How to set a maximum execution time for a mysql qu

I would like to set a maximum execution time for sql queries like set_time_limit() in php. How can I do ?

回答1:

I thought it has been around a little longer, but according to this,

MySQL 5.7.4 introduces the ability to set server side execution time limits, specified in milliseconds, for top level read-only SELECT statements.

SELECT 
MAX_EXECUTION_TIME = 1000 --in milliseconds
* 
FROM table;

Note that this only works for read-only SELECT statements.

Update: This variable was added in MySQL 5.7.4 and renamed to max_execution_time in MySQL 5.7.8. (source)

回答2:

If you're using the mysql native driver (common since php 5.3), and the mysqli extension, you can accomplish this with an asynchronous query:

<?php

// Here's an example query that will take a long time to execute.
$sql = "
    select *
    from information_schema.tables t1
    join information_schema.tables t2
    join information_schema.tables t3
    join information_schema.tables t4
    join information_schema.tables t5
    join information_schema.tables t6
    join information_schema.tables t7
    join information_schema.tables t8
";

$mysqli = mysqli_connect('localhost', 'root', '');
$mysqli->query($sql, MYSQLI_ASYNC | MYSQLI_USE_RESULT);
$links = $errors = $reject = [];
$links[] = $mysqli;

// wait up to 1.5 seconds
$seconds = 1;
$microseconds = 500000;

$timeStart = microtime(true);

if (mysqli_poll($links, $errors, $reject, $seconds, $microseconds) > 0) {
    echo "query finished executing. now we start fetching the data rows over the network...\n";
    $result = $mysqli->reap_async_query();
    if ($result) {
        while ($row = $result->fetch_row()) {
            // print_r($row);
            if (microtime(true) - $timeStart > 1.5) {
                // we exceeded our time limit in the middle of fetching our result set.
                echo "timed out while fetching results\n";
                var_dump($mysqli->close());
                break;
            }
        }
    }
} else {
    echo "timed out while waiting for query to execute\n";
    var_dump($mysqli->close());
}

The flags I'm giving to mysqli_query accomplish important things. It tells the client driver to enable asynchronous mode, while forces us to use more verbose code, but lets us use a timeout(and also issue concurrent queries if you want!). The other flag tells the client not to buffer the entire result set into memory.

By default, php configures its mysql client libraries to fetch the entire result set of your query into memory before it lets your php code start accessing rows in the result. This can take a long time to transfer a large result. We disable it, otherwise we risk that we might time out while waiting for the buffering to complete.

Note that there's two places where we need to check for exceeding a time limit:

The actual query execution
while fetching the results(data)

You can accomplish similar in the PDO and regular mysql extension. They don't support asynchronous queries, so you can't set a timeout on the query execution time. However, they do support unbuffered result sets, and so you can at least implement a timeout on the fetching of the data.

For many queries, mysql is able to start streaming the results to you almost immediately, and so unbuffered queries alone will allow you to somewhat effectively implement timeouts on certain queries. For example, a

select * from tbl_with_1billion_rows

can start streaming rows right away, but,

select sum(foo) from tbl_with_1billion_rows

needs to process the entire table before it can start returning the first row to you. This latter case is where the timeout on an asynchronous query will save you. It will also save you from plain old deadlocks and other stuff.

ps - I didn't include any timeout logic on the connection itself.

回答3:

You can find the answer on this other S.O. question:

MySQL - can I limit the maximum time allowed for a query to run?

a cron job that runs every second on your database server, connecting and doing something like this:

SHOW PROCESSLIST

Find all connections with a query time larger than your maximum desired time

Run KILL [process id] for each of those processes

回答4:

Please rewrite your query like

select /*+ MAX_EXECUTION_TIME(1000) */ * from table

回答5:

pt_kill has an option for such. But it is on-demand, not continually monitoring. It does what @Rafa suggested. However see --sentinel for a hint of how to come close with cron.