Question

In MySQL I have to check whether select query has returned any records, if not I insert a record. I am afraid though that the whole if-else operation in PHP scripts is NOT as atomic as I would like, i.e. will break in some scenarios, for example if another instance of the script is called where the same record needs to be worked with:

if(select returns at least one record)
{
    update record;
}
else
{
    insert record;
}

I did not use transactions here, and autocommit is on. I am using MySQL 5.1 with PHP 5.3. The table is InnoDB. I would like to know if the code above is suboptimal and indeed will break. I mean the same script is re-entered by two instances and the following query sequence occurs:

1. instance 1 attempts to select the record, finds none, enters the block for insert query
2. instance 2 attempts to select the record, finds none, enters the block for insert query
3. instance 1 attempts to insert the record, succeeds
4. instance 2 attempts to insert the record, fails, aborts the script automatically

Meaning that instance 2 will abort and return an error, skipping anything following the insert query statement. I could make the error not fatal, but I don’t like ignoring errors, I would much rather know if my fears are real here.

Update: What I ended up doing (is this ok for SO?)

The table in question assists in a throttling (allow/deny, really) amount of messages the application sends to each recipient. The system should not send more than X messages to a recipient Y within a period Z. The table is [conceptually] as follows:

create table throttle
(
    recipient_id integer unsigned unique not null,
    send_count integer unsigned not null default 1,
    period_ts timestamp default current_timestamp,
    primary key (recipient_id)
) engine=InnoDB;

And the block of [somewhat simplified/conceptual] PHP code that is supposed to do an atomic transaction that maintains the right data in the table, and allows/denies sending message depending on the throttle state:

function send_message_throttled($recipient_id) /// The 'Y' variable
{
    query('begin');

    query("select send_count, unix_timestamp(period_ts) from throttle where recipient_id = $recipient_id for update");

    $r = query_result_row();

    if($r)
    {
        if(time() >= $r[1] + 60 * 60 * 24) /// The numeric offset is the length of the period, the 'Z' variable
        {/// new period
            query("update throttle set send_count = 1, period_ts = current_timestamp where recipient_id = $recipient_id");
        }
        else
        {
            if($r[0] < 5) /// Amount of messages allowed per period, the 'X' variable
            {
                query("update throttle set send_count = send_count + 1 where recipient_id = $recipient_id");
            }
            else
            {
                trigger_error('Will not send message, throttled down.', E_USER_WARNING);
                query('rollback');
                return 1;
            }
        }
    }
    else
    {
        query("insert into throttle(recipient_id) values($recipient_id)");
    }

    if(failed(send_message($recipient_id)))
    {
        query('rollback');
        return 2;
    }

    query('commit');
}

Well, disregarding the fact that InnoDB deadlocks occur, this is pretty good no? I am not pounding my chest or anything, but this is simply the best mix of performance/stability I can do, short of going with MyISAM and locking entire table, which I don’t want to do because of more frequent updates/inserts vs selects.

Answer 1

It seems like you already know the answer to the question, and how to solve your problem. It is a real problem, and you can use one of the following to solve it:

• SELECT … FOR UPDATE
• INSERT … ON DUPLICATE KEY UPDATE
• transactions (don’t use MyIsam)
• table locks