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1. WORDPRESS MYSQL DEADLOCK UPDATE
2. WORDPRESS MYSQL DEADLOCK MODS
3. WORDPRESS MYSQL DEADLOCK CODE
4. WORDPRESS MYSQL DEADLOCK LICENSE
5. WORDPRESS MYSQL DEADLOCK FREE

Obviously the usual caveats around indexing apply, have too many or excessively large indexes on your tables can hurt write performance so make sure that an index is the right way forward for you. A deadlock can occur when transactions lock rows in multiple tables (through statements such as UPDATE or SELECT. Hopefully the above illustrates how the use of an index can help prevent blocking and ultimately, deadlock situations from occurring. So now we can see that we’re once again scanning the index and now we’re back to the blocking situation. Just to prove that the scan is the cause of the block, we can add FORCESCAN to our query and see what happens. So by adding a covering index we can avoid our session getting blocked and prevent the deadlock from occurring. Notice anything different? Because we’ve built a covering index, we can now perform a seek on the index and avoid the locked record altogether. Because of that, we’ll now see a lock on the new index as well.īut why is our SELECT not getting blocked? Let’s have a look at that execution plan now… Let’s think about the update first, now be aware that because we’ve added an index, our update as also got to update that index too. Let’s create that and try to recreate our original deadlock situation… What about the following index… CREATE INDEX ix_Name_Surname_INCLUDE If we can make it easier for SQL to find that row, we might be able to avoid that block and in turn, avoid the deadlock situation. It can’t go any further at that point so ends up getting blocked. So what that means is that SQL is scanning the clustered index from top to bottom until it hits the locked row. Let’s quickly check the execution plan and see what’s happening…Ī clustered index scan. Now lets run our SELECT statement from the second session and see what happens. The first thing that happens is the UPDATE statement takes out a lock on the row it’s updating. Now for the big question, can we solve this using an index? Before we look at that, let’s have a look at what’s going on inside our ‘name’ table during this situation. Let’s run the following from session 2 SELECT Street, CityĪ pretty straight forward deadlock scenario, I’m not going to explain that here, there’re plenty of resources explaining how and why this happens out there. That’s to be expected of course, session 2 is holding an exclusive lock on ‘Name’ thanks to the UPDATE that it’s not yet committed. Now, back to the first session and I’ll run the following select statement… SELECT ForeName, Surname A pretty standard situation in SQL and not at all sinister. We’re now in a place where we’ve got two sessions each holding an exclusive row lock in their respective tables.

WORDPRESS MYSQL DEADLOCK CODE

On the first session, I’ll run the following code BEGIN TRANSACTION Now I’ll open up two sessions on that database and create a simple deadlock situation. I’ll then populate the pair of them with 500 rows. I’m going to start by creating a couple of rather simple tables. Let’s create a rather simple, contrived deadlock situation. So, can we solve a deadlocking issue with an index? The reaction I got was along the lines of, ‘What, how can in index solve a deadlocking issue?’

WORDPRESS MYSQL DEADLOCK MODS

WORDPRESS MYSQL DEADLOCK LICENSE

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WORDPRESS MYSQL DEADLOCK FREE

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