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Database Panel

The Database Panel shows the queries executed by the page and the backtrace which initiated the call.

Duplicates & Excessive Callers

To assist in debugging and performance optimization the DB panel 'Queries' tab shows "duplicates" and the 'Callers' tab the source code origin including the number of calls from the same backtrace.

Let's dive into their use and differences based on some examples.
Note: for the sake of simplicity the caller in the examples below is a single file instead of the full backtrace.

Duplicates occur when the exact same query is executed (could be the same or different source code). E.g:

File:Line Query "Duplicates" "No. of Calls"
User.php:20 SELECT * FROM user WHERE id=123; 2 + 1 for User.php:20
Test.php:55 SELECT * FROM user WHERE id=123; 2 + 1 for Test.php:55

Results in 2 duplicates (SELECT * FROM user WHERE id=123;) and 2 callers (User.php:80 making 1 call and Test.php:55 making 1 call).

No. of Calls increase when the exact same source code makes a DB call (could be the same or different queries). E.g:

File:Line Query "Duplicates" "No. of Calls"
User.php:75 SELECT * FROM user WHERE id=123; 0 + 1 for User.php:75
User.php:75 SELECT * FROM user WHERE id=456; 0 + 1 for User.php:75
User.php:75 SELECT * FROM user WHERE id=789; 0 + 1 for User.php:75

Results in 0 duplicates and 1 caller (User.php:75) making 3 calls.

A combination of both duplicates and repeating calls is also possible E.g:

File:Line Query "Duplicates" "No. of Calls"
User.php:80 SELECT * FROM user WHERE id=123; 2 + 1 for User.php:80
User.php:80 SELECT * FROM user WHERE id=456; 0 + 1 for User.php:80
User.php:80 SELECT * FROM user WHERE id=789; 0 + 1 for User.php:80
Test.php:60 SELECT * FROM user WHERE id=123; 2 + 1 for Test.php:60

Results in 2 duplicates (SELECT * FROM user WHERE id=123;) and 2 callers (User.php:80 making 3 calls and Test.php:60 making 1 call).

Configuring Excessive Callers Threshold

Repeated DB calls from the same backtrace is often an indication that code needs to be optimized (e.g. related models need to be eager loaded). In order to be marked an "Excessive Caller" the number of DB calls needs to exceed the excessiveCallerThreshold, these "Excessive Callers" will be highlighted on the 'Callers' tab.

In case you have implemented your own logic for DB query execution, for example extended the yii\db\Command, that code would be seen as the caller and likely be incorrectly considered an "Excessive Caller".
In order to avoid this you can add files and/or folders to the ignoredPathsInBacktrace list.

$config['modules']['debug'] = [
    'class' => 'yii\debug\Module',
    'panels' => [
        'db' => [
            'class' => 'yii\debug\panels\DbPanel',
            'criticalQueryThreshold' => 1000, // Show warning in case the total number of queries exceed this number.
            'excessiveCallerThreshold' => 10, // Increase the "Excessive Caller" threshold
            'ignoredPathsInBacktrace' => [
                '@common/components/db/MyCustomDbCommand', // Ignore custom DB Command 
            ],
        ],
    ],
];

Note: Both $criticalQueryThreshold and $excessiveCallerThreshold can be disabled by setting their value to null.
Changes in $excessiveCallerThreshold will only be reflected in new requests.