Nov 192015
 

I was always challenged when my customers asked me what tables and columns are referenced by a stored procedure which was written many years ago by the guy who left the company 5 years ago. When I Google the solution, I was always told that sys.sql_dependencies and sys.sql_expression_dependencies can tell. At the end of the day, I figured that depending on the complexity of the procedure, those 2 views couldn’t always give me accurate information as needed, dynamic SQLs for instance. Even worse, my customer also asked me if a table was accessed by anyone and what columns were referenced. I realized that I have to write something to get it done.
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Apr 092012
 

Stream Aggregate is the most efficient physical operator for value aggregation and generating distict rows. Stream here refers to record stream. Give you a typical example here. When you use SqlDataReader to retrieve reuslt from a query, the rows are read one by one by calling SqlDataReader.Read() method. You can say that you are streaming records from SQL Server to your client. If you perform aggregates in the stream, for instance, you are asked to count number of rows in the stream. In this example, you only need to increase the value with 1 to a variable in your application whenever Read() is involked with true returned. This is called Stream Aggregate.

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Apr 062012
 

In my last post, I described Hash Joins which is showing as Hash Match physical operator in the graphical execution plan. Another place hash match is used is for aggregation. When columns in group by clause do have have indexes or SQL Server cannot determine whether the rows are sorted or not, SQL Server will perform a Hash match to get aggregates. If distinct keywork appears on the select list, but there is no indexes on the selected columns, a hash match will be used as well. This operation is called hash aggregate.

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Apr 042012
 

Hash is used in 2 most commonly seen physical operators, Hash Join and Hash Aggregate. Those 2 happen when there are no any other alternatives (merge, nested loop, or stream aggregate) which can be used for more efficient operation. For instance, when SQL Server joins 2 tables together but none of them has an index. SQL Server has no idea whether the joining keys are sorted or not.  In most of the case for such scenarios, hash join will take place.  As its name, hash join uses hash algorithm to encode the joining keys from both side, compares the hashed values, and produce the result. This sounds very complex — yes, it is a very heavy operator.
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Mar 122012
 

Merge is a physical operator joining 2 sets together into one. Similar to Nested Loop, it can implement all logical join operations, such as outer join and inner join. Different from Nested Loop, Merge needs 2 input sets which are sorted on the joining keys. For instance, there are 2 piles of papers. The first pile includes customers’ basic information. Each paper has and only has one customers information. The first pile is sorted by customers’ ID. The second pile includes customers’ purchase information. Every customer might have 0 to many purchase orders. The second pile is sorted by customer’s ID. While merging taking place, operator takes one page from the first pile, Customer1 for instance, to compare the page from the second pile. If matched, return the combined information, then take the next page from the second pile and compare again. Until no more pages on the second pile can mach the current page from the first pile, which also means no more pages for Customer1 in the second pile, then the operator takes the second page from the first pile, and repeat this operation again and again until all the pages in the first pile get processed. This is a very effecient operation.

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Mar 052012
 

The answer of the question I asked in my last post is Statistics. Query Optimizer is a cost(and rule) based optimizer. It calculates the costs for each operator based on its formulars behind and get the total estimated cost of query. If there are more alternatives to implement the same logic, SQL Server will know the cost of each alternative, then it can pickup a most efficient one to run. However, databases nowadays are usually complicated. Very frequently, implementing a logic for data accessing can have millions of alternatives. Getting cost for each and find the cheapest best one is just so time consuming. It doesn’t make sense to take a day to find a best plan to execute where the returning of the query can be done in 10 minutes by using the wrost plan. There are definitely some rules behind. We will come back to the rule in the future. Now let’s see how SQL Server gets estimated number of rows on the data it maniputes on.

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Feb 292012
 

Relationships amoung multiple sets can always be interpreted into multiple relationships of 2 sets – joins. Conceptually, you can have 2 types of joins, Inner Join and Outer Join. Inner joining returns the conjunction of two sets, such as Inner Join, Cross Apply, and Intersect in SQL Server. Outer join will return full set from one or both of the inputs with or without relations between each other such as Left Outer join, Right Outer Join, Full Outer Join, Cross Join, and Outer Apply. In SQL Server, if rows are only returned from one set not the other, it’s called Semi Join, such as Exists, IN (subset). If returning rows do not exist in another set, it’s called Anti Semi Join, such as NOT Exists, Except, Not In (subset). They are all conceptual. Nested loop is a PHYSICAL operator, it supports any of types of joins described above.

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Feb 272012
 

At the end of last post, I gave you a puzzle and also mentioned to give the answer of it.

use AdventureWorks2008R2

--set statistics io on -- this is the hint
select CustomerID, SalesOrderID
from Sales.SalesOrderHeader with (index=[IX_SalesOrderHeader_CustomerID])
where CustomerID = 29974 and SalesOrderID = 45785

select CustomerID, SalesOrderID
from Sales.SalesOrderHeader
where CustomerID = 29974 and SalesOrderID = 45785

Look at the query plan, the cost of both are the same. Does it mean the performance of those 2 are the same? If not, Which statement generates better plan?

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Feb 242012
 

Seek operator presents both physical and logical operator. It can only apply to an index, clustered index or none clustered index. It’s the most efficient operation to reach a record in an index by key. As we know, indexes are organized in a B-Tree, each record in the non-leaf level of B-Tree includes a the first key(s) in the page of next level and the pointer (File:Page:Slot) of the page in the next level.

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