executor: fix show global variables return session variables also

[erwadba]

What problem does this PR solve?

Issue Number: close #18517

Problem Summary: To fix show global variables statement return session variables.

What is changed and how it works?

What's Changed:
Rewrite the function fetchShowVariables() to distinguish global var from session var.
How it Works:

Related changes

n/a

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

mysql> select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v4.0.0-beta.2-993-g18b4e2b1a-dirty
Edition: Community
Git Commit Hash: 18b4e2b1a106d19db590da4af741e22ed08a5bf1
Git Branch: showvar
UTC Build Time: 2020-08-20 08:16:31
GoVersion: go1.13.8
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.04 sec)

mysql>  show global Variables like "tidb_mem_quota_topn";
Empty set (0.15 sec)

mysql> show  Variables like "tidb_mem_quota_topn";
+---------------------+-------------+
| Variable_name       | Value       |
+---------------------+-------------+
| tidb_mem_quota_topn | 34359738368 |
+---------------------+-------------+
1 row in set (0.01 sec)

mysql>

Side effects

n/a

Release note

• fix show global variables return session variables also

[erwadba]

@fzhedu PTAL

[erwadba]

@ti-srebot /run-all-test

[ti-srebot]

/run-all-test

[erwadba]

@ti-srebot /run-all-tests

[ti-srebot]

/run-all-tests

[erwadba]

@ti-srebot /rebuild

[erwadba]

@ti-srebot /run-all-tests

[ti-srebot]

/run-all-tests

[erwadba]

@ti-srebot /run-common-test

[ti-srebot]

/run-common-test

[erwadba]

@ti-srebot /run-all-tests

[erwadba]

@qw4990 PTAL

[erwadba]

@ti-srebot /run-all-tests

[qw4990]

@erwadba Please pick the issue again.

[erwadba]

/pick-up

[ti-challenge-bot]

Pick up success.

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[qw4990]

/pick-up

Please comment /pick-up in this issue(#18517).

[erwadba]

picked

[qw4990]

/reward 300

[ti-challenge-bot]

Reward success.

[ti-challenge-bot]

@erwadba, Congratulations, you get 300 in this PR, and your total score is 300 in high-performance challenge program.

[qw4990]

It's merged. Thanks for your contribution :D @erwadba

[ti-srebot]

cherry pick to release-4.0 in PR #20764

[ti-challenge-bot]

@erwadba You did not submit PR within 7 days, so give up automatically.