## Finding cache candidates

A statement should be considered for caching if it is executed often and has a long run time. Cache candidates are found by creating a list of statements sorted by the product of the number of executions multiplied by the statements run time. The function mysqlnd_qc_get_query_trace_log() returns a query log which help with the task.

Collecting a query trace is a slow operation. Thus, it is disabled by default. The PHP configuration directive `mysqlnd_qc.collect_query_trace` is used to enable it. The functions trace contains one entry for every query issued before the function is called.

Example #1 Collecting a query trace

    mysqlnd_qc.enable_qc=1
    mysqlnd_qc.collect_query_trace=1

`<?php
/* connect to MySQL */
$mysqli = new mysqli("host", "user", "password", "schema", "port", "socket");
/* dummy queries to fill the query trace */
for ($i = 0; $i < 2; $i++) {
$res = $mysqli->query("SELECT 1 AS _one FROM DUAL");
$res->free();
}
/* dump trace */
var_dump(mysqlnd_qc_get_query_trace_log());
?>`

The above examples will output:

    array(2) {
      [0]=>
      array(8) {
        ["query"]=>
        string(26) "SELECT 1 AS _one FROM DUAL"
        ["origin"]=>
        string(102) "#0 qc.php(7): mysqli->query('SELECT 1 AS _on...')
    #1 {main}"
        ["run_time"]=>
        int(0)
        ["store_time"]=>
        int(25)
        ["eligible_for_caching"]=>
        bool(false)
        ["no_table"]=>
        bool(false)
        ["was_added"]=>
        bool(false)
        ["was_already_in_cache"]=>
        bool(false)
      }
      [1]=>
      array(8) {
        ["query"]=>
        string(26) "SELECT 1 AS _one FROM DUAL"
        ["origin"]=>
        string(102) "#0 qc.php(7): mysqli->query('SELECT 1 AS _on...')
    #1 {main}"
        ["run_time"]=>
        int(0)
        ["store_time"]=>
        int(8)
        ["eligible_for_caching"]=>
        bool(false)
        ["no_table"]=>
        bool(false)
        ["was_added"]=>
        bool(false)
        ["was_already_in_cache"]=>
        bool(false)
      }
    }

Assorted information is given in the trace. Among them timings and the origin of the query call. The origin property holds a code backtrace to identify the source of the query. The depth of the backtrace can be limited with the PHP configuration directive `mysqlnd_qc.query_trace_bt_depth`. The default depth is `3`.

Example #2 Setting the backtrace depth with the `mysqlnd_qc.query_trace_bt_depth` ini setting

    mysqlnd_qc.enable_qc=1
    mysqlnd_qc.collect_query_trace=1

`<?php
/* connect to MySQL */
$mysqli = new mysqli("host", "user", "password", "schema", "port", "socket");
$mysqli->query("DROP TABLE IF EXISTS test");
$mysqli->query("CREATE TABLE test(id INT)");
$mysqli->query("INSERT INTO test(id) VALUES (1), (2), (3)");
/* dummy queries to fill the query trace */
for ($i = 0; $i < 3; $i++) {
$res = $mysqli->query("SELECT id FROM test WHERE id = " . $mysqli->real_escape_string($i));
$res->free();
}
$trace = mysqlnd_qc_get_query_trace_log();
$summary = array();
foreach ($trace as $entry) {
if (!isset($summary[$entry['query']])) {
$summary[$entry['query']] = array(
"executions" => 1,
"time" => $entry['run_time'] + $entry['store_time'],
);
} else {
$summary[$entry['query']]['executions']++;
$summary[$entry['query']]['time'] += $entry['run_time'] + $entry['store_time'];
}
}
foreach ($summary as $query => $details) {
printf("%45s: %5dms (%dx)\n",
$query, $details['time'], $details['executions']);
}
?>`

The above examples will output something similar to:

                        DROP TABLE IF EXISTS test:     0ms (1x)
                        CREATE TABLE test(id INT):     0ms (1x)
        INSERT INTO test(id) VALUES (1), (2), (3):     0ms (1x)
                 SELECT id FROM test WHERE id = 0:    25ms (1x)
                 SELECT id FROM test WHERE id = 1:    10ms (1x)
                 SELECT id FROM test WHERE id = 2:     9ms (1x)