Writing Better T-SQL: Top-Down Design May Not be the Best Choice

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Writing Better T-SQL: Top-Down Design May Not be the Best Choice

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Henry B. Stinson SSC Eights! Points: 949 More actions · Answer 1

I did not see the use of CTE included in the comparisons.

Jeff Moden SSC Guru Points: 1004402 More actions · Answer 2

Mickey Stuewe (6/2/2015)
I'm looking forward to you reading your article.
Mickey

Heh... I've gotta ask... slip or something else, Mickey?

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

Mickey Stuewe SSC Eights! Points: 879 More actions · Answer 3

LOL. That's what happens when I'm trying to type to fast. 🙂

Mickey

Jeff Moden (6/3/2015)
Mickey Stuewe (6/2/2015)
I'm looking forward to you reading your article.
Mickey
Heh... I've gotta ask... slip or something else, Mickey?

Mickey Stuewe
Sr Database Developer
My blog
Follow me on twitter: @SQLMickey
Connect with me on LinkedIn
--------------------------------------------------------------------------
I laugh loudly and I laugh often. Just ask anyone who knows me.

Jeff Moden SSC Guru Points: 1004402 More actions · Answer 4

Getting back to it, you've got a good touch for writing. You should do it more often.

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

Mickey Stuewe SSC Eights! Points: 879 More actions · Answer 5

Thanks Jeff. I plan on it. 🙂

Mickey

Mickey Stuewe
Sr Database Developer
My blog
Follow me on twitter: @SQLMickey
Connect with me on LinkedIn
--------------------------------------------------------------------------
I laugh loudly and I laugh often. Just ask anyone who knows me.

Leo Peysakhovich Hall of Fame Points: 3973 More actions · Answer 6

Nice article. Thanks for taking time to write it.

But why temp tables were analyzed the way it should not be used. It is very bad way to use

select column(s)

into #table

from table.

If you create temp table up front with column's definition and proper primary key then you will see completely different picture.

Leo Peysakhovich

Jeff Moden SSC Guru Points: 1004402 More actions · Answer 7

Leo Peysakhovich (6/4/2015)
Nice article. Thanks for taking time to write it.
But why temp tables were analyzed the way it should not be used. It is very bad way to use
select column(s)
into #table
from table.
If you create temp table up front with column's definition and proper primary key then you will see completely different picture.
Leo Peysakhovich

"It Depends". Ostensibly, you only put into the Temp Table exactly what you need and there's no need for a PK or a Clustered Index because you're going to use everything in the Temp Table.

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

Leo Peysakhovich Hall of Fame Points: 3973 More actions · Answer 8

Jeff Moden (6/4/2015)
Leo Peysakhovich (6/4/2015)
Nice article. Thanks for taking time to write it.
But why temp tables were analyzed the way it should not be used. It is very bad way to use
select column(s)
into #table
from table.
If you create temp table up front with column's definition and proper primary key then you will see completely different picture.
Leo Peysakhovich
"It Depends". Ostensibly, you only put into the Temp Table exactly what you need and there's no need for a PK or a Clustered Index because you're going to use everything in the Temp Table.

"It depends". But based on my experience, I have seen many examples where this technique improve stored procedure performance dramaticaly. And it is never get it worth. With statement "select column into #temp from table" I have seen performance problems very often. So, I am advocating even for the temp tables - create them up front and with at least dummy (identity) column as a PK.

Jeff Moden SSC Guru Points: 1004402 More actions · Answer 9

I've never seen such problems as you mention with SELECT/INTO Leo but that doesn't mean they don't exist. Thanks for the warning.

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

Bill Talada SSChampion Points: 11954 More actions · Answer 10

Top-down design usually results in failure; see any centralized government.

Bottom-up design is used by evolution and in my opinion is a thousand times more intelligent than humans.

The two extremes are monolithic SELECT statements or discrete procedural steps. The article is one dimensional, only concerned about CPU. I believe many dimensions need to be considered such as disk, ease of maintenance, stability, clarity.

Most of the queries I support are joining 30 tables with complex filtering criteria. These need to be broken up since the optimizer gets overwhelmed at 8 tables in my experience. I've had huge success with modularizing monolithic queries by using derived tables for every table in a query. It leads to predictability and better plans.

Stan Kulp-439977 SSChampion Points: 10062 More actions · Answer 11

One trick I use is to write stored procedures using temporary tables, then when the stored procedure is finished and working go back through it and turn the references to temporary tables into derived tables by replacing the temporary tables with the code used to create the temporary tables.

That's the best of both worlds.

jbsedivy SSC Enthusiast Points: 150 More actions · Answer 12

Agree with other posters that once you hit large datasets, temp tables are the way to go. And to solve a very complex problem, sometimes those temp table results are used more than once in a later process.

aalcala SSC Enthusiast Points: 157 More actions · Answer 13

A worthwhile article, but I feel the need to trot out my hobbyhorse.

You write about the "best" solution, without defining best. If I read the article correctly, by "best" you mean "running in the least amount of time".

Having spent more years developing software than most of your readers have been alive, my experience is that (assuming the proper result set is returned) "best" is more often appropriately defined as "easy to maintain", which often means "easy to understand at a glance, often by a relatively inexperienced person". With that definition, breaking things down into a series of steps is usually the "better" choice. It is easier to rewrite an easy to understand script to gain more performance (if/when needed) than it is to train up new people to quickly read and understand "sophisticated" scripts sufficiently reliably to perform maintenance.

Summary: if you called this "Writing Faster T-SQL" I would not complain, but you didn't, and I am.

sknox SSChampion Points: 12388 More actions · Answer 14

Stan Kulp-439977 (8/12/2016)
One trick I use is to write stored procedures using temporary tables, then when the stored procedure is finished and working go back through it and turn the references to temporary tables into derived tables by replacing the temporary tables with the code used to create the temporary tables.
That's the best of both worlds.

That's funny. I do almost the opposite.

I'll write using CTEs to begin with, because I can still order them within the WITH statement to provide logical flow, but I don't have to write create...insert statements for them all.

So I get a functional query very quickly. Then I do a performance pass or two where I find the CTEs which affect performance the most and convert them to temp tables.

I'm done when I run out of time or find a good enough query. Wait, where have I heard that before?

rchantler SSCrazy Points: 2128 More actions · Answer 15

Your example states your procedure will return customer count for each employee but your temp table holds customer count by store. Am I missing something? Is there only 1 employee per store?

I tend to fall in with the 'temp table, divide and conquer' crowd. For me this approach evolved over time as my group learned to deal with performance issues. Testing alternatives with realistic record counts is my main takeaway from your very interesting, useful discussion-provoking article.