Logical Query Processing- Fuzzy Query with Order By Clause

Today it happened, I was called by my fellow programmer, he saw me a piece of T-SQL Code and asks few question on it, I will try to replicate the same code here.

There was a query somehow like this

select sub_group,sub_group,name,name from #temp

where sub_group = 'Furniture'

order by sub_group

And the query was failing, and asked me to justify the why it failing. That’s good, I find it interesting to explain the logic behind the scene through Logical Query Processing.  Yeah ;) I didn’t ask tea this time for this: P

Let’s formulate the same scenario by creating a sample table.

create table #temp

(id int identity(1,1),

sub_group varchar(10),

name varchar(10))

Command(s) completed successfully.

Lets insert few records in it.

insert into #temp

select 'Furniture','chair' union all

select 'Vechile','Maruti' union all

select 'Furniture','Desk' union all

select 'Furniture','Dine' union all

select 'Vechile','Honda' union all

select 'H.Vechile','Mahindra'

Now we can have the same Select query. Lets fire the query and see  the output.

select sub_group,sub_group,name,name from #temp

where sub_group = 'Furniture'

order by sub_group

Msg 209, Level 16, State 1, Line 32

Ambiguous column name 'sub_group'.

Now lets take the error, and dig the background of it. Now it says Ambiguous Column name.

If you see in the select list, we have called this column two times, now if we go through logical query processing, it says for this query.

1^st from clause gets executed i.e #temp and record set will build up say RS1.

2^nd where by clause gets executed which will filter the data and remain with the data which has ‘Furniture’ as sub_group and create a new record set say RS2

3^rd Now the Select clause will gets executed with form one more column of same name as sub_group. So the new record set will have this many column “sub_group, sub_group, name, name” say this record set as RS3

4^th Now Oder By clause gets the chance, it operate on RS3 record set, now if you see in query, the order by clause is on sub_group, so when it tries to order the RS3 data, it will found two column with same name sub_group, and optimizer find it difficult which column to pick up, thus it throws Ambiguous column error.

Solution to this problem is: make an alias to one of the sub_group column.

select sub_group,sub_group SB ,name,name from #temp

where sub_group = 'Furniture'

order by sub_group

Run the query and you will get your result.

sub_group  SB         name       name

---------- ---------- ---------- ----------

Furniture  Furniture  chair      chair

Furniture  Furniture  Desk       Desk

Furniture  Furniture  Dine       Dine

You just have to make time for your coding and keep it balance J

 

Post Reference: Vikram Aristocratic Elfin Share

A Celebrity among all Execution Plan Operators – KEYLOOKUP

I will start with this contour, this operator is crazy on changing its name, and it has changed its name three times in the last three release of SQL Server. From BookMark Lookup to Cluster Index Seek and now it is Key Lookup. I am personally comfortable with Key Lookup, which make more sense to me to the way it works.

Few facts I would like to point out here about KeyLook Up.

• It typically comes with Non Clustered Index.
• It fetches values via cluster index when the required values are not present in (Cover in) Non Clustered Index.
•  It perform one of the expensive IO operation, I will take on this further in sometime  :)

I want to take it further to explain more on this Celebrity operator, when rows are requested by the query with predicate, optimizer look into non-clustered index applied on column participate in where predicate to retrieve the requested data. If the all requested data is not found in Non Clustered Index then it go to data page where the data is stored and retrieve it.

Now in above situation, if clustered Index present on table, it goes to Clustered index to get the physical location of data and this operation is called as KEYLOOKUP and if there is no clustered index present on table then there is something called RID lookup comes into picture.

Now it’s time to get yourself relaxed and have popcorn with cup of tea, ah I m addicted to Tea, you can have CokeJ

Ya, now we can take up our point for more understanding “It perform one of the expensive IO operation”

It is one of the most expensive operations because for each value in non clustered index it goes to clustered index to retrieve the asked data.

Lets stimulate a condition where we have Keylookup operator comes into picture in execution plan.

I am using AdventureWorks2008R2 database and Person.Address Table to stimulate the condition, let’s see how many different indexes are present on Person.Address table

select * from sys.indexes where object_id = OBJECT_ID('Person.Address')

Table   name                         type_desc

------- ---------------------------  ---------------

Address PK_Address_AddressID         CLUSTERED

Address AK_Address_rowguid           NONCLUSTERED

Address IX_Address_AddressLine1_    

        AddressLine2_City_           

        StateProvinceID_PostalCode   NONCLUSTERED

Address IX_Address_StateProvinceID   NONCLUSTERED

So here we can see, we have 4 indexes, one cluster and 3 non clustered index. Lets disable all non clustered indexes first.

alter index AK_Address_rowguid

on Person.Address DISABLE

go

alter index IX_Address_AddressLine1_AddressLine2_City_StateProvinceID_PostalCode

on Person.Address DISABLE

go

alter index IX_Address_StateProvinceID

on Person.Address DISABLE

go

Command(s) completed successfully.

Now lets fire the query

select AddressID, City, StateProvinceID from Person.Address where City = 'Las Vegas'

Here we are creating a non clustered index on City column which is used in where predicate. Once it s done, again we will check the query Plan.

CREATE NONCLUSTERED INDEX ix_PersonAddress_City

ON [Person].[Address] ([City])

Now lets query the same select statement to see how this newly created index perform, is it  better then the above clustered index scan method

select AddressID, City, StateProvinceID from Person.Address where City = 'Las Vegas'

To be honest, seeing Keylookup in Execution plan excites SQL guy, because he has a good chance to improve further the performance of Query by simply creating Covering Index. I will take covering index in separate article.

Here Key Lookup operator is used to read the value of AddressID, StateProvinceID from by referencing to clustered Index, since this two column are not included in Non Clustered Index.

But at the same time Key Lookup Operator will worsen your cost, if the number of rows to return is more because for each rows it has to switch to Clustered Index to get the value. So that moments scan would be more efficient strategy then Key Lookup. But yes we have the option of Covering those column in Non Clustered Index which prompt Key Lookup by creating Covering Index.

         

If I can love someone, I need to love her as I love my Programming with SQL Server! :)

 

Post Reference: Vikram Aristocratic Elfin Share

Listing Next 5 Sunday

Again I got an opportunity to write an interesting piece of College Code in response to the question posted on one of my SQL Server forum. The problem statement was

“I want a list of next five Sunday, how I can achieve this through T-SQL”

Ya its very simple, just a couple of Date and String function, blend it with college logic, have your tea with Parle G and that’s all code will all set to fly high. Enjoy Sunday J

So here I am sharing a piece of code which will do the rest, the code is self explanatory, So I don’t think I should buy my time to catch up anything from code to explain.

declare @dt datetime

declare @howMany int

declare @cntr int

set @cntr = 0

set @howMany = 6

set @dt = getdate()

create table #temp

(

SundayCol datetime,

WeekDayName varchar(10)

)

while (datename(w,@dt) not like 'Sunday')

begin

       set @dt= dateadd(dd,1,@dt)

end

--select datename(w,@dt)

insert into #temp

select @dt, Datename(w,@dt)

while (@cntr < @howMany)

begin

       set @dt = dateadd(dd,7,@dt)

       insert into #temp

       select @dt, Datename(w,@dt)

       set @cntr = @cntr + 1

end

select convert(varchar(10),SundayCol,103), WeekDayName from #temp

drop table #temp

Execute it and output you will get is next 6 Sunday. Enjoy your Sunday J

Date       WeekDayName

---------- -----------

29/06/2014 Sunday

06/07/2014 Sunday

13/07/2014 Sunday

20/07/2014 Sunday

27/07/2014 Sunday

03/08/2014 Sunday

10/08/2014 Sunday

The popping up of Sunday every week makes me happy and my potential double up to buys more time for my articles on SQL! J

 

Post Reference: Vikram Aristocratic Elfin Share