title: What is the meaning of the "Missing Index Impact %" in a SQL Server 2008 execution plan?
body: <p>I was just examining an estimated execution plan in SSMS. I noticed that a query had query cost of 99% (relative to the batch). I then examined the plan displayed below. That cost was almost entirely coming from a "Clustered Index Delete" on table A. However, the Missing Index recommendation is for Table B. And the Missing Index Impact is said to be 95%.  </p>
  
  <p>The query is a <code>DELETE</code> statement (obviously) which relies on a nested loops <code>INNER JOIN</code> with <code>TableB</code>. If nearly all the cost according to the plan is coming from the <code>DELETE</code> operation, why would the index suggestion be on Table B which -- even though it was a scan -- had a cost of only 0%? Is the impact of 95% an impact against the neglible cost of the scan (listed as on 0%) and not the overall cost of the query (said to be nearly ALL of the batch)? </p>
  
  <p>Please explain IMPACT if possible. Here is the plan:
  <img src="https://i.stack.imgur.com/arqrK.png" alt="Execution Plan"></p>