MySQL查询语句权限Bug修复

[feiazifeiazi]

Bug现象：

执行以下SQL时报错，不符合预期。

select * from (
    select 1 as n
    union
    select 2
) as num

  报错信息：

 无法校验查询语句权限，请联系管理员，错误信息：'Schema'

  "schema": snode["Source"]["Schema"]["O"] or db_name,
              ~~~~~~~~~~~~~~~^^^^^^^^^^
KeyError: 'Schema'

Bug描述：

goInception解析后的语法树，不存在Schema节点，导致报错。

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.08%. Comparing base (29bc308) to head (03aae5c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2778   +/-   ##
=======================================
  Coverage   78.08%   78.08%           
=======================================
  Files         122      122           
  Lines       17311    17311           
=======================================
  Hits        13518    13518           
  Misses       3793     3793           

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