SELECT Expression Execution Order

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Today I want to share with you an interesting observation I made about SELECT expression execution order.

I was working on writing a dynamic SQL query that would transform the following piece of JSON:

{"data":[["a","b","c"],["d","e","f"]]}

Into a query that looked like this:

SELECT 'a' AS Row0Column0, 'b' AS Row0Column1, 'c' AS Row0Column2 
UNION ALL
SELECT 'd' AS Row1Column0, 'e' AS Row1Column1, 'f' AS Row1Column2 

Normally I would use something like OPENJSON and PIVOT to transform the original data into a table result set, but in this instance I my requirements dictated that I needed to build the code as a series of SELECT and UNION ALL statements.

The first step in building this query was using OPENJSON to parse the JSON data into rows and value arrays:

SELECT
    *
FROM
(
SELECT
    rows.[key] AS RowNumber,
    rows.[Value] AS RowArray
FROM 
    OPENJSON(N'{"data":[["a","b","c"],["d","e","f"]]}','$.data') rows
) r
CROSS APPLY OPENJSON(r.RowArray) v
ORDER BY
    r.RowNumber,
    v.[key]

image

This first query was a good start. I then added a variable @RowQuery and started building my dynamic SQL code to generate my SELECT and UNION ALL statements:

DECLARE 
    @RowQuery varchar(max)

/* TOP is here to get the ORDER BY to work as expected */
SELECT TOP 134960239460263
    @RowQuery =  COALESCE (@RowQuery + ' ','') + '''' + v.[value] + ''' as [Row'+r.RowNumber+'Column'+v.[key]+']'
FROM
(
SELECT
    rows.[key] AS RowNumber,
    rows.[Value] AS RowArray
FROM 
    OPENJSON(N'{"data":[["a","b","c"],["d","e","f"]]}','$.data') rows
) r
CROSS APPLY OPENJSON(r.RowArray) v
ORDER BY
    r.RowNumber,
    v.[key]

PRINT 'SELECT ' + @RowQuery;

image-1

At this point I had the row/column numbering correct, but I still needed to add a UNION ALL SELECT before the start of each row.

I thought, "Oh, this is easy. Since the dynamic SQL I'm building is basically a loop, I need to check for a change in the RowNumber column's value to identify I'm on a new row. If I am, I can insert the UNION ALL SELECT text and I'll be all set":

DECLARE 
    @RowQuery varchar(max),
    @CurrentRow int = 0;

/* TOP is here to get the ORDER BY to work as expected */
SELECT TOP 134960239460263
    @RowQuery =  COALESCE (@RowQuery + '','') + IIF(r.RowNumber > @CurrentRow, CHAR(10)+'UNION ALL'+CHAR(10)+'SELECT ', ', ')+'''' + v.[value] + ''' as [Row'+r.RowNumber+'Column'+v.[key]+']',
    @CurrentRow = IIF(r.RowNumber > @CurrentRow, r.RowNumber, @CurrentRow)
FROM
(
SELECT
    rows.[key] AS RowNumber,
    rows.[Value] AS RowArray
FROM 
    OPENJSON(N'{"data":[["a","b","c"],["d","e","f"]]}','$.data') rows
) r
CROSS APPLY OPENJSON(r.RowArray) v
ORDER BY
    r.RowNumber,
    v.[key]

/*remove the first comma and add an initial SELECT */
PRINT STUFF(@RowQuery,1,1,'SELECT'); 

image-2

Success! But as I was celebrating my dynamic SQL victory, I realized I was making an assumption about SQL Server that I had never thought about before:

The above query only works because SQL Server is executing the variables in the SELECT list sequentially. I'm incrementing @CurrentRow only after processing my @RowQuery variable, and this logic only works correctly if SQL Server executes the variable expressions in the order they appear in the SELECT list. If SQL Server was executing items in the SELECT list in reverse or random order, @CurrentRow could potentially get set BEFORE @RowQuery was evaluated, causing the logic of adding "UNION ALL SELECT" in the right location to fail.

This surprised me because I don't usually think about the column execution order of a query. Normally column expressions in the SELECT statement are independent of each other so the order that the columns are executed in doesn't really matter. But in this example, the column execution order does matter and it's reassuring to see SQL Server do what I assumed it was doing.

Now, I can't guarantee this always works. I tried but failed to think of a scenario where SQL Server wouldn't execute the columns in sequential order. While the query seemed to work as expected in all of the tests I ran, I'll leave this observation open ended in case anyone has ever encountered a scenario or has any ideas of when SQL Server doesn't process SELECT statement expressions in the order they are listed.

Displaying Long Values in SSMS

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I write a lot of dynamic SQL and frequently encounter variables that contain many characters:

DECLARE @LongValue NVARCHAR(MAX) = CAST('' AS NVARCHAR(MAX)) + 
N'SELECT
    ''A'' AS AShortValue,
    '''+REPLICATE(N'A',4000)+''' as ALongValue
ORDER BY 1';

This variable is 4059 characters long, and when I execute it it runs great.

SELECT LEN(@LongValue); -- 4059 characters
EXEC(@LongValue);

2018-08-01_12-17-24 A homage to one of my favorite Uncyclopedia entries.

If my programmatically built query had an error in it, the first thing I'd want to do when debugging it would be to see the the text of the entire @LongValue variable.

I could do this by just saying SELECT @LongValue, and while recent versions of SSMS will display the whole value for me, it completely loses my formatting which stinks (and is especially bad if there are any comments prefixed with --  in the query):

2018-08-01_12-25-54 Need a ultra HD wide display to fit this all on one screen.

I can say PRINT @LongValue, which will keep the formatting, but it will get trimmed at 4,000 characters (notice the missing ORDER BY):

2018-08-01_12-32-45

Some Better Ways

Erik Darling posts one solution to this problem in his T-SQL Tuesday #104 entry (as well as some other problems/solutions for lengthy SQL variables). Specifically he links to a SQL string printing script that will loop through the lengthy variable and print everything while maintaining formatting:

2018-08-01_12-32-45 Not perfectly formatted, but good enough.

And while I like using that stored procedure on my primary server, I'm too lazy to install it every where I need it.

Instead, I have a couple of go-to solutions that work on all SQL Server instances 2008 forward.

Solution 1: CAST to XML
SELECT CAST(@LongValue AS XML) AS LongValue

Casting the long variable to XML allows SSMS to generate a clickable, single-row result that preserves formatting:

2018-08-01_12-44-37 IntelliSense complains but I'm OK with it

The only downside to this approach is that certain charaters, like "<" and ">", can't be converted to XML:

2018-08-01_12-46-43

Solution 2: FOR XML PATH

A slight variation on solution 1, we can get similar results using FOR XML PATH:

SET @LongValue = '<' + @LongValue -- Let's add in an invalid character
SELECT @LongValue FOR XML PATH('')

2018-08-01_12-50-20 FOR XML PATH is one of the most abused SQL Server functions.

In this solution, the "<" is escaped to "<", which isn't perfect but at least my variable can be displayed with formatting intact.  A quick find and replace for any escaped characters and I'm good to go.

Good Enough

These techniques aren't perfect, but for purposes of debugging dynamically generated code they are good enough.

Maybe one day SSMS will print longer strings or include a syntax formatter and I won't care nearly as much.

And if not, I'll happily continue to abuse FOR XML to do things other than generate XML documents.

Building Dynamic Table-Driven Queries

MJ-t-sql-Tuesday

This post is a response to this month's T-SQL Tuesday #104 prompt by me! T-SQL Tuesday is a way for SQL Server bloggers to share ideas about different database and professional topics every month.

This month's topic is asking what code would you hate to live without?


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When given the choice between working on new projects versus maintaining old ones, I'm always more excited to work on something new.

That means that when I build something that is going to used for years to come, I try to build it so that it will require as little maintenance as possible in the future.

One technique I use for minimizing maintenance is making my queries dynamic.  Dynamic queries, while not right for every situation, do one thing really well: they allow you to modify functionality without needing a complete rewrite when your data changes.  The way I look it, it's much easier to add rules and logic to rows in table than having to modify a table's columns or structure.

To show you what I mean,let's say I want to write a query selecting data from model.sys.database_permissions:

SELECT class
      ,class_desc
      ,major_id
      ,minor_id
      ,grantee_principal_id
      ,grantor_principal_id
      ,type
      ,permission_name
      ,state
      ,state_desc
  FROM model.sys.database_permissions

Writing the query as above is pretty simple, but it isn't flexible in case the table structure changes in the future or if we want to programmatically write some conditions.

Instead of hardcoding the query as above, here is a general pattern I use for writing dynamic table-driven queries.  SQL Server has the handy views sys.all_views and sys.all_columns that show information about what columns are stored in each table/view:

2018-07-03_21-00-45

Using these two views, I can use this dynamic SQL pattern to build the same exact query as above:

-- Declare some variables up front
DECLARE 
    @FullQuery nvarchar(max),
    @Columns nvarchar(max),
    @ObjectName nvarchar(128)

-- Build our SELECT statment and schema+table name
SELECT 
    @Columns = COALESCE(@Columns + ', ', '') + '[' + c.[name] + ']',
    @ObjectName = QUOTENAME(s.name) + '.' + QUOTENAME(o.name)
FROM 
    sys.all_views o  
    INNER JOIN sys.schemas s
        ON o.schema_id = s.schema_id
    INNER JOIN sys.all_columns c
        ON o.object_id = c.object_id
WHERE 
    o.[name] = 'database_permissions'
ORDER BY
    c.column_id 

-- Put all of the pieces together an execute
SET @FullQuery = 'SELECT ' + @Columns + ' FROM ' + @ObjectName

EXEC(@FullQuery)

The way building a dynamic statement like this works is that I build my SELECT statement as a string based on the values stored in my all_columns view.  If a column is ever added to this view, my dynamic code will handle it (I wouldn't expect this view to change that much in future versions of SQL, but in other real-world tables I can regularly expect changing data).

Yes, writing certain queries dynamically like this means more up front work.  It also means some queries won't run to their full potential (not necessarily reusing plans, not tuning every individual query, needing to be thoughtful about SQL injection attacks, etc...).  There are A LOT of downsides to building queries dynamically like this.

But dynamically built queries make my systems flexible and drastically reduce the amount of work I have to do down the road.  In the next few weeks I hope to go into this type of dynamically built, table-driven process in more detail (so you should see the pattern in the example above get reused soon!).

Why Is My VARCHAR(MAX) Variable Getting Truncated?

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Sometimes SQL Server doesn't do what you tell it to do.

Normally that's ok - SQL is a declarative language after all, so we're supposed to tell it what we want it to do, not how we want it done.

And while that's fine for most querying needs, it can become really frustrating when SQL Server decides to completely disregard what you explicitly asked it to do.

Why Is My VARCHAR(MAX) Truncated to 8000 Characters?

A prime example of this is when you declare a variable as VARCHAR(MAX) because you want to assign a long string to it.  Storing values longer than 8000 characters long is the whole point of VARCHAR(MAX), right?

DECLARE @dynamicQuery VARCHAR(MAX);

SET @dynamicQuery = REPLICATE('a',8000) + 'b'

SELECT @dynamicQuery as dynamicQueryValue, LEN(@dynamicQuery) AS dynamicQueryLength

If we look at the above query, I would expect my variable @dynamicQuery to be 8001 characters long; it should be 8000 letter 'a's followed by a single letter 'b'.  8001 characters total, stored in a VARCHAR(MAX) defined variable.

But does SQL Server actually store all 8001 characters like we explicitly asked it to?

No:

2018-05-13_18-31-03

First we can see that the LEN() of our variable is only 8000 - not 8001 - characters long!

2018-05-13_18-31-28

Copying and pasting our resulting value into a new query window also shows us that there is no character 'b' at position 8001 like we expected.

The Miserly SQL Server

The reason this happens is that SQL Server doesn't want to store something as VARCHAR(MAX) if none of the variable's components are defined as VARCHAR(MAX).  I guess it doesn't want to store something in a less efficient way if there's no need for it.

2018-05-12_06-44-53

However, this logic is flawed since we clearly DO want to store more than 8000 characters.  So what can we do?

Make Something VARCHAR(MAX)

Seriously, that's it.  You can do something like CAST the single character 'b' as VARCHAR(MAX) and your @dynamicQuery variable will now contain 8001 characters:

2018-05-14_18-06-44

But casting a single character as VARCHAR(MAX) isn't very intuitive.

Instead, I recommend casting a blank as VARCHAR(MAX) and prefixing it to the start of your variable string.  Leave yourself a comment for the future and hopefully you'll remember why this superfluous looking piece of code is needed:

-- using CAST('') to force SQL to define
-- as varchar(MAX)
SET @dynamicQuery =  CAST('' AS varchar(MAX))
    + REPLICATE('a',8000)+ 'b'