Concurrency Week How to Delete Just Some Rows from the Really Big Table

Say you’ve got a dining table with millions or huge amounts of rows, and you also need certainly to delete some rows. Deleting ALL of them is quick and that is easy do TRUNCATE TABLE – but things have much harder if you want to delete a small percentage of them, state 5%.

It’s especially painful if you want to do archiving that is regular, like deleting the oldest 1 month of data from a dining table with ten years of information in it.

The trick is making a view that contains the very best, state, 1,000 rows that you would like to delete

Make sure that there’s an index to support your view

After which deleting from the view, maybe not the dining table

Allowing you nibble down deletes in faster, smaller chunks, all while avoiding ugly dining table hair. Just keep operating the DELETE statement until no rows are kept that match. It won’t necessarily be faster general than simply taking one lock and calling it a but it’ll be much more concurrency-friendly day.

Wanna see it for action? No? Then simply copy/paste my code, straight put it into manufacturing as if you constantly do, and obtain back once again to work. For the rest of you, continue reading.

Demoing Fast Ordered Deletes

To demo this system, I’m going to use the cloud setup for our Mastering Query Tuning classes

  • An 8-core, 60GB RAM VM with all the information & log files on ephemeral (fast) SSD
  • The Stack Overflow general public database as of 2017-Aug
  • The dbo ments table – which has 60M rows, 20GB within the index that is clustered
  • I’ve created 5 indexes that are nonclustered total about 5GB of area ( to really make the deletes just a little tougher and more like real-world tables)

The Comments table has A creationdate field, and let’s say I need to delete the oldest comments – we’re likely to delete most of the ones from 2008 and 2009

Responses by 12 months

2008 & 2009 possessed a total of 1,387,218 remarks – but that’s no more than 2.3percent for the table’s rows that are overall.

First, the plain ol’ DELETE.

I possibly could decide to try just deleting them outright

It requires 39 seconds. Here’s exactly what the actual execution plan (PasteThePlan) seems like

DELETE dbo ments WHERE CreationDate cause we’re deleting so numerous rows, SQL Server does a number of sorting, and people sorts also end up spilling to TempDB.

Plus, it is having a big dining table lock since it works. That’s no g d, specially on big tables.

If you’re able to get away with a 39-second table lock and task in TempDB, the plain ol’ DELETE method is okay. But let’s pretend you’re working in an environment that is mission-critical a 39-second dining table lock is going for the question, and also you desire a faster back ground technique.

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Demoing Fast Requested Deletes

Like we talked about at the start of the odyssey, develop a view

Be sure that there’s an index to support your view

And then deleting from the view, not the table

It operates almost immediately (because we’ve got an index to guide it), and here’s the program

Fast ordered deletes plan

In the beginning, it l ks just like the plain DELETE plan, but l k closer, and there’s one thing missing

Similar to me aided by the tequila – no spills

There’s bangs that are no yellow there’s less kind operators and they’re perhaps not spilling to disk. Likewise, the memory grant with this query is way reduced

  • Plain DELETE memory grant 118MB (only 64MB of which gets utilized, however it spills to disk anyhow because not every operator can leverage the grant that is full you can find out about grant fractions from Joe Obbish)
  • Fast Ordered Delete memory grant 1.8MB (only 472KB of which got used)

The grants are lower because we’re managing less data, which is also evidenced by the STATISTICS IO output

  • Ordinary DELETE logical reads 25,022,799 in the Comments dining table (plus another 4.1M in the worktables)
  • Fast Ordered Delete logical reads 24,732 on the Comments table, plus 2K on the worktables – but that’s with me TOP that is using 1,000 the view. If We change it out to TOP 10,000, then the reads jump to 209,163. Still means better than 25,022,799 though, but it raises a g d point….

If you wish to repeat this regularly, tune it.

You’ll mess around with

  • The number of rows in the view (say, 1K, 5K, 10K, etc)
  • The wait time between deletions

That way you can find the spot that is sweet your own personal deletes considering your server’s horsepower, concurrency demands off their inquiries ( some of that will be attempting to simply take table locks on their own), the amount of information you will need to delete, etc. Use the practices Michael J. Swart describes in be careful When Scripting Batches.

For more learning on this topic, read Microsoft SQLCat on Fast requested Deletes – Wayback device copy because Microsoft removed a lot of pages during one of their yearly shuffles that are corporate. You are able to tell it’s old because…MySpace, yeah.

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