[Feature] Add feature to send session variables to JDBC external table

[boosheen]

Why I'm doing:

I want to be able to send session variables to MySQL JDBC External tables.

This feature lets users send session variables to MySQL JDBC External Tables by setting a new session variable jdbc_external_table_session_variables.

For example, running this query against external table external_query_table

SELECT /*+ SET_VAR(jdbc_external_table_session_variables="@my_user_variable='abc',sql_select_limit=5") */ 
* from external_query_table;

Will get translated to this query sent to the external table

SELECT 
/*+ SET_USER_VARIABLE("@my_user_variable='abc') */ 
/*+ SET_VAR(sql_select_limit=5) */
* from external_table;

Also added new property DATABASE_TYPE to JDBC external table resource creation. This way, we can detect if DATABASE_TYPE is starrocks and set user defined variables as well to target starrocks cluster (MySQL does not support user defined variable query hint).

CREATE EXTERNAL RESOURCE external_resource_starrocks
PROPERTIES (
    "type" = "jdbc",
    "driver_class" = "com.mysql.cj.jdbc.Driver",
    "driver_url" = "mysql-connector-j-8.4.0.jar",
    "jdbc_uri" = "jdbc:mysql://127.0.0.1:9030/mydb",
    "user" = "user",
    "password" = "password",
    "database_type" = "starrocks"
);

What I'm doing:

Fixes #53356

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• Interface/UI changes: syntax, type conversion, expression evaluation, display information
• Parameter changes: default values, similar parameters but with different default values
• Policy changes: use new policy to replace old one, functionality automatically enabled
• Feature removed
• Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 3.4
  • 3.3
  • 3.2
  • 3.1
  • 3.0

[CLAassistant]

All committers have signed the CLA.

[boosheen]

I will raise a new PR with documentation if this is accepted 🙏

[sonarcloud]

Quality Gate failed

Failed conditions
C Reliability Rating on New Code (required ≥ A)

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[github-actions]

[Java-Extensions Incremental Coverage Report]

✅ pass : 0 / 0 (0%)

[github-actions]

[BE Incremental Coverage Report]

❌ fail : 0 / 6 (00.00%)

file detail

	path	covered_line	new_line	coverage	not_covered_line_detail
🔵	src/connector/jdbc_connector.cpp	0	6	00.00%	[52, 55, 56, 58, 59, 60]

[zombee0]

Question: if there are more than one starrocks table in a query, which one should take the session variable?

[boosheen]

Question: if there are more than one starrocks table in a query, which one should take the session variable?

Hi @zombee0 good question. By default, the session variables will be sent to all external tables with MySQL protocol in a query (based on the external resource connection pool). It makes the implementation and query syntax simpler for the end user, e.g. if they want to set the same sql_select_limit to all possible external tables

I have considered adding support for user to specify specific resources to send session variables to, but that would be in a new feature/PR as the query input would be more complex.