Improvement on subquery handling in ShardingRouteDecorator

Requisites

1. Have the basic learning about ShardingStatementValidator
2. Java coding

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Aim-1

1. This issue is to do some refactoring works to make the subquery validating cohesive.

How-1

1. Create a classShardingSelectStatementValidator implements ShardingStatementValidator.
2. Move the content of checkSubqueryShardingValues in ShardingRouteDecorator to the preValidate of ShardingSelectStatementValidator.

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Once How-1 finishes, you can raize a PR. Moreover, there is a further to-do for Aim-2.

Aim-2

1. Review the validating logic and make all the subqueries in the one database instance run well. FYI, currently, subquery only with the same sharding key can work well, which limits the subquery supported scope where we can reach.

How-2

1. Review the preValidate function and fill the postValidate function in ShardingSelectStatementValidator to make the subqueries SQL routed in the same instance work well.
2. Fix the @Ignore tests in SubqueryRouteTest
3. Add a test case with a subquery SQL containing different sharding column names, like using order_id and item_id together.

[wgy8283335]

@strongduanmu hi, I’ve modified the codes according to your suggestion.

[strongduanmu]

I’d like to take care of aim-2.

@wgy8283335 Welcome ! 👍