MySQL 8 - user variables within expressions is deprecated [UDF calls with lot of parameters]

Question

I have this:

SELECT
   @foo1 := UDF1(0, a, b, c, d) AS Foo1,
   @foo2 := UDF1(1, a, b, c, d) AS Foo2,
   @foo3 := UDF1(2, a, b, c, d) AS Foo3,
   @foo4 := UDF1(3, a, b, c, d) AS Foo4,
   @foo5 := UDF2( @foo1, @foo2, @foo3, @foo4) AS Foo5,
   @foo6 := UDF3( @foo1, @foo2, @foo3, @foo4) AS Foo6,
   @foo8 := UDF4( @foo5, @foo7, x, y, z) AS Foo8
FROM MyTable;

As you can see it's quite complicated and a, b, c, d, x, y and z are field names which are quite long [The names express their functionality].

I receive now this error message on MySQL 8.0.20:

X Setting user variables within expressions is deprecated and will be removed in a future release. Consider alternatives: 'SET variable=expression, ...', or 'SELECT expression(s) INTO variables(s)'.

OK, this is the wrong place to discuss if it makes sense that @var := value is deprecated, so I have to move on and I want to assure that the program doesn't stop working if the next MySQL update is installed.

I could solve it like this:

SELECT
   UDF1(0, a, b, c, d) AS Foo1,
   UDF1(1, a, b, c, d) AS Foo2,
   UDF1(2, a, b, c, d) AS Foo3,
   UDF1(3, a, b, c, d) AS Foo4,
   UDF2(UDF1(0, a, b, c, d), UDF1(1, a, b, c, d), UDF1(2, a, b, c, d), UDF1(3, a, b, c, d)) AS Foo5,
   UDF3(UDF1(0, a, b, c, d), UDF1(1, a, b, c, d), UDF1(2, a, b, c, d), UDF1(3, a, b, c, d) ) AS Foo5,
   UDF3(UDF2(UDF1(0, a, b, c, d) , UDF1(1, a, b, c, d) , UDF1(2, a, b, c, d) , UDF1(3, a, b, c, d) ) ,
        UDF3(UDF1(0, a, b, c, d), UDF1(1, a, b, c, d), UDF1(2, a, b, c, d), UDF1(3, a, b, c, d) ), x, y, z) AS Foo6
FROM MyTable;

Honestly, doesn't this hurt and, what I find worst, it becomes so unreadable and changing any call I have to maintain many times --> buggy.

Also, in the current version the length of the SELECT grows from 2'334 bytes to 3'504 bytes.

---

I am trying to work with a temporary table but to fill the table is a quite long and [useless] complicated SELECT using LEFT JOIN as @foo5 depends on @foo1-4 and @foo6 depends on @foo5.

This works, but I am wondering if there may be another solution I am not capable to see. The suggested SELECT expression(s) INTO variables(s) I don't understand how this should help in my case.

As I wrote in the beginning, I don't understand why this feature is deprecated as it apparently can solve lot of troubles and makes complicated SELECT statements simpler...

Any suggestions?

Answer 1

WITH 
cte1 AS ( SELECT UDF1(0, a, b, c, d) AS Foo1,
                 UDF1(1, a, b, c, d) AS Foo2,
                 UDF1(2, a, b, c, d) AS Foo3,
                 UDF1(3, a, b, c, d) AS Foo4,
                 x, y, z
          FROM MyTable ),
cte2 AS ( SELECT Foo1, Foo2, Foo3, Foo4, x, y, z,
                 UDF2(Foo1, Foo2, Foo3, Foo4) AS Foo5,
                 UDF3(Foo1, Foo2, Foo3, Foo4) AS Foo6
          FROM cte1 ),
SELECT Foo1, Foo2, Foo3, Foo4, Foo5, Foo6,
       UDF4(Foo5, Foo6, x, y, z ) AS Foo8
FROM cte2;

Answer 2

I can give you a more concrete example as to what was actually deprecated.

Seven years ago, I wrote the answer to Get the rank of a user in a score table

I make use of variables to find out the rank of a score table and assign the same value for rank in the event that more than one user has the same score (i.e. in case there are ties):

SET @rnk=0; SET @rank=0; SET @curscore=0;
SELECT score,ID,rank FROM
(
    SELECT AA.*,BB.ID,
    (@rnk:=@rnk+1) rnk,
    (@rank:=IF(@curscore=score,@rank,@rnk)) rank,
    (@curscore:=score) newscore
    FROM
    (
        SELECT * FROM
        (SELECT COUNT(1) scorecount,score
        FROM scores GROUP BY score
    ) AAA
    ORDER BY score DESC
) AA LEFT JOIN scores BB USING (score)) A;

This SQL Statement is an expression and will not work in MySQL 8.0.

Try to imagine trying rewriting this without variables. I feel your pain !!!

Aggressive queries that use variables in this fashion is what is being deprecated.

SUGGESTION

Assuming a,b,c,d,x,y,z are columns from MyTable

You simply rewrite it as

SELECT
   UDF1(0, a, b, c, d)
   UDF1(1, a, b, c, d)
   UDF1(2, a, b, c, d)
   UDF1(3, a, b, c, d)
INTO
    @foo1,@foo2,@foo3,@foo4
FROM MyTable;
SELECT
    UDF2( @foo1, @foo2, @foo3, @foo4)
    UDF3( @foo1, @foo2, @foo3, @foo4)
INTO
    @foo5,@foo6;
SELECT UDF4( @foo5, @foo7, x, y, z)
    INTO @foo8
FROM MyTable;

GIVE IT A TRY !!!