Unexpected plan for insert plain values

[zhongzc]

What type of bug is this?

Incorrect result

What subsystems are affected?

Frontend

What happened?

The plain values plan is wrapper with an unexpected MergeScan

What operating system did you use?

Ubuntu 22.04 x64

Relevant log output and stack trace

No response

How can we reproduce the bug?

mysql> create table t(ts timestamp time index default current_timestamp, v int32);
Query OK, 0 rows affected (0.01 sec)

mysql> explain insert into t(v) values (1)\G
*************************** 1. row ***************************
plan_type: logical_plan
     plan: Dml: op=[Insert] table=[t]
  Projection: CAST(column1 AS Int32) AS v
    MergeScan [is_placeholder=false]
1 row in set (0.01 sec)