Improving a query UPDATE using large mysql databases - php

Question

I'm trying to attempt updating my quite robust database (nearly 3 million rows) with following query:
$length = strlen($this);
$query = "UPDATE database
SET row_to_update='1'
WHERE row='{$this}'
AND row_length='{$length}'
LIMIT 1";
It gets words ($this) from a file (quite a lot of them) and then searches for a match. If found, it updates row_to_update with value 1 (set none as default).
Every row_length contains already value of length of certain cell, which I thought might speed up process significantly. Sadly it didn't.
It manages only ~30k queries in 8h. That's slow, to say the least!
Is there any way, I could improve this bit of inefficient code?

Answer 1

Try to collect a bunch of values you're looking for and use
UPDATE table SET row_to_update='1' WHERE row IN ({$my_values});
You can use EXPLAIN <your_query> and EXPLAIN EXTENDED .. to check if it uses indexes or not and adjust the query or create indexes to speed it up. Play with SELECT with the same WHERE conditions that way.
Much more you can get using:
SET profiling = 1;
<your query goes here>
SHOW PROFILES;
SHOW PROFILE FOR QUERY 1;
Be carefull with it if it's not on dev. env.
Consider as well to fill temp table with the values you're interested in and use it that way:
UPDATE table SET row_to_update='1' WHERE row in (SELECT values FROM my_temp_table);
when you get there than you can improve it to:
UPDATE table INNER JOIN temp_table ON table.row = temp_table.row SET row_to_update = '1';
EXAMPLES:
As you asked for examples. Lat say example table represents your original one with lot of data in it. In this example I'll use just 4 rows:
mysql> select * from example;
+----+------+
| id | data |
+----+------+
| 1 | a |
| 2 | b |
| 3 | c |
| 4 | d |
+----+------+
4 rows in set (0.00 sec)
Let say that you're looking for ids of rows that has data= 'a', 'b', or 'c'
You can do this in 3 ways:
1) SELECT ... IN (list)
mysql> select id from example where data in ('a', 'b', 'c');
+----+
| id |
+----+
| 1 |
| 2 |
| 3 |
+----+
3 rows in set (0.00 sec)
2) SELECT ... IN (SELECT ... FROM temp_table)
mysql> select * from temp_table;
+----+------+
| id | data |
+----+------+
| 10 | foo~ |
| 11 | a |
| 12 | bar |
| 13 | baz |
| 14 | b |
| 15 | c |
+----+------+
6 rows in set (0.00 sec)
mysql> select id from example where data in (SELECT data from temp_table);
[..]
3 rows in set (0.00 sec)
3) SELECT ... INNER JOIN temp_table ...
mysql> select example.id from example inner join temp_table on example.data = temp_table.data;
[..]
3 rows in set (0.01 sec)
And when you'll be ready use UPDATE with the same conditions to apply changes you like.