Add support for JOIN pushdown in Exasol Connector

[skyglass]

Description

• Add support for JOIN pushdown in Exasol Connector

Additional context and related issues

• Implement toWriteMapping in ExasolClient for DECIMAL type
• Implement basic convertPredicate in ExasolClient

[skyglass]

Thank you very much for your review, @ebyhr ! 👍

I have implemented integration tests, as requested.
In order to make the integration tests work, I also had to add basic implementation of convertPredicate method, which is also a prerequisite for the upcoing JOIN_PUSHDOWN feature.

Unfortunately, I couldn't implement convertPredicate in a separate PR, because the implemented integration tests require both toWriteMapping and convertPredicate methods.

The integration tests testToWriteMappingForDecimalType have been implemented in TestExasolConnectorTest

toWriteMapping and convertPredicate are triggered for LEFT JOIN queries when assertJoinConditionallyPushedDown check is done in integration tests.

Different column types are used to provide different values for toWriteMapping parameters.
Unfortunately, there is no way to check the business-logic in more details for these integration tests. These tests only make sure that the checks are successful and the assertJoinConditionallyPushedDown and expectJoinPushdownOnEmptyProjection are successfully supported for LEFT JOIN queries with different column types. By triggering toWriteMapping and convertPredicate these integration tests indirectly test these methods.

For more fine-grained assertions, please, check TestExasolToWriteMapping unit tests.

toWriteMapping is also triggered for INSERT queries, but because Exasol Connector is read-only and doesn't support INSERT queries, we don't need to cover toWriteMapping with the tests for INSERT queries.

Basic implementations of toWriteMapping and convertPredicate are prerequisites for enabling the upcoming JOIN_PUSHDOWN feature.

In the next PR I will enable testJoinPushdown() in BaseJdbcConnectorTest, which requires basic implementations of toWriteMapping and convertPredicate to make testJoinPushdown() work and to enable JOIN_PUSHDOWN feature.

[pj-spoelders]

Hi @ebyhr ,
Can you take another look at this PR please?
A mutual customer of ours is looking to use this optimization.

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[skyglass]

@chenjian2664, thank you very much for your feedback and clarification! 👍
I actually didn't know that SUPPORT_JOIN_PUSHDOWN automatically enables all sub flags, but I checked the TestPostgreSqlConnectorTest and it only has SUPPORT_JOIN_PUSHDOWN flag, without any sub-flags.

Therefore, I have removed all sub-flags and kept only SUPPORT_JOIN_PUSHDOWN flag, as you suggested.

The PR was updated, please review again.

[skyglass]

Thank you very much for your review, @chenjian2664 !
I have answered your comments, hopefully it resolves the current PR.
Our customers would be very happy if we could continue further support of join and aggregate pushdown, but, unfortunately, this unresolved PR is blocking us from further development.

.

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[github-actions]

Closing this pull request, as it has been stale for six weeks. Feel free to re-open at any time.

[skyglass]

Any updates on this PR?

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This pull request has gone a while without any activity. Ask for help on #core-dev on Trino slack.