[GitHub] [beam] kennknowles opened a new issue, #19176: Support conjunction clause when it's only equi-join

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kennknowles opened a new issue, #19176:
URL: https://github.com/apache/beam/issues/19176

   conjunction_clause: function_call(function_parameter, ...) | field_access | column
   function_parameter: function_call | field_access
   
   In Beam, equi-join is implemented by CoGBK, which requires both join inputs (assume binary join) to build PCollection of KV<Row, Row\>, where the key is join key.
   
   For equi-join, conjunction clause is essentially an equation. In order to build KV<Row, Row\>, it requires that columns from different sides of equation should come from different join input. For example, a **** b = 2 cannot be used to build join key but a = 2 - b can. So rewriting is required for clauses when it does not satisfy this property. 
   
   It also implies that not every clause is rewritable. Say the clause is f(a, b) = 3, in which a is from left input and b is from right input. If this function f is not splittable, such that we cannot move a or b to right side of equation, then we cannot support this clause in BeamSQL's  join.
   
   
   
   Imported from Jira [BEAM-7151](https://issues.apache.org/jira/browse/BEAM-7151). Original Jira may contain additional context.
   Reported by: amaliujia.
   Subtask of issue #19175


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