mysql query very slow

Question

I have a very slow query because using ORDER BY. Now i understand why it is slow but i have no idea how i make it faster.

The table got like 13,000,000 records. (the reason why it is slow)

Tables 30 Rows 13,591,548 Data 1.3 GiB Indexes 265.2 MiB Total 1.5 GiB Overhead 96 B

$sql3 =  mysql_query("SELECT * FROM config LIMIT 1");
$row3 = mysql_fetch_assoc($sql3);
$log=$row3['con_log'];
$sql2 = mysql_query("SELECT sl_start,sl_id FROM scan_list2 WHERE sl_scanned='0' ORDER BY sl_id LIMIT 40");
while($row2 = mysql_fetch_assoc($sql2)){
echo "crk|".$row2['sl_start']."|".$log."-";
$sl_id = $row2['sl_id'];
mysql_query("UPDATE scan_list2 SET sl_scanned='1' WHERE sl_id='$sl_id'");

Is there any better solution for this problem?

Answer 1

First review your table indexes.

In php you don't need to perform one update for each 40 row, run your update then select and after, perform a loop to show information about rows from select , like:

mysql_query("UPDATE scan_list2 SET sl_scanned = '1' WHERE sl_scanned = '0' ORDER BY sl_id LIMIT 40");

$sql3 =  mysql_query("SELECT * FROM config LIMIT 1");
$row3 = mysql_fetch_assoc($sql3);

$log=$row3['con_log'];

$sql2 = mysql_query("SELECT sl_start,sl_id FROM scan_list2 WHERE sl_scanned = '1' ORDER BY sl_id DESC LIMIT 40");

while($row2 = mysql_fetch_assoc($sql2)){
    echo "crk|".$row2['sl_start']."|".$log."-";
}

Answer 2

Try instead change your UPDATE query to update all the records in one update by adding JOIN

UPDATE scan_list2 
INNER JOIN
        (
            SELECT sl_start,sl_id 
            FROM scan_list2 
            WHERE sl_scanned='0' 
            ORDER BY sl_id 
            LIMIT 40
        ) b ON scan_list2.sl_id = b.sl_id 
SET sl_scanned='1'