Now I have issue with Update Statistics is taking to long to complete
as mentioned in my 1st post.
Since it's running after the index rebuild, make sure that (if it's a maint plan) you have the selection set for column statistics only, not all.
@GilaMonster: How do I troubleshoot hitting IO bottlenecks?
I suggest you open your favourite search engine and do some reading. It's a large topic.
What does the update Statistics job do internally?
Depends what options you're running it with. In essence, reads part or all of a table, calculates distributions of row values and updates the stored histogram.
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
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