DELETE doesn't delete data in some cases when the database name is capitalized

[winoros]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop database if exists test1;
create database TEST1;
use TEST1;
create table t(a int);
create table t1(a int);
insert into t values(1);
insert into t1 values(1);

delete a from t a where exists (select 1 from t1 where a.a = t1.a);

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

The row in t is deleted successfully.

3. What did you see instead (Required)

No row is deleted.

4. What is your TiDB version? (Required)

Latest master and 4.0.8.

[ti-srebot]

Please edit this comment or add a new comment to complete the following information

Not a bug

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2. Add notes to indicate why it is not a bug

Duplicate bug

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2. Add the link to the original bug

Bug

Note: Make Sure that 'component', and 'severity' labels are added
Example for how to fill out the template: #20100

1. Root Cause Analysis (RCA) (optional)

The code treats a string which can have upper case char as a string only has lower case char. This behavior leads to unexpected DELETE behavior.

2. Symptom (optional)

The rows should be deleted aren't deleted.

3. All Trigger Conditions (optional)

use USE stetament choose a database whose original name in its CREATE DATABASE statement is not in pure lower representation.

Then deleting the tables in this database may not delete any data.

4. Workaround (optional)

5. Affected versions

[v3.0.1:v3.0.19], [v4.0.1:v4.0.8]

6. Fixed versions

v3.0.20, v4.0.9