Revert "executor: fix show global variables return session variables also (#19341)"

[bb7133]

This reverts commit cc7a383..

What problem does this PR solve?

Issue Number: close #24326 for TiDB v5.0

Problem Summary:

• Manual test (add detailed scripts or steps below)

The following code is used to do the tests:

       Class.forName("com.mysql.jdbc.Driver").newInstance();    
        String url = "jdbc:mysql://address=(protocol=tcp)(host=127.0.0.1)(port=4000)(user=root)/test?allowMultiQueries=true";    
        Connection conn = DriverManager.getConnection(url, "", "");    
        DatabaseMetaData meta = conn.getMetaData();    
        System.out.println("Driver version: " + meta.getDriverVersion());  

        int connId = 0;    
        while (connId < 20) {    
            try {    
                long startTime = System.currentTimeMillis();    
                conn = DriverManager.getConnection(url, "", "");    
                long duration = System.currentTimeMillis() - startTime;    
                System.out.println("Time for onConnection #" + connId + ": " + duration);    
                connId ++;    
            } catch (Exception e) {    
                System.out.println("Exception: "+e);    
                e.printStackTrace();    
            } finally {    
                conn.close();
            }
        }

After reverting the commit, duration of connection reduced.

Side effects

• This PR brings show global variables statement returns session variables also #18517 back.

Documentation

• To be added

Release note

• Revert executor: fix show global variables return session variables also #19341 to avoid show session variables is very slow on first run #24326.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• djshow832
• morgo

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[bb7133]

/run-integration-test

[bb7133]

/run-integration_test

[bb7133]

/rebuild

[bb7133]

PTAL @morgo , the revert work is not trivial because of some succeeding PRs(#21988, #22284...)

[morgo]

LGTM, but I've added the compatibility breaker tag. I think we should change the compatibility message to:

• The output of SHOW VARIABLES now only includes session scoped variables. This change was introduced to address a performance issue; in TiDB 5.1 and above the output of SHOW VARIABLES returns all variables again.

[bb7133]

LGTM, but I've added the compatibility breaker tag. I think we should change the compatibility message to:

• The output of SHOW VARIABLES now only includes session scoped variables. This change was introduced to address a performance issue; in TiDB 5.1 and above the output of SHOW VARIABLES returns all variables again.

Thank you!

[djshow832]

/merge

[djshow832]

/merge cancel

[djshow832]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: afcf2e8

[ti-chi-bot]

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