Question

I was informed by someone senior in our company today that the PHP code I have written for performing prepared statements on a MySQL database is “inefficient” and “too taxing on our server”. Since then I find myself in the difficult position of trying to understand what he meant and then to fix it. I have no contact to said person for four days now so I am asking other developers what they think of my code and if there are any areas that might be causing bottlenecks or issues with server performance.

My code works and returns the results of my query in the variable $data, so technically it works. There is another question though as to whether it is efficient and written well. Any advice as to what that senior employee meant or was referring to? Here is the method I use to connect and query our databases.

(Please note, when I use the word method I do not mean a method inside a class. What I mean to say is this how I write/structure my code when I connect and query our databases.)

<?php

// Create database object and connect to database
$mysqli=new mysqli();
$mysqli->real_connect($hostname, $username, $password, $database);

// Create statement object
$stmt=$mysqli->stmt_init();

// Prepare the query and bind params
$stmt->prepare('SELECT `col` FROM `table` WHERE `col` > ?');
$stmt->bind_param('i', $var1);

// Execute the query
$stmt->execute();

// Store result
$stmt->store_result();

// Prepare for fetching result
$rslt=array();
$stmt->bind_result($rslt['col']);

// Fetch result and save to array
$data=array();
while($stmt->fetch()){
    foreach($rslt as $key=>$value){
        $row[$key]=$value;
    } 
    $data[]=$row;
}

// Free result
$stmt->free_result();

// Close connections
$stmt->close();
$mysqli->close();

?>

Any advice or suggestions are useful, please do contribute and help out even if you are only guessing. Thanks in advance 🙂

Answer 1

There are two types of code that may be inefficient, the PHP code and the SQL code, or both.

For example, the SQL is a problem if the `col` column isn’t indexed in the database. This puts lots of load on the database because the database has to scan very many rows to answer queries. If `col` isn’t indexed in the given query, then all of the rows in the table would be scanned. Also, if the value passed in isn’t very selective, then many rows will have to be examined, perhaps all of the rows, as MySQL will choose a table scan over an index scan when many rows will be examined. You will need to become familiar with the MySQL EXPLAIN plan feature to fix your queries, or add indexes to the database to support your queries.

The PHP would be a problem if you followed something like the pattern:

select invoice_id from invoices where customer_id = ?
for each invoice_id
   select * from line_items where invoice_id = ?   

That kind of pattern will lead to “over querying” the database, which puts extra load on it.
Instead use a join:

select li.* from invoices i join line_items li using (invoice_id)

Ask your database administrator to turn on the slow query log and then process it with pt-query-digest

You can use pt-query-digest to report on queries that are expensive (take a long time to execute) and also to use it to report by frequency to detect over querying.