MySQL 5.7.5+ get first row for the groups

问题

I have a legacy application in which Group By has been used along with non aggregate columns to fetch the first row for each group. The query is as following:

SELECT
    columnPrimaryKey,
    column1,
    column2,
    column3
FROM
    (SELECT
        columnPrimaryKey,
        column1,
        column2,
        column3
    FROM testTable
    ORDER BY column2
) AS tbl
GROUP BY column3

Recently, the version was updated to 5.7.22 and now the above query is not returning expected results even after disabling the ONLY_FULL_GROUP_BY mode.

Yes, I can rewrite the query as following to work according to the new behavior as:

SELECT
    x.columnPrimaryKey,
    x.column1,
    x.column2,
    x.column3
FROM tableName AS x INNER JOIN (
    SELECT
        MIN( column2 ) AS column2,
        column3
    FROM tableName
    GROUP BY column3
) AS y ON x.column2 = y.column2 AND x.column3 = y.column3;

Unfortunately, that's not an option for now. The only option I see is to downgrade to 5.7.5 earlier.

Fiddle 5.7 with 'ONLY_FULL_GROUP_BY' disabled and unexpected results:

https://www.db-fiddle.com/f/8VjB7XpkobWVyXpPvUaGt2/0

Fiddle 5.6 with default modes and expected results:

https://www.db-fiddle.com/f/8VjB7XpkobWVyXpPvUaGt2/1

My question is: Is there any way to disable this behavior of random selection so that the legacy code works without rewriting them or downgrading?

Any suggestions greatly appreciated!

回答1:

Your ORDER BY in the derived table subquery is ignored in MySQL 5.7.

See https://dev.mysql.com/doc/refman/5.7/en/derived-table-optimization.html

The optimizer propagates an ORDER BY clause in a derived table or view reference to the outer query block if these conditions are all true:

The outer query is not grouped or aggregated.

The outer query does not specify DISTINCT, HAVING, or ORDER BY.

The outer query has this derived table or view reference as the only source in the FROM clause.

Otherwise, the optimizer ignores the ORDER BY clause.

Your outer query has a JOIN and a GROUP BY, so it doesn't qualify to propagate the ORDER BY, therefore it ignores the ORDER BY.

This optimizer behavior is controlled by the optimizer switch derived_merge. You can disable it.

Demo:

mysql [localhost] {msandbox} (test) > select @@version;
+-----------+
| @@version |
+-----------+
| 5.7.21    |
+-----------+

mysql [localhost] {msandbox} (test) > SELECT     columnPrimaryKey,     column1,     column2,     column3 FROM     (SELECT         columnPrimaryKey,         column1,         column2,         column3     FROM testTable     ORDER BY column2 ) AS tbl GROUP BY column3;
+------------------+----------------+---------+---------+
| columnPrimaryKey | column1        | column2 | column3 |
+------------------+----------------+---------+---------+
|                1 | Some Name 8-4  |       4 |       8 |
|                6 | Some Name 9-1  |       1 |       9 |
|                8 | Some Name 10-2 |       2 |      10 |
+------------------+----------------+---------+---------+

mysql [localhost] {msandbox} (test) > set optimizer_switch = 'derived_merge=off';
Query OK, 0 rows affected (0.00 sec)

mysql [localhost] {msandbox} (test) > SELECT     columnPrimaryKey,     column1,     column2,     column3 FROM     (SELECT         columnPrimaryKey,         column1,         column2,         column3     FROM testTable     ORDER BY column2 ) AS tbl GROUP BY column3;
+------------------+----------------+---------+---------+
| columnPrimaryKey | column1        | column2 | column3 |
+------------------+----------------+---------+---------+
|                5 | Some Name 8-1  |       1 |       8 |
|                6 | Some Name 9-1  |       1 |       9 |
|                8 | Some Name 10-2 |       2 |      10 |
+------------------+----------------+---------+---------+

来源：https://stackoverflow.com/questions/51969174/mysql-5-7-5-get-first-row-for-the-groups

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