asked
Is there any tool to explain how a query executes in SQL Server
Hi Is there any tool which explains how a query executes in SQL Server. For example, SELECT NAME, (SELECT [DESC] FROM PRODUCT LEFT OUTER JOIN PRODUCTDESC ON PRODUCT.PDID = PRODUCTDESC.PDID) FROM PRODUCT `enter code here` in the above query, how data is fetched, how data is filtered, when join is executed such kind of things can any tool explain? i heard about query analyzer but its complex is there any simple tool :)
Pick up any tsql book or read any number of thousands of blogs about tuning sql.
Display actual execution plan in the toolbar is a good start (or use: SET STATISTICS XML ON ) I also use Set statistics io on; Set statistics time on; You can also get the execution plans from the cache by the DMV: SELECT * FROM sys.[dm_exec_query_stats] AS DEQS CROSS APPLY sys.[dm_exec_query_plan]([DEQS].[plan_handle]) AS DEQP ORDER BY [DEQS].[last_execution_time] This will give you a lot of information about the queries executed. You can sort and find slow or resource intense queries by this query. When you find the "worst" queries you can have a look at the execution plan to see whats wrong with it, and whats wrong always depends, but you can start to look for SCANS or KEY lookups, estimated cost, and the size of the arrows (the wider, the more data from each step). But to fully understand you should read the books from @grant fritchey.
In sql server management studio, instead of executing the query; hit Ctrl+L and you'll ser the estimated query plan.
+1 actual and estimated may differ, but you should always show estimated before you execute anything to see if you will get any result within a couple of seconds and not a couple of hours
To start understand the query plans, you can read article [Understanding More Complex Query Plans][1] by @Grant Fritchey and even There are also links to his excelent book **Dissecting SQL Server Execution Plans**. [1]:
http://www.simple-talk.com/sql/performance/understanding-more-complex-query-plans/
Of course you can also refer the MSDN: [Logical and Physical Operators Reference][1] [1]:
http://msdn.microsoft.com/en-us/library/ms191158(v=SQL.105).aspx
In addition to the other answers, if you want a detailed explanation of each of the showplan operators then the series of article that Simple Talk has here
http://www.simple-talk.com/search/default.aspx?search=showplan+operator+of+the+week is a great reference
That series takes a lot of time to get through, but it is well worth reading if you are trying to understand what the plan means.
