MySQL performance hit accessing table simultaneously - php

Question

Following code is a mock-up of my real code. I'm getting a big performance hit when myFunction is called. myTable is no more than a few hundred rows, but calling myFunction adds ~10 seconds execution time. Is there something inherently wrong with trying to access a row of a table inside a loop already accessing that table?
<select>
<?php
$stmt = SQLout ("SELECT ID,Title FROM myTable WHERE LEFT(Title,2) = ? ORDER BY Title DESC",
array ('s', $co), array (&$id, &$co_title));
while ($stmt->fetch()) {
if (myFunction($id)) // skip this function call and save 10 seconds
echo '<option value="' . $co_title . '">' . $co_title . '</option>';
}
$stmt->close();
function myFunction ($id) {
$stmt = SQLout ("SELECT Info FROM myTable WHERE ID = ?",
array ('i', $id), array (&$info));
if ($stmt->fetch()) {
$stmt->close();
if ($info == $something)
return true;
}
return false;
}
?>
SQLout is basically:
$sqli_db->prepare($query);
$stmt->bind_param;
$stmt->execute();
$stmt->bind_result;
return $stmt;

Answer 1

What you're doing is sometimes called the "N+1 queries" problem. You run the first (outer) query 1 times, and it returns N rows. Then you run N subordinate queries, one for each row returned by the first query. Thus N+1 queries. It causes a lot of overhead.
This would have far better performance if you could apply the "something" condition in SQL:
$stmt = SQLout ("SELECT ID,Title FROM myTable
WHERE LEFT(Title,2) = ? AND Info = ... ORDER BY Title DESC",
array ('s', $co), array (&$id, &$co_title));
In general, it's not a good idea to run queries in a loop that depends on how many rows match the outer query. What if the outer query matches 1000000 rows? That means a million queries inside the loop will hit your database for this single PHP request.
Even if today the outer query only matches 3 rows, the fact that you've architected the code in this way means that six months from now, at some unpredictable time, there will be some search that results in a vast overhead, even if your code does not change. The number of queries is driven by the data, not the code.
Sometimes it's necessary to do what you're doing, for instance of the "something" condition is complex and can't be represented by an SQL expression. But you should try in all other cases to avoid this pattern of N+1 queries.

Answer 2

So, if you have a "few hundred rows" in the table, you might be calling myFunction a few hundred times, depending on how many rows are returned in the first query.
Check the number of rows that first query is returning to make sure it meets your expectations.
After that, make sure you have an index on myTable.ID.
After that, I would start looking into system/server level issues. On slower systems, say a laptop hard drive, 10 queries per second might be all you can get.

Answer 3

Try something like this:
$stmt = SQLout ("SELECT ID,Title, Info FROM myTable WHERE LEFT(Title,2) = ? ORDER BY Title DESC",
array ('s', $co), array (&$id, &$co_title, &$info));
while ($stmt->fetch()) {
if (myFunction($info)) // skip this function call and save 10 seconds
echo '<option value="' . $co_title . '">' . $co_title . '</option>';
}
$stmt->close();
function myFunction ($info) {
if ($info == $something)
return true;
}
return false;
}