Performance tuning with MDB2

December 9th, 2006. Tagged: mdb2, mysql, PEAR, php

This is a follow-up to Lars' comment about the PEAR book. In the MDB2 chapter I showed an example how you can create custom debug handlers in MDB2 and then gave a suggestion about a useful application of this functionality for performance tuning. Basically the idea is that your custom debug handler collects all queries that are executed during the life of a given script. Then, once the script finishes execution, the debug handler reports the stats that it has collected. In the book, the example is how you count the number of times each distinct query is executed, this way you can spot problems caused by the OO abstraction. For example, say you have a come class Users that has a method loadUser(), which abstracts the database work. While debugging with the custom error handler, you might figure out that without noticing, you're calling this method in a few places and it makes the same repeating query(queries) over and over again. So you can now optimize/cache results and so on.

The suggestion I made in the book is that in addition to counting, you might want to try executing all SELECTs again, just to see how much time they take and you can execute them once again, prepending them with EXPLAIN to get some details on possible room for improvement.

Now here's one solution to this suggestion. What you can see in this script is:

  • Setting up MDB2
  • Declaring a custom debug handler class
  • "Attaching" it to the MDB2 instance
  • Registering it for execution at the end of each script
  • Testing it (creating a DB, table, some queries)

I hope you like it and try it out.

Here's the result of executing this script, you can see what you get back.

Room for improvement

Obviously, the method dumpInfo() can be improved. First, it can print out a nice table, instead of lazy print_r(). Then, it can include some logic, my idea is for it to "understand" the EXPLAIN results and to give you a hint by using colors, for exampe green background for queries that are OK, yellow for warnings and red for queries that definitelly need some work. Could be nice, no?

Test script

Kinda longish, but I hope I added enough comments. I also hope I didn't introduce any syntax errors while formatting it for posting here, chopping long lines, etc.

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<?php
 
// PEAR error handling setup
require_once 'PEAR.php';
function pearError ($e)
{
  echo '<pre>';
  echo $e->getMessage().': '.$e->getUserinfo();
  echo '</pre>';
}
PEAR::setErrorHandling(
  PEAR_ERROR_CALLBACK,
  'pearError'
);
 
// creating MDB2 instance
require_once 'MDB2.php';
$dsn = 'mysql://root:test@localhost';
$mdb2 =& MDB2::factory($dsn);
$mdb2->setFetchMode(MDB2_FETCHMODE_ASSOC);
 
// The custom error handler
//
// It will collect all the queries being executed
// in the script, the collection is done by the
// collectInfo() method.
// Once the script finishes executing, we'll call
// the method executeAndExplain() which will
// execute all unique SELECTs once again
// in order to give us an info of how much time
// each query takes.
// Then executeAndExplain() will execute again
// all SELECTs, this time prepending an EXPLAIN
// so that we can get valuable
// optimization-related information
// Not only that but instead of simple EXPLAIN,
// we can use EXPLAIN EXTENDED and after that
// we can call SHOW WARNINGS -
// this will give us even more optimization hints
//
// http://dev.mysql.com/doc/refman/5.1/en/explain.html
// http://dev.mysql.com/doc/refman/5.1/en/show-warnings.html
//
class Explain_Queries
{
  // how many queries were executed
  var $query_count = 0;
  // which queries and their count
  var $queries = array();
  // results of EXPLAIN-ed SELECTs
  var $explains = array();
  // the MDB2 instance
  var $db = false;
 
  // constructor that accepts MDB2 reference
  function Explain_Queries(&$db) {
    $this->db = $db;
  }
 
  // this method is called on every query
  function collectInfo(
    &$db,
    $scope,
    $message,
    $is_manip = null)
  {
    // increment the total number of queries
    $this->query_count++;
    // the SQL is a key in the queries array
    // the value will be the count of how
    // many times each query was executed
    @$this->queries[$message]++;
  }
 
  // print the debug information
  function dumpInfo()
  {
    echo '<h3>Queries on this page</h3>';
    echo '<pre>';
    print_r($this->queries);
    echo '</pre>';
    echo '<h3>EXPLAIN-ed SELECTs</h3>';
    echo '<pre>';
    print_r($this->explains);
    echo '</pre>';
  }
 
  // the method that will execute all SELECTs
  // with and without an EXPLAIN and will
  // create $this->explains array of debug
  // information
  // SHOW WARNINGS will be called after each
  // EXPLAIN for more information
  function executeAndExplain() {
 
    // at this point, stop debugging
    $this->db->setOption('debug', 0);
    $this->db->loadModule('Extended');
 
    // take the SQL for all the unique queries
    $queries = array_keys($this->queries);
    foreach ($queries AS $sql) {
 
      // for all SELECTs…
      $sql = trim($sql);
      if (stristr($sql,"SELECT") !== false){
        // note the start time
        $start_time = array_sum(
            explode(" ", microtime())
        );
        // execute query
        $this->db->query($sql);
        // note the end time
        $end_time = array_sum(
            explode(" ", microtime())
        );
        // the time the query took
        $total_time = $end_time - $start_time;
 
        // now execute the same query with
        // EXPLAIN EXTENDED prepended
        $explain = $this->db->getAll(
          'EXPLAIN EXTENDED ' . $sql
        );
 
        $this->explains[$sql] = array();
        // update the debug array with the
        // new data from
        // EXPLAIN and SHOW WARNINGS
        if (!PEAR::isError($explain)) {
          $this->explains[$sql]['explain'] = $explain;
          $this->explains[$sql]['warnings'] =
               $this->db->getAll('SHOW WARNINGS');
        }
 
        // update the debug array with the
        // count and time
        $this->explains[$sql]['time'] = $total_time;
      }
    }
  }
}
 
// instance of the custom debug handler
$my_debug_handler = new Explain_Queries($mdb2);
// set debug option
$mdb2->setOption('debug', 1);
// set debug handler to the method that
// collects all queries
$mdb2->setOption(
  'debug_handler',
  array($my_debug_handler, 'collectInfo')
);
// register functions to be executed on shut down
// after the script has finished execution.
// Now that the show's over, it's the time to
// report what happened in this script db-access-wise
// First shutdown function executes the
// SELECTs again, the other one prints the results
register_shutdown_function(
  array($my_debug_handler, 'executeAndExplain')
);
register_shutdown_function(
  array($my_debug_handler, 'dumpInfo')
);
 
 
//
//
// At this point all MDB2 setup is done,
// time for the actual script to do something
//
//
 
// load the DB manager module
$mdb2->loadModule('Manager');
 
// drop database if it exists
// temporarily change the PEAR error handling
PEAR::pushErrorHandling(PEAR_ERROR_RETURN);
$mdb2->dropDatabase('test_db_explain');
PEAR::popErrorHandling();
 
// create and set a new database
$mdb2->createDatabase('test_db_explain');
$mdb2->setDatabase('test_db_explain');
 
// create table "events" from a definition array
// the table has event ID, name and date/time
$definition = array (
  'id' => array (
    'type' => 'integer',
    'unsigned' => 1,
    'notnull' => 1,
    'default' => 0,
  ),
  'name' => array (
    'type' => 'text',
    'length' => 255
  ),
  'datetime' => array (
    'type' => 'timestamp'
  )
);
 
$mdb2->createTable('events', $definition);
 
// create a primary key - the ID field
$definition = array (
  'primary' => true,
  'fields' => array (
    'id' => array()
  )
);
$mdb2->createConstraint(
  'events',
  'myprimekey',
  $definition
);
 
// load the class that has some static helper
// functions to work with MDB2's cross-RDBMS
// date format
MDB2::loadFile('Date');
 
// INSERT
// some data to insert into the events table
$data = array(
  // using MDB2-managed sequences
  'id'     => $mdb2->nextId('events'),
  'name'     => "Breakfast a Tiffany's",
  'datetime'   => MDB2_Date::unix2Mdbstamp(
    strtotime('Jan 15, 2007')
  )
);
// The "datetime" value shows how you can use
// any date format you wish as long as you're
// able to get a unix timestamp out of it
// In this case I'm using strtotime()
// Then there is a call to MDB2's date helper
// to get the MDB2 timestamp
 
// auto insert
// for the autoExecute() method we need to
// load the Extended module
$mdb2->loadModule('Extended');
$result = $mdb2->autoExecute(
  'events',
  $data,
  MDB2_AUTOQUERY_INSERT
);
 
//
// Time to SELECT something
//
// Using date helpers again
$start_date = MDB2_Date::date2Mdbstamp(0,0,0,12,31,1980);
$end_date   = MDB2_Date::date2Mdbstamp(0,0,0,12,31,2020);
$sql = 'SELECT * FROM %s WHERE %s > %s AND %s < %s';
$sql = sprintf(
  $sql,
  $mdb2->quoteIdentifier('events'),   // quote table name
  $mdb2->quoteIdentifier('datetime'), // quote field name
  $mdb2->quote($start_date, 'date'),  // quote data as date
  $mdb2->quoteIdentifier('datetime'), // quote field name
  $mdb2->quote($end_date,   'date')   // quote data as date
);
$res = $mdb2->getAll($sql); // execute
 
//
// * Bad practice code follows *
// Just some more inserts and selects, bad
// practice because these queries are not
// necessarily portable accross various RDBMS,
// lacking proper quoting and preparation
for ($i = 2; $i < 31; $i++) {
  $mdb2->query(
    'INSERT INTO events VALUES('
    . $i
    . ', "test event", "2005-05-05 00:00:00")'
  );
}
$res = $mdb2->getCol(
  'SELECT DISTINCT datetime FROM events'
);
$res = $mdb2->getRow(
  'SELECT * FROM events WHERE id
  IN (SELECT id FROM events WHERE id > 1)'
);
?>

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