Manual does not describe Innodb_current_row_locks in any useful way

[Valerii Kravchuk]

Manual page about extended SHOW ENGINE INNODB STATUS in Percona Server 5.6 (http://www.percona.com/doc/percona-server/5.6/diagnostics/innodb_show_status.html#transactions) is very brief while describing new status variables and their correspondence to sections of the InnoDB status output. Check this example (TRANSACTION section specifically):

...
------------
TRANSACTIONS
------------
Trx id counter 76600
Purge done for trx's n:o < 76600 undo n:o < 0 state: running but idle
History list length 586
LIST OF TRANSACTIONS FOR EACH SESSION:
---TRANSACTION 76596, not started
MySQL thread id 39, OS thread handle 0x7fef74317700, query id 159 localhost root cleaning up
---TRANSACTION 76595, not started
MySQL thread id 38, OS thread handle 0x7fef74358700, query id 161 localhost root init
show engine innodb status
---TRANSACTION 76597, ACTIVE 106 sec
4 lock struct(s), heap size 1184, 7 row lock(s), undo log entries 6
MySQL thread id 40, OS thread handle 0x7fef742d6700, query id 153 localhost root cleaning up
TABLE LOCK table `test`.`t3` trx id 76597 lock mode IX
RECORD LOCKS space id 1416 page no 3 n bits 72 index `PRIMARY` of table `test`.`t3` trx id 76597 lock_mode X
RECORD LOCKS space id 1416 page no 3 n bits 72 index `PRIMARY` of table `test`.`t3` trx id 76597 lock_mode X
RECORD LOCKS space id 1416 page no 3 n bits 72 index `PRIMARY` of table `test`.`t3` trx id 76597 lock_mode X locks rec but not gap
----------------------------
END OF INNODB MONITOR OUTPUT
============================

1 row in set (0,00 sec)

mysql> show global status like 'Innodb_current_row%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| Innodb_current_row_locks | 3 |
+--------------------------+-------+
1 row in set (0,00 sec)

Now, tell my by the manual (or from any other source) what the value of Innodb_current_row_locks shows in this case and how it is related to anything about row locks? Is it a count of RECORD LOCKS above, something else?

[Valerii Kravchuk (valerii-kravchuk)]

Please, make sure to provide example of transactions section that will allow to show and explain values of all status variables mentioned at http://www.percona.com/doc/percona-server/5.6/diagnostics/innodb_show_status.html#transactions

[Valerii Kravchuk (valerii-kravchuk)]

I've spent some time checking the code to find out what this variable really shows:

[openxs@centos ~]$ cd git/percona-server/
[openxs@centos percona-server]$ grep -rni Innodb_current_row_locks *
doc/source/ps-variables.rst:788: * - :variable:`Innodb_current_row_locks`
doc/source/diagnostics/innodb_show_status.rst:432:.. variable:: Innodb_current_row_locks
storage/innobase/include/srv0srv.h:1047: ulint innodb_current_row_locks;
storage/innobase/srv/srv0srv.cc:1763: export_vars.innodb_current_row_locks
storage/innobase/handler/ha_innodb.cc:809: (char*) &export_vars.innodb_current_row_locks, SHOW_LONG},

So, the only place where it's referred in the code (5.6.27 from GitHub) is in srv0srv.cc:

/******************************************************************//**
Function to pass InnoDB status variables to MySQL */
UNIV_INTERN
void
srv_export_innodb_status(void)
/*==========================*/
{
...
        export_vars.innodb_current_row_locks
                = lock_sys->rec_num;
...

First, let's check when srv_export_innodb_status() is called:

[openxs@centos percona-server]$ grep -rn srv_export_innodb_status *
storage/innobase/include/srv0srv.h:812:srv_export_innodb_status(void);
storage/innobase/srv/srv0mon.cc:1435:srv_export_innodb_status() for related global counters used by
storage/innobase/srv/srv0srv.cc:1577:srv_export_innodb_status(void)
storage/innobase/handler/ha_innodb.cc:13330: srv_export_innodb_status();
[openxs@centos percona-server]$

So, this is where it's called:

/************************************************************************//**
Here we export InnoDB status variables to MySQL. */
static
void
innodb_export_status()
/*==================*/
{
        if (innodb_inited) {
                srv_export_innodb_status();
        }
}

Let's check when innodb_export_status() is called:

[openxs@centos percona-server]$ vi +13330 storage/innobase/handler/ha_innodb.cc
[openxs@centos percona-server]$ grep -rn innodb_export_status *
storage/innobase/handler/ha_innodb.cc:13326:innodb_export_status()
storage/innobase/handler/ha_innodb.cc:16429: innodb_export_status();
[openxs@centos percona-server]$

So, here it is:

/****************************************************************//**
Callback function for accessing the InnoDB variables from MySQL:
SHOW VARIABLES. */
static
int
show_innodb_vars(
/*=============*/
        THD* thd,
        SHOW_VAR* var,
        char* buff)
{
        innodb_export_status();
        var->type = SHOW_ARRAY;
        var->value = (char*) &innodb_status_variables;

        return(0);
}

So, we can assume it's called every time when SHOW VARIABLES is called. Now, let's check where lock_sys->rec_num comes from:

[openxs@centos percona-server]$ grep -rn 'lock_sys->rec_num' *
...
storage/innobase/lock/lock0lock.cc:619: lock_sys->rec_num = 0;
storage/innobase/lock/lock0lock.cc:1869: lock_sys->rec_num++;
storage/innobase/lock/lock0lock.cc:2511: lock_sys->rec_num--;
storage/innobase/lock/lock0lock.cc:2562: lock_sys->rec_num--;
storage/innobase/srv/srv0srv.cc:1764: = lock_sys->rec_num;

Creates a new record lock and...

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-1599