SQL Math更新性能优化

Question

I am running simple update over >70 Mio. Records (MS-SQL Server 2008)

UPDATE T1 SET COSTS=AMOUNT*0.003

and it takes up to 8 Hours.

Is there an simple "cheap" way to improve math performance of updates like this?

And since there is no WHERE clause or compares, just multiply every record with value, creating an INDEX will have no effect... or?

Thank you very much in advance for your time.

Answer 1

Writing 70 Mio rows does take time. Adding an index would make things even worse, because the index has to be updated along with the table.

Is this update really necessary? Can't you solve the task with a view or computed column or whatever?

Answer 2

I fully agree with the above comments. The issue is not match calculations, instead it is the I/O of the database engine.

I just want to suggested computed columns. You can add a "virtual" column to the table that is calculated from other values in the same row. This means that you might be able to dispense with the update altogether. Just alter the table to add a column:

alter table add costs_new as amount * 0.03.

To keep the current name, you need to change the old name:

exec sp_rename 't1.costs', 'costs_old', 'column';
alter table add costs as amount*0.03;

This may be a quick-and-dirty way to do what you need.

Answer 3

Your problem is not math (calculation of that value), but the fact that sql server needs to read every record and write it to the HDD again. Since calculation itself is really cheap, you can just have costs as calculated field wherever you need it.