PHP MySql Query performance with variable declaration - php

Question

Alright - this may be a dumb question, but my desire for perfection is fueling me, so, if it is stupid and pointless, just let me know.
I have a mysql query where I fetch rows from a database table, and I need the value in some shape or form for all the columns I collect. Look at the following sample code (this could use an array to gather output instead - the fundamental question should be the same):
$query="SELECT A,B,C,D FROM table1 WHERE 'X'='Y'";
$result=mysql_query($query,$resource);
while($row=mysql_fetch_assoc($result)){
$var1=$row['A']; $var2=$row['B'];
echo '<li class="' . $row['C'] . '" id="' . $row['D'] . '">';
}
My question is as follows:
I only need $row['A'] and $row['B'] once since they are the same value for every row in this case, but I do need them once. The other values will be different for every row, and I need them as well, as in the example.
Is there a problem with continuously setting the variable throughout the loop? Or is it better to use if(!isset...? or is there some other way to do this? Or is the performance hit so minimal as to make this question irrelevant?
Thanks

Answer 1

why not just set the variables outside the loop.
$result=mysql_query($query,$resource);
$result_new = mysql_fetch_assoc($result);
$var1 = $result_new['A']; //considering $result_new['A'] always exists
$var2=$result_new['B']; //considering $result_new['B'] always exists
while($row=$result_new){
echo '<li class="' . $row['C'] . '" id="' . $row['D'] . '">';
}

Answer 2

Really don't bother with that. Unless you're looping over millions of records.
If you need more speed, don't echo in the while loop:
$result=mysql_query($query,$resource);
$string ='';
if ($result) {
$row=mysql_fetch_assoc($result);
$string .= '<li class="' . $row['C'] . '" id="' . $row['D'] . '">';
$var1=$row['A'];
$var2=$row['B'];
while($row=mysql_fetch_assoc($result)){
$var1=$row['A']; $var2=$row['B'];
$string .= '<li class="' . $row['C'] . '" id="' . $row['D'] . '">';
}
}
echo $string;
Two more things. Don't use mysql_*: Use of this extension is discouraged!
And (like I thought someone else said) if a/b is always the same, you should rethink you're database structure.
Edit:
To give a real answer to your question:
I did a small test.
Setting the value on each loop (1,000 times) takes 0.00027s. Checking inside the loop if the value was already set takes 0.00030s. So just setting it each time is even faster than checking.

Answer 3

the performance hit in this scenario is irrelevant, in fact i would speculate if if(isset..) guards would take a tiny bit longer - only scenario I can think of, where this question would matter would be a situation where you assign (larger) objects or arrays to $var1 and somehow force php to make a copy, not a reference
if it bothers you for aesthetic reasons (I could relate ;)), you could use a do - while loop:
//assuming you always get at least one result
$result=mysql_query($query,$resource);
$row=mysql_fetch_assoc($result);
$var1=$row['A']; $var2=$row['B'];
do {
// the loop stuff - stringify or echo the list
} while ($row=mysql_fetch_assoc($result)) ;

Answer 4

you can do it with a do-while expression
$query="SELECT A,B,C,D FROM table1 WHERE 'X'='Y'";
$result=mysql_query($query,$resource);
if($row=mysql_fetch_assoc($result)) {
$var1=$row['A']; $var2=$row['B'];
do {
echo '<li class="' . $row['C'] . '" id="' . $row['D'] . '">';
} while($row=mysql_fetch_assoc($result));
}