Friday, May 22, 2020

PostgreSQL adds FETCH FIRST WITH TIES.. just like TOP n WITH TIES in SQL Server

PostgreSQL 13 Beta 1 was released yesterday, you can read the release notes here

https://www.postgresql.org/about/news/2040/

One thing that caught my eye was this statement in the release notes

PostgreSQL 13 brings more convenience to writing queries with features like FETCH FIRST WITH TIES, which returns any additional rows that match the last row.

This is I guess exactly like TOP WITH TIES in SQL Server. I believe this has been around in SQL Server since at least version 7.  How many times have I used it in code that was deployed in the last 20 years?  I believe I have used WITH TIES only once. It does make for great interview questions and SQL puzzles  :-)


So let's take a quick look at how TOP WITH TIES works in SQL Server.  The first thing we will do is look at what Books On Line says about TOP


WITH TIES Returns two or more rows that tie for last place in the limited results set. You must use this argument with the ORDER BY clause. WITH TIES might cause more rows to be returned than the value specified in expression. For example, if expression is set to 5 but two additional rows match the values of the ORDER BY columns in row 5, the result set will contain seven rows.You can specify the TOP clause with the WITH TIES argument only in SELECT statements, and only if you've also specified the ORDER BY clause. The returned order of tying records is arbitrary. ORDER BY doesn't affect this rule.

Time to get started and write some code to see this in action

First create this table of students and insert some data

CREATE TABLE #TopExample(GradeAverage int, Student varchar(100))
INSERT #TopExample VALUES(99.00,'Plato'),
      (98,'Socrates'),
      (95,'Diogenes the Cynic'),
      (94,'Antisthenes'),
      (94,'Demetrius'),
      (50,'Denis')

As you can see, I am not a very good student  :-(

If you do a regular TOP 4 query like this

SELECT TOP 4 GradeAverage, Student 
FROM #TopExample  
ORDER BY GradeAverage DESC

You will get back these results

GradeAverage Student
99          Plato
98          Socrates
95          Diogenes the Cynic
94          Demetrius


As you can see we are missing another student with a grade of 94, this is Antisthenes

This is easily fixed by adding WITH TIES to the query

SELECT TOP 4 WITH TIES GradeAverage, Student 
FROM #TopExample 
ORDER BY GradeAverage DESC

Now, you will get back these results, as you can see, you now have 5 rows and both rows with a grade average of 94 are included

GradeAverage Student
99          Plato
98          Socrates
95          Diogenes the Cynic
94          Demetrius
94          Antisthenes


Another way to do the same as WITH TIES is by using DENSE_RANK.  That query looks like this

;WITH c AS (SELECT DENSE_RANK() OVER (ORDER BY GradeAverage DESC) AS dens, 
 GradeAverage,Student 
 FROM #TopExample)

SELECT GradeAverage, Student 
FROM c WHERE dens <=4
ORDER BY GradeAverage DESC 

You will get back these same results again, you now have 5 rows and both rows with a grade average of 94 are included as well
GradeAverage Student
99          Plato
98          Socrates
95          Diogenes the Cynic
94          Demetrius
94          Antisthenes

Using DENSE_RANK is bit more code, but if portability is a concern, it might be a better choice

There you go a post about a feature you will never use :-)

If you want to ran all the queries in one shot here is all the code



CREATE TABLE #TopExample(GradeAverage int, Student varchar(100))
INSERT #TopExample VALUES(99.00,'Plato'),
      (98.00,'Socrates'),
      (95.00,'Diogenes the Cynic'),
      (94.00,'Antisthenes'),
      (94.00,'Demetrius'),
      (50.00,'Denis')

SELECT TOP 4 GradeAverage, Student 
FROM #TopExample  
ORDER BY GradeAverage DESC

SELECT TOP 4 WITH TIES GradeAverage, Student 
FROM #TopExample 
ORDER BY GradeAverage DESC

;WITH c AS (SELECT DENSE_RANK() OVER (ORDER BY GradeAverage DESC) AS dens, 
 GradeAverage,Student 
 FROM #TopExample)

SELECT GradeAverage, Student 
FROM c WHERE dens <=4
ORDER BY GradeAverage DESC 


DROP TABLE #TopExample


And here is what it all looks like in SSMS, code and output



PostgreSQL adds FETCH FIRST WITH TIES.. just like TOP n WITH TIES in SQL Server Query in SSMS

Wednesday, May 06, 2020

You know about waitfor delay but did you know there is a waitfor time?


I was looking at some code I wrote the other day and noticed the WAITFOR command.. This got me thinking. How many times have I used WAITFOR in code, probably as much as I have used NTILE  :-)

I looked at the documentation for WAITFOR and notice there is TIME in addition to DELAY.  Oh that is handy, I always rolled my own ghetto-style version by calculating how long it would be until a specific time and then I would use that in the WAITFOR DELAY command

Why would you use the WAITFOR command?

The WAITFOR command can be used to delay the execution of command for a specific duration or until a specific time occurs. From Books On Line, the description is as follows:



Blocks the execution of a batch, stored procedure, or transaction until either a specified time or time interval elapses, or a specified statement modifies or returns at least one row.

WAITFOR   
{  
    DELAY 'time_to_pass'   
  | TIME 'time_to_execute'   
  | [ ( receive_statement ) | ( get_conversation_group_statement ) ]   
    [ , TIMEOUT timeout ]  
}  



Arguments
DELAY
Is the specified period of time that must pass, up to a maximum of 24 hours, before execution of a batch, stored procedure, or transaction proceeds.

'time_to_pass'
Is the period of time to wait. time_to_pass can be specified either in a datetime data format, or as a local variable. Dates can't be specified, so the date part of the datetime value isn't allowed. time_to_pass is formatted as hh:mm[[:ss].mss].

TIME
Is the specified time when the batch, stored procedure, or transaction runs.

'time_to_execute'
Is the time at which the WAITFOR statement finishes. time_to_execute can be specified in a datetime data format, or it can be specified as a local variable. Dates can't be specified, so the date part of the datetime value isn't allowed. time_to_execute is formatted as hh:mm[[:ss].mss] and can optionally include the date of 1900-01-01.


WAITFOR with a receive_statement or get_conversation_group_statement is applicable only to Service Broker messages. I will not cover those in this post



I must admit that I only use these commands a couple of times a year when running something ad-hoc.  In code,  I will use WAITFOR DELAY when doing a back fill of data,  and the table is replicated. In that case I will batch the data and after each batch is completed I will pause for a second or so. The reason I am doing this is because I don't want to increase replication latency, after all, I am a nice guy


WAITFOR TIME 

Let's take a look how you would use the WAITFOR command. I will start with WAITFOR TIME


The command is very easy.. if you want the print command to run at 09:57:16, you would do the following

WAITFOR TIME '09:57:16'
PRINT 'DONE  ' 

The seconds are optional, if you want it to run at 9 hours and 57 minutes, you can do the following

WAITFOR TIME '09:57'
PRINT 'DONE  ' 

One thing to know is that you can't grab the output from a time data type and use that in your WAITFOR TIME command. The following will blow up

SELECT CONVERT(time,  getdate()) --'09:57:16.9600000'

WAITFOR TIME '09:57:16.9600000'

Msg 148, Level 15 , State 1, Line 32
Incorrect time syntax in time string '09:57:16.9600000' used with WAITFOR.

What you need to do is strip everything after the dot.

We need the command to be the following


WAITFOR TIME '09:57:16'

There are two ways to accomplish this... first way is by using PARSENAME, I blogged about that function several times, the first time here: Ten SQL Server Functions That You Hardly Use But Should

All you have to tell SQL Server which part you want, if you use PARSENAME,1 you will get everything after the dot, if you use PARSENAME,2 you will get everything before the dot.


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2
SELECT  PARSENAME('09:57:16.9600000',2), 
PARSENAME('09:57:16.9600000',1)

This returns  the following

09:57:16     9600000


The easiest way would have been to just use time(0) instead

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SELECT CONVERT(time,  getdate()) ,--'09:57:16.9600000'
CONVERT(time(0),  getdate())  --'09:57:16

Below is a complete example that will wait for 10 seconds to run the PRINT statement on line 12 if you run the whole code block in 1 shot.

Also notice that I use a variable with the WAITFOR TIME command on line 9. The caveat with that is that the variable can't be a time datatype. This is why I use a varchar datatype and store the value of the time data type in it. The reason I use the time datatype in my procs is so that I don't have to do a lot of validations when someone is calling the proc. If they pass in a string that can't be converted.. the proc won't even run... it will fail right at the proc call itself


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DECLARE @DelayTime time(0)= GETDATE()
PRINT @DelayTime

SELECT @DelayTime =DATEADD(second,10,@DelayTime)

PRINT @DelayTime

DECLARE @d varchar(100) = @DelayTime -- you have to use varchar in command
WAITFOR TIME @d 

-- Run your command here
PRINT 'DONE  ' + CONVERT(varchar(100),CONVERT(time(0),  getdate()))


What is printed is the following


10:49:48
10:49:58
DONE  10:49:58

Now when would you really use WAITFOR TIME?  You can accomplish the same with a scheduled job, the only time I use WAITFOR TIME is if I want a quick count of want to run something at a specific time but I know I won't be at my desk and I can't create a job without a ticket

But you also have to be aware that if your connection gets lost to the SQL Server instance, your command won't execute


WAITFOR DELAY

The WAITFOR DELAY command is similar to the WAITFOR TIME command, instead of waiting for a time, the command pauses for a specific period

Like I said before,  I use WAITFOR DELAY as well as a batch size in my back fill procs. Both can be passed in, if you do a load during a weekday, your delay would be longer than on a weekend.


Sometimes I need to see how many rows are getting inserted every minute.. or something similar
I will then combine WAITFOR DELAY and the batch terminator with a count number to execute the batch of statements  more than once

Here is such an example, it will run the INSERT statement 20 times, it will pause 1 minute between each execution


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INSERT #temp(SomeCol, SomeTimeStamp)
SELECT COUNT(*), GETDATE() FROM sometable 
WAITFOR DELAY '00:01:00'
GO 20


That's all for this post.

Do you use the WAITFOR command, if so, what do you use it for?