I'm running some simple scripts to test possible solutions for an integrity problem I'm solving. Suppose I have a table my_table
|foo |
|1 |
And I have these two snippets:
// db_slow.php
<?php
$db = new PDO('mysql:host=localhost;dbname=my_playground;charset=utf8', 'root', '');
echo 'starting transaction<br />';
$db->beginTransaction();
$stmt = $db->query('select * from my_table for update');
$rows = $stmt->fetchAll();
echo 'count tables: ', count($rows), '<br />';
if (count($rows) == 1) {
sleep(10);
$db->query('insert into my_table(foo) VALUES(2)');
}
$db->commit();
echo 'done';
// db_fast.php
<?php
$db = new PDO('mysql:host=localhost;dbname=my_plyaground;charset=utf8', 'root', '');
echo 'starting transaction<br />';
$db->beginTransaction();
$stmt = $db->query('select * from my_table for update');
$rows = $stmt->fetchAll();
echo 'count tables: ', count($rows), '<br />';
if (count($rows) == 1) {
$db->query('insert into my_table(foo) VALUES(3)');
}
$db->commit();
echo 'done';
db_slow.php
has a 10 second delay to simulate a race condition.
As I understand, select ... for update
locks all rows it selects. If I run db_slow
then db_fast
, db_fast
also has a 10 second delay, as it's waiting for db_slow
as I expect.
However, what I don't get is this is the output:
// db_slow.php
starting transaction
count tables: 1
done
// db_fast.php
starting transaction
count tables: 2
done
And my_table
|foo |
|1 |
|2 |
As I understand, select ... for update
locks all rows that are selected for that transaction. So this is what I expect:
- db_slow: select row 1 and lock it
- db_slow: see that it's only 1 row and wait
- db_fast: try to select row 1, see that it's locked, wait
- db_slow: insert row with '2'
- db_fast: continues because row 1 is unlocked
- db_fast: only selected 1 row, so it inserts '3'
- End up with
foo: 1, 2, 3
The output and delay described above seems to confirm steps 1, 2, 3, 4. It seems like db_fast
is running select after trying to obtain a lock? I thought it would select the one row, then lock or wait.
Somewhat related question:
When I run this with select ... lock in share mode
I end up with
// db_slow.php
starting transaction
count tables: 1
done
// db_fast.php
starting transaction
count tables: 1
done
And my_table
|foo |
|1 |
|3 |
Why is db_slow
not inserting a row '2' even when it thinks there's only 1 row in the table (the condition to insert a row)?