MySQL queries within a foreach loop - performance issue? - php

Question

The array showcasef holds 20 items per page. I do 3 different queries within the foreach loop, which is 60 queries (just for the loop, there's additional queries too).
<?php
foreach($showcasef as $itemf){
$sf_id = $itemf['sf_id'];
$sf_url = $itemf['sf_url'];
$sf_title = $itemf['sf_title'];
$sf_urltitle = post_slug($sf_title);
// Fetch number of favs
$stmt = $conn->prepare("SELECT COUNT(f_id) FROM favourites WHERE f_showcaseid=?");
$stmt->bind_param("i", $sf_id);
$stmt->execute();
$stmt->bind_result($numfFavs);
$stmt->fetch();
$stmt->close();
// Fetch class
$stmt = $conn->prepare("SELECT avg(r_class) FROM ranks WHERE r_showcaseid=?");
$stmt->bind_param("i", $sf_id);
$stmt->execute();
$stmt->bind_result($sf_class);
$stmt->fetch();
$stmt->close();
// Fetch number of classes
$stmt = $conn->prepare("SELECT COUNT(r_class) FROM ranks WHERE r_showcaseid=?");
$stmt->bind_param("i", $sf_id);
$stmt->execute();
$stmt->bind_result($numfClasses);
$stmt->fetch();
$stmt->close();
?>
Render HTML here
<?php } ?>
Will this be a severe performance issue, or are these particular queries relatively simple? If I keep the columns indexed, should it perform okay with millions of rows (potentially)? Or can the queries be optimized/simplified?
Here's how I get the showcasef:
$stmt = $conn->prepare("SELECT s_id,s_url,s_title FROM showcase WHERE s_userid=? ORDER BY s_date DESC LIMIT $skippingFactor, 20");
$stmt->bind_param("i", $u_id);
$stmt->execute();
$stmt->bind_result($sf_id,$sf_url,$sf_title);
while($stmt->fetch())
{
$showcasef[] = [
'sf_id' => $sf_id,
'sf_url' => $sf_url,
'sf_title' => $sf_title
];
}
$stmt->close();

Answer 1

A few suggestions here.
Reuse prepared statements
You are creating three prepared statements inside the loop. Why don't you create your statements only once, and then reuse them using multiple binds?
<?php
$stmt1 = $conn->prepare("SELECT COUNT(f_id) FROM favourites WHERE f_showcaseid=?");
$stmt1->bind_param("i", $sf_id);
$stmt1->bind_result($numfFavs);
$stmt2 = $conn->prepare("SELECT avg(r_class) FROM ranks WHERE r_showcaseid=?");
$stmt2->bind_param("i", $sf_id);
$stmt2->bind_result($sf_class);
$stmt3 = $conn->prepare("SELECT COUNT(r_class) FROM ranks WHERE r_showcaseid=?");
$stmt3->bind_param("i", $sf_id);
$stmt3->bind_result($numfClasses);
foreach($showcasef as $itemf) {
$sf_id = ...
$stmt1->execute();
$stmt1->fetch();
/* if the fetch succeedes then $numfFavs will contain the count */
$stmt2->execute();
...
$stmt3->execute();
..
}
$stmt1->close();
$stmt2->close();
$stmt3->close();
Use a single query to Count the rows and calculate the average
You can combine the second and third statement a single SQL query:
SELECT COUNT(r_class) AS cnt, AVG(r_class) AS average
FROM ranks
WHERE r_showcaseid=?
Use a single query instead a foreach loop
With the previous suggestions you can get better performances. But are you really sure you need a foreach loop?
If your IDs are returned by another query, instead of a foreach loop is better to use a subquery:
SELECT f_showcaseid, COUNT(f_id)
FROM favourites
WHERE f_showcaseid IN (SELECT id FROM ... WHERE ...)
GROUP BY f_showcaseid
or you can provide a list of IDs to the query:
SELECT f_showcaseid, COUNT(f_id)
FROM favourites
WHERE f_showcaseid IN (?,?,?,?,?)
GROUP BY f_showcaseid
(you can dynamically create the list of ? if the number of IDs is not fixed)

Answer 2

You could do this in a single query I think.
Something like the following:-
SELECT f_showcaseid, COUNT(f_id), avg(r_class), COUNT(r_class)
FROM ranks WHERE r_showcaseid IN (".implode(',', $showcasef).")
GROUP BY f_showcaseid
Of course, to use parameters you would need to do that a bit more elegantly:-
<?php
$stmt = $conn->prepare("SELECT f_showcaseid, COUNT(f_id), avg(r_class), COUNT(r_class)
FROM ranks WHERE r_showcaseid IN (".implode(',', str_split(str_repeat('?', count($showcasef)), 1)).")
GROUP BY f_showcaseid");
foreach($showcasef as $itemf)
{
$stmt->bind_param("i", $itemf['sf_id']);
}
$stmt->execute();
$stmt->bind_result($numfClasses);
$stmt->fetch();
$stmt->close();
?>