InnoDB事务-不起作用

Question

I wanted to try out transactions and how they work practically. So I decided to write two scripts in order to test the main function of transactions (handling simultaneous accesses to the database)

I already asked a question here on stackOverflow, and the following was an Edit of that question. But I read through the rules again and I thought it might be wrong to post it under my original question because it's something different. So I ask this in a new question:

My Code (and the database table is set to InnoDB):

On page1.php:

$db->query("START TRANSACTION;");

$db->query("SET AUTOCOMMIT = 0;");
try {
$i = 0;
while ($i <= 120000000) {
    $i++;
}
var_dump($db->query("INSERT INTO test VALUES (NULL, 'testvalue')"));
$db->query("COMMIT;");
}
catch (Exception $e) {
$db->query("ROLLBACK;");
echo $e->getMessage();
}

the query -method works. It just queries the string. And the while -loop is just for me. I need a bit time to go to the other browser tab to load page2.php:

$db->query("START TRANSACTION;");
$db->query("SET AUTOCOMMIT = 0;");
try {
    // outputs an array with the data
    var_dump($db->query("SELECT * FROM test", "assoc"));
    $db->query("COMMIT;");
}
catch (Exception $e) {
    $db->query("ROLLBACK");
    echo $e->getMessage();
}

With the SELECT I get an array with all of the values inside the database table, which was empty at first.

Now I opened page1.php which will insert new data into the database . But first it runs through the loop, which takes about 3-4 seconds. Meanwhile, I open up page2.php .

From my understanding, page2.php should have waited for page1.php to complete its transaction ?? But it just loads as usual and outputs an empty Array.

When I refresh page2.php after page1.php finished loading, I get the correct output with the new data.

Where is my mistake? I don't quite understand it.

EDIT: Here is another one I tried:

page1.php

$db->query("SET AUTOCOMMIT = 0;");
$db->query("START TRANSACTION;");
try {

    //print_r($db->query("DELETE FROM test;", "affected"));
    $i = 200;
    while ($i <= 700) {
        var_dump($db->query("INSERT INTO test VALUES ({$i}, 'testvaluetestvaluetestvaluetestvaluetestvalue')"));
        $i++;
    }

    $db->query("COMMIT;");
}
catch (Exception $e) {
    $db->query("ROLLBACK;");
    echo $e->getMessage();
}

page2.php

$db->query("SET AUTOCOMMIT = 0;");
$db->query("START TRANSACTION;");

try {
    var_dump($db->query("SELECT * FROM test", "assoc"));
    $db->query("COMMIT;");
}
catch (Exception $e) {
    $db->query("ROLLBACK");
    echo $e->getMessage();
}

While page1.php is not completed, page 2 should output nothing , but it outputs the first 70 rows (depending on how fast I reloaded)

Answer 1

transactions try to avoid locking tables / rows wherever possible to improve concurrency. that is a good thing.

what they are for is to ensure that a set of sql statements all execute as an atomic unit.

meaning if an error occurs all the queries within the transaction are rolled back.

how strict / aggressive the locking is can be controlled with isolation modes, more infos in the mysql documentation.

so it sounds like you are misunderstanding theire purpose, the are not a semaphore mechanism.

Answer 2

This is the expected behavior by transactions... What you expected is a pessimistic locking mechanism, but every relational database uses optimistic locking and transaction isolation to make thing faster.

You can read more about this in the pgsql manual . I know your question is about mysql, but it does not really matter, because it's about concurrency control conceptions: ACID properties and transaction isolation levels, etc...