERT/DELETE/UPDATE etc.  to you. In this blog I will show you how it will look like.

Existing statistics

Before jumping into the new feature, let's take a look at existing statistics data. PostgreSQL already gives you tuple access data like number of tuples inserted in a database (see pg_stat_database manual). Before Pgpoool-II 4.1 or before gives number of SELECT sent to PostgreSQL in "show pool_nodes" command.

How is like the new statistics?

A new command called "show backend_stats" provides the statistics data like number of NSERT, UPDATE and DELETE issued since Pgpool-II started up. So it's not like PostgreSQL's pg_stat_database in that PostgreSQL provides number of tuples accessed, while Pgpool-II provides number SQL commands issued.  If you want number of tuples based statistics you can always access PostgresQL's statistics data.

 Here is an example of the new statistics data:

test=# show pool_backend_stats;
 node_id | hostname | port  | status |  role   | select_cnt | insert_cnt | update_cnt | delete_cnt | ddl_cnt | other_cnt | panic_cnt | fatal_cnt | error_cnt 
---------+----------+-------+--------+---------+------------+------------+------------+------------+---------+-----------+-----------+-----------+-----------
 0       | /tmp     | 11002 | up     | primary | 12         | 10         | 30         | 0          | 2       | 30        | 0         | 0         | 1
 1       | /tmp     | 11003 | up     | standby | 12         | 0          | 0          | 0          | 0       | 23        | 0         | 0         | 1
(2 rows) 

select_cnt, insert_cnt, update_cnt, delete_cnt, ddl_cnt, other_cnt are the numbers of SQL command issued since Pgpool-II started up. What is "other_cnt"? Well, it is a count of: CHECKPOINT/DEALLOCATE/DISCARD/EXECUTE/EXPLAIN/LISTEN/LOAD/LOCK/NOTIFY/PREPARE/SET/SHOW/Transaction commands/UNLISTEN. If an SQL does not belong to SELECT/INSERT/UPDATE/DELETE/other, then it is counted as DDL.

Please note that these counts are counted up even if an SQL command fails or roll backed.

panic_cnt, fatal_cnt and error_cnt are number of errors returned from PostgreSQL.  They are counts of PANIC, FATAL or ERROR. These error statistics is handy because PostgreSQL does not provide error statistics.

Summary

The "show pool_backend_stats" gives you at a glance view of whole cluster statistics on number SQL commands issued. Moreover, error counts, which does not exists in PostgreSQL could be useful for those who are interested in database or application failures.

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