How can joins impact the efficiency and performance of a PHP application's database design?

Joins can impact the efficiency and performance of a PHP application's database design by increasing the complexity of queries and potentially slowing down data retrieval. To mitigate this impact, it's important to optimize queries by using indexes, limiting the number of joins, and considering denormalization where appropriate. 

// Example of optimizing a query with indexes
$query = "SELECT * FROM users WHERE email = 'example@email.com'";
$result = mysqli_query($connection, $query);

// Create index on the email column for faster retrieval
$index_query = "CREATE INDEX email_index ON users (email)";
mysqli_query($connection, $index_query);

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