feat(StoneDB 8.0): parent_lex->result is nullptr in "select ... from" clause without "into outfile/dumpfile". (#557)

[lujiashun]

[summary]
1 "SELECT ... INTO OUTFILE/DUMPFILE",parent_lex->result is not nullptr;
2 "SELECT ... FORM ",normal clause,parent_lex->result is nullptr;
3 parent_lex->result can be used to check the dumpfile,just to check null value;

Summary about this PR

Issue Number: close #557

 auto exchange =
      static_cast<Query_result_to_file *>(selects_list->parent_lex->result)->get_sql_exchange();
  if (exchange) {
    is_dumpfile = exchange->dumpfile;
    return exchange->file_name;
  }

1 "SELECT ... INTO OUTFILE/DUMPFILE",parent_lex->result is not nullptr;
2 "SELECT ... FORM ",normal clause,parent_lex->result is nullptr;
3 parent_lex->result can be used to check the dumpfile,just to check null value;

Tests Check List

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Changelog

• New Feature
• Bug Fix
• Improvement
• Performance Improvement
• Build/Testing/CI/CD
• Documentation
• Not for changelog (changelog entry is not required)

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features

[mergify]

Thanks for the contribution!
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[RingsC]

LGTM

[DandreChen]

LGTM