MySQL – How ORDER BY FIELD() Works Internally

fieldsMySQLorder-by

I understand how ORDER BY clause works and how the FIELD() function works.
What i want to understand is how the both of them work together to sort.
How are the rows retrieved and how is the sort order derived

+----+---------+
| id |  name   |
+----+---------+
|  1 | stan    |
|  2 | kyle    |
|  3 | kenny   |
|  4 | cartman |
+----+---------+ 

SELECT * FROM mytable WHERE id IN (3,2,1,4) ORDER BY FIELD(id,3,2,1,4)

The query above will result in 

+----+---------+
| id |  name   |
+----+---------+
|  3 | kenny   |
|  2 | kyle    |
|  1 | stan    |
|  4 | cartman |
+----+---------+

something similar to saying ORDER BY 3, 2, 1, 4

QUESTIONS

  • How does this work internally?
  • How does MySQL get the rows, and calculate the sort order ?
  • How does MySQL know it has to sort by the id column ?

Best Answer

For the record

SELECT * FROM mytable WHERE id IN (1,2,3,4) ORDER BY FIELD(id,3,2,1,4);

should work as well because you do not have to order the list in the WHERE clause

As for how it works,

  • FIELD() is a function that returns the index position of a comma-delimited list if the value you are searching for exists.

  • The ORDER BY values are evaluated by what FIELD() returns

You can create all sorts of fancy orders

For example, using the IF() function

SELECT * FROM mytable
WHERE id IN (1,2,3,4)
ORDER BY IF(FIELD(id,3,2,1,4)=0,1,0),FIELD(id,3,2,1,4);

This will cause the first 4 ids to appear at the top of the list, Otherwise, it appears at the bottom. Why?

In the ORDER BY, you either get 0 or 1.

  • If the first column is 0, make any of the first 4 ids appear
  • If the first column is 1, make it appear afterwards

Let's flip it with DESC in the first column

SELECT * FROM mytable
WHERE id IN (1,2,3,4)
ORDER BY IF(FIELD(id,3,2,1,4)=0,1,0) DESC,FIELD(id,3,2,1,4);

In the ORDER BY, you still either get 0 or 1.

  • If the first column is 1, make anything but the first 4 ids appear.
  • If the first column is 0, make the first 4 ids appear in the original order

YOUR ACTUAL QUESTION

If you seriously want internals on this, goto pages 189 and 192 of the Book

MySQL Internals

for a real deep dive.

In essence, there is a C++ class called ORDER *order (The ORDER BY expression tree). In JOIN::prepare, *order is used in a function called setup_order(). Why in the middle of the JOIN class? Every query, even a query against a single table is always processed as a JOIN (See my post Is there an execution difference between a JOIN condition and a WHERE condition?)

The source code for all this is sql/sql_select.cc

Evidently, the ORDER BY tree is going to hold the evaluation of FIELD(id,3,2,1,4). Thus, the numbers 0,1,2,3,4 are the values being sorted while carrying a reference to the row involved.

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