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⚡ Optimize random question selection to avoid ORDER BY rand() #12

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⚡ Optimize random question selection to avoid ORDER BY rand() #12

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40 changes: 33 additions & 7 deletions quiz_system_git/quiz.php
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Original file line number Diff line number Diff line change
Expand Up @@ -68,12 +68,38 @@
// but better to use binding or ensure it is int.
$total_questions = (int)$total_questions;

$stmtQ = $pdo->prepare("SELECT * FROM questions WHERE quiz_id=:quizID ORDER BY rand() LIMIT :limit");
$stmtQ->bindValue(':quizID', $final_quiz_ID);
$stmtQ->bindValue(':limit', $total_questions, PDO::PARAM_INT);
$stmtQ->execute();

if ($stmtQ->rowCount() < 1) {
// Optimized: Fetch IDs first, shuffle in PHP, then fetch rows.
// This avoids ORDER BY RAND() which is slow on large datasets.
$stmtIDs = $pdo->prepare("SELECT id FROM questions WHERE quiz_id=:quizID");
$stmtIDs->bindValue(':quizID', $final_quiz_ID);
$stmtIDs->execute();
$all_ids = $stmtIDs->fetchAll(PDO::FETCH_COLUMN);

shuffle($all_ids);
$selected_ids = array_slice($all_ids, 0, $total_questions);

$sorted_questions = [];
if (!empty($selected_ids)) {
// Fetch the questions for the selected IDs
$inQuery = implode(',', array_map('intval', $selected_ids));
$stmtQ = $pdo->query("SELECT * FROM questions WHERE id IN ($inQuery)");
$fetched_rows = $stmtQ->fetchAll(PDO::FETCH_ASSOC);

// Map them by ID for easy lookup
$rows_map = [];
foreach ($fetched_rows as $row) {
$rows_map[$row['id']] = $row;
}

// Reconstruct the array in the shuffled order
foreach ($selected_ids as $id) {
if (isset($rows_map[$id])) {
$sorted_questions[] = $rows_map[$id];
}
}
}

if (empty($sorted_questions)) {
$user_msg = 'Hey, weird, but it seems there are no questions in this quiz!';
header('location: index.php?user_msg='.urlencode($user_msg));
exit();
Expand All @@ -83,7 +109,7 @@
$m_display_ID = 1;

//looping through the questions and adding them on the page
while($m_row = $stmtQ->fetch()){
foreach($sorted_questions as $m_row){
//initializing the options
$m_answers='';

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