Empty slow query text for a stmt that reads tidb_mdl_info

[ekexium]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Unknown. This seems to be an internal SQL.

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

Slow query should display the query text.

3. What did you see instead (Required)

The query text is empty.

4. What is your TiDB version? (Required)

master(8d9e67b)

[ekexium]

cc @wjhuang2016

[wjhuang2016]

It's an internal SQL, but it should not be empty for internal SQL?

[ekexium]

It's an internal SQL, but it should not be empty for internal SQL?

Yes that's my expectation. Do you wanna take a look?

[wjhuang2016]

It's an internal SQL, but it should not be empty for internal SQL?

Yes that's my expectation. Do you wanna take a look?

I don't have an idea for it.

[ekexium]

Some other cases.