Handle exception for empty tables in elasticsearch

[K-Naga-Vivek]

Add a catch block to handle the exception when a table in Elasticsearch does not have any columns. Previously, this situation would cause an internal error in Presto.

The code iterates through the mappings (columns) but lacks an exception handler for cases where an element is not found. So, catching the NoSuchElementException to throw the appropriate error.

Motivation and Context

#23849

Test Plan

Added the related testcase to the code change.

Tested and working as excepted.

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Release Notes

Please follow release notes guidelines and fill in the release notes below.

== RELEASE NOTES ==

Elasticsearch Changes
* Improve handling of exceptions for empty tables in Elasticsearch :pr:`23850`

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: K-Naga-Vivek (4a1df65)

[steveburnett]

Thanks for the release note entry! Nit suggestion to help it follow the Order of changes phrasing in the Release Notes Guidelines.

== RELEASE NOTES ==

Elasticsearch Changes
* Improve handling of exceptions for empty tables in Elasticsearch :pr:`23850`

[tdcmeehan]

Suggested change

	throw new PrestoException(ELASTICSEARCH_QUERY_FAILURE, "Relation does not have any columns");
	throw new PrestoException(ELASTICSEARCH_QUERY_FAILURE, "Relation does not have any columns", e);

[tdcmeehan]

Can you explain where this is thrown? I'm just wondering if it's because of the iteration on line 496.

[K-Naga-Vivek]

Yeah, the issue occurs on the line mappings = mappings.elements().next();. When there are no properties, the flow enters the condition and proceeds to iterate to the next element, which leads to a NoSuchElementException.

[tdcmeehan]

So instead of catching an exception, can you instead check if !hasNext() and throw?

[K-Naga-Vivek]

@tdcmeehan, as suggested, I’ve updated the code to return an empty list instead of throwing an error when encountering tables without columns. This change resolves the failing test case and functions as expected. Now, if a SELECT * query is run on a table with no columns, the StatementAnalyzer throws an error: SELECT * not allowed from relation that has no columns as intended.

Could you please review the changes?

[tdcmeehan]

Why does this connector throw at all if there are no columns? Can't it return a table with no columns?

[K-Naga-Vivek]

The connector is currently set to return an empty list, but Presto has a check in the StatementAnalyzer that throws an error if a table has no columns.

[tdcmeehan]

Can we let the exception occur in the analyzer, instead of throwing within the Elastic Search connector?

[K-Naga-Vivek]

@tdcmeehan We are not throwing an exception within the connector. I've updated the code so that it now returns an empty list. The exception is now only thrown by the Analyzer.

[tdcmeehan]

Got it. I was confused because I saw this message also in the Elastic client.

[prestodb-ci]

Saved that user @K-Naga-Vivek is from IBM