ddl: show columns should not change the memory table info

[GMHDBJD]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

CREATE VIEW test.v AS SELECT 1
explain test.v

2. What did you expect to see? (Required)

the tableinfo from infoschema.TableByID should not changed

3. What did you see instead (Required)

the tableinfo from infoschema.TableByID changed

4. What is your TiDB version? (Required)

master