PHP select the biggest `id` value from MySQL table - php

Question

I have a table with more than 300 000 rows and I need to select the highest value for the column 'id'. Usually, I will do like this:
SELECT id FROM my_table ORDER BY id DESC
... but this will cause slow queries and I don't want to use it. Is there a different way to solve this situation? id is auto increment and primary key.
Later Edit: It seems my full code is quite bad written, as I deduct from your comments. Below I posted a sample of the code I'm working and the tables. Can you suggest a proper way to insert the last ID+1 of table_x in two tables (including table_x itself). I have to mention that the script will be running more than once.
TABLE_X TABLE_Y
------------ ----------
id_x | value id_y | id_x
------------ ----------
1 | A 1 | 3
2 | B
3 | C
<?php
for($i=0; $i<10; $i++){
$result_x = mysql_query('SELECT id_x FROM table_x ORDER BY id_x DESC');
$row_x = mysql_fetch_array($result_x);
$next = $row_x['id_x'] + 1;
mysql_query('INSERT INTO table_x(id_x) VALUES("'.$next.'")');
mysql_query('INSERT INTO table_y(id_x) VALUES("'.$next.'")');
}
?>

Answer 1

Slightly better:
SELECT id FROM my_table ORDER BY id DESC LIMIT 1
Significantly better:
SELECT MAX(id) FROM my_table

Answer 2

Here is the right code you have to use.
mysql_query('INSERT INTO table_x(id_x) VALUES(NULL)');
$id = mysql_insert_id();
mysql_query("INSERT INTO table_y(id_x) VALUES($id)");

Answer 3

Depending on the context either
SELECT id FROM my_table ORDER BY id DESC LIMIT 1
or mysql_insert_id() in PHP or (SELECT LAST_INSERT_ID()) in MySQL

Answer 4

As other said, you should use the MAX operator:
SELECT MAX( id ) FROM my_table ORDER BY id DESC
As a general rule of thumb, always reduce the amount of records returned from the database. The database always is faster than your application program when operating on result sets.
In case of slow queries, please give EXPLAIN a try:
EXPLAIN SELECT id FROM my_table ORDER BY id DESC
vs.
EXPLAIN SELECT MAX( id ) FROM my_table ORDER BY id DESC
EXPLAIN ask MySQL's query optimizer how it sees the query. Look in the documentation, to learn how to read its output.
PS: I really wonder, why you need MAX(id). Even if your application gets the value back from the database, it is useless: Another process might just during the next CPU cycle have inserted a new record - and MAX(id) isn't valid any more.

Answer 5

I guess it is slow because you retrieve all 300 000 rows. Add LIMIT 1 to the query.
SELECT id FROM my_table ORDER BY id DESC LIMIT 1
Or use the MAX() operator.