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An execution plan is basically a road map that graphically or textually shows the data retrieval methods chosen by the SQL Server query optimizer for a stored procedure or ad-hoc query and is a very useful tool for a developer to understand the performance characteristics of a query or stored procedure since the plan is the one that SQL Server will place in its cache and use to execute the stored procedure or query. From within Query Analyzer is an option called “Show Execution Plan” (located on the Query drop-down menu). If this option is turned on it will display query execution plan in separate window when query is ran again.
MS SQL SERVER SERVICE, SQL AGENT SERVICE, DTC (Distribution transac co-ordinator)
Clustered Index is slightly faster than Non Clustered Index. This is because, when a Non Clustered Index is used there is an extra look up from the Non Clustered Index to the table, to fetch the actual rows.

Clustered Index - Only one Clustered Index per table. A clustered index contains all of the data for a table in the index, sorted by the index key. Phone Book is an example for Clustered Index.

Non Clustered Index - You can have multiple Non Clustered Indexes per table. Index at the back of a book is an example for Non Clustered Index.

A Trigger is a database object that is attached to a table. In many aspects it is similar to a stored procedure. As a matter of fact, triggers are often referred to as a "special kind of stored procedure." The main difference between a trigger and a stored procedure is that the former is attached to a table and is only fired when an INSERT, UPDATE or DELETE occurs.

After Triggers : Fired after Insert, Update and Delete operations on a table.

Instead of Triggers : Fired instead of Insert, Update and Delete operations on a table.

Triggers make use of two special tables called inserted and deleted. The inserted table contains the data referenced in an INSERT before it is actually committed to the database. The deleted table contains the data in the underlying table referenced in a DELETE before it is actually removed from the database. When an UPDATE is issued both tables are used. More specifically, the new data referenced in the UPDATE statement is contained in inserted table and the data that is being updated is contained in deleted table.

No, we cannot insert data in table if Clustered Index is disabled because Clustered Indexes are in fact original tables which are physically ordered according to one or more keys (Columns).

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