Description

!!!NOTE!!! Please ignore most file changes as they're all changing instance method to static method call on DSL. The only change matters is in FunctionRepository which becomes a singleton after this.

Problem Statement

The performance issue is improved in #944, this PR is focused on the functionality and our own DSL language adoption problem.

Currently, FunctionRepository is managed by Spring container and inject to class who needs it. DSL depends on the function repository to resolve function signature as well. This causes the function repository or DSL instance passed around and leads to "ugly" code as below:

  private final DSL dsl = new ExpressionConfig().dsl(new ExpressionConfig().functionRepository());

  dsl.equal(
    DSL.ref("integer_value", INTEGER),
    DSL.literal(integerValue(1)))),

Besides, the bigger problem is it blocks high level physical operator from building custom logic on top of the expression system. For example, the new WindowAssigner needs to calculate the lower bound of a window as below. The current coupling with Spring container make it hard to implement:

  // Assign a numeric or datetime window to the value according to its type
  Window assignWindow(ExprValue value) {
    ...
    ExprValue lowerBound = rounding.round(value);
    ExprValue windowSize = ...
    ExprValue upperBound = 
      lowerBound.isNumber()
        ? DSL.add(lowerBound, windowNumberSize)
        : DSL.adddate(lowerBound, windowIntervalSize)
  }

Solution

After double check, make FunctionRepository singleton as well as DSL. This makes both can be reused anywhere.

TODO

1. May need to move storage engine function register code elsewhere, because it is supposed to be a static list for each storage engine. Added TODO comment for now.

Further Thoughts

1. Will this impact storage engine register its own function in future? <= Probably not.
2. Should basic arithmetic operation built-in ExprValue? ex. val1.add(val2) rather than DSL.add(val1, val2).valueOf(null) <= Not sure. Maybe we should.

Issues Resolved

#944

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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