Deleting 10 million rows from table

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Deleting 10 million rows from table

vsuresh

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January 30, 2015 at 4:49 am

#397950

A lot of unwanted rows got entered into a table due to a faulty script. This caused the database to grow by 6GB. I need to delete these rows. The table has a unique non-clustered index only, no keys. I tried deleting the rows in chunks.
After deleting about a million rows, I noticed that though the row count has decreased, the table size is growing. I am confused. I stopped deleting the rows till I can figure out why the table size is not decreasing. Can someone please explain to me what is going on. Thanks.

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ChrisM@Work SSC Guru Points: 186127 More actions · Answer 1

vsuresh (1/30/2015)
A lot of unwanted rows got entered into a table due to a faulty script. This caused the database to grow by 6GB. I need to delete these rows. The table has a unique non-clustered index only, no keys. I tried deleting the rows in chunks.
After deleting about a million rows, I noticed that though the row count has decreased, the table size is growing. I am confused. I stopped deleting the rows till I can figure out why the table size is not decreasing. Can someone please explain to me what is going on. Thanks.

If you are saying the table is a heap, i.e. no clustered index, then you are observing expected behaviour:

http://sqlmag.com/blog/curious-case-empty-heap-table

https://technet.microsoft.com/en-us/library/ms189245(v=sql.105).aspx

^{“Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw}

For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden

vsuresh SSC Veteran Points: 225 More actions · Answer 2

Thank you for replying. Do you know why the table size would increase though? If the pages are not getting deallocated because it is a heap, at least wouldn't it remain the same. I would just like to understand better, hence the question. Thanks again.

Jeff Moden SSC Guru Points: 1003888 More actions · Answer 3

Aren't people also adding rows at the same time?

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

vsuresh SSC Veteran Points: 225 More actions · Answer 4

I did this during off hours when no one else is accessing that table, hence I'm puzzled.