fix sel field

[wjhuang2016]

What problem does this PR solve?

For pingcap/tidb#27320

What is changed and how it works?

Something is missing when refactoring.

Check List

Tests

• Manual test (add detailed scripts or steps below)

mysql> WITH RECURSIVE cte AS (SELECT 5 UNION SELECT @c:=@c+1 FROM cte WHERE @c<3) SELECT * FROM cte;
+---+
| 5 |
+---+
| 5 |
+---+
1 row in set (0.00 sec)
mysql> select 1  union select 2;
+---+
| 1 |
+---+
| 1 |
| 2 |
+---+
2 rows in set (0.00 sec)

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

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[ti-chi-bot]

In response to a cherrypick label: new pull request created: #1315.