add func user()

[TCeason]

in MySQL only has function user()

We has already impl current_user so we can use this function.

 select user();
+----------------+
| user()         |
+----------------+
| root@localhost |
+----------------+
1 row in set (0.00 sec)

I hereby agree to the terms of the CLA available at: https://databend.rs/dev/policies/cla/

Summary

Changelog

• New Feature

Related Issues

Fixes #5023

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1 Ignored Deployment

Name	Status	Preview	Updated
databend	⬜️ Ignored (Inspect)		May 25, 2022 at 11:44AM (UTC)

[mergify]

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[TCeason]

And have a question, I try to add a test on stateless test but ther has some err: test_statements_in_legacy_suites

'root'@mysqldb 19:44:26 [(none)]> set enable_planner_v2 = 1;
Query OK, 0 rows affected (0.04 sec)
Read 0 rows, 0.00 B in 0.002 sec., 0 rows/sec., 0.00 B/sec.

'root'@mysqldb 19:44:31 [(none)]> select user();
ERROR 1105 (HY000): Code: 1005, displayText = error: 
  --> SQL:1:8
  |
1 | select user()
  | ------ ^^^^ expected `*`, `IS`, `IN`, `BETWEEN`, `+`, `-`, or 46 more ...
  | |       
  | while parsing `SELECT ...`

[BohuTANG]

cc @andylokandy for help, thanks.

[BohuTANG]

We can do it in another PR as #5562 (comment)