SQL Execution Plan High Cost 70%

Question

Im running a query that is taking 2 seconds but it should perform better, so I run the Execute Plan details from SQL Managemenet Studio and I found a "step" in the process that the Cost is 70%.

Then, I right click on the item and I found an option that says "Missing Index Details", after I clicked that then a query with a recommendation is generated:

/*
Missing Index Details from SQLQuery15.sql - (local).application_prod (appprod (58))
The Query Processor estimates that implementing the following index could improve the query cost by 68.8518%.
*/

/*
USE [application_prod]
GO
CREATE NONCLUSTERED INDEX [<Name of Missing Index, sysname,>]
ON [dbo].[cloud_document] ([isactivedocument])
INCLUDE ([objectuid])
GO
*/

So my question is exactly what happens if I execute the query? Is it going to affect my database, is there any sideback or sideeffects after applying that?

Thanks a lot and appreciate in advance.

Answer 1

The literal meaning is telling you to build an index on isactivedocument of table [dbo].[cloud_document], from which I assume you are using isactivedocument as a condition to filter the table, and select the column objectuid. Something like

select objectuid, ... from [dbo].[cloud_document] where isactivedocument = 0/1

Note that the "Clustered Index Scan (70%)" doesn't mean it's a problem of the clustered index. It means you don't have index on isactivedocument, then sql engine has to scan clustered index to find what you want. That's why it's taking so much pressure. Once you create index on isactivedocument, check out the plan again, and you'll see the node becomes "index seek", which is a much faster way to find out what you want.

Usually if your database stress mainly comes from query, new index doesn't do much harm to your system. Just go ahead and create the index. But of course you need to keep index quantities as less as possible.