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[skip ci] Add document for visitor in segcore (#9162) 

Signed-off-by: xiaofan-luan <xiaofan.luan@zilliz.com>
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Xiaofan 2021-10-03 21:17:55 +08:00 committed by GitHub
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docs/design_docs/segcore/visitor.md
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# Visitor Pattern
Visitor Pattern is used in segcore for parse and execute Execution Plan.
1. Inside `${core}/src/query/PlanNode.h` , contains physical plan for vector
1. `FloatVectorANNS` FloatVector search execution node
2. `BinaryVectorANNS` BinaryVector search execution node
2. `${core}/src/query/Expr.h` contains physical plan for expression
1. `TermExpr` support operation like `col in [1, 2, 3]`
2. `RangeExpr` support constant compare with data column like `a >= 5` `1 < b < 2`
3. `CompareExpr` support compare with different columns, like `a < b`
4. `LogicalBinaryExpr` support and/or
5. `LogicalUnaryExpr` support not
Currently , under `${core/query/visitors}` directory, there are following visitors
1. `ShowPlanNodeVisitor` print PlanNode in json
2. `ShowExprVisitor` Expr -> json
3. `Verify...Visitor` validate ...
4. `ExtractInfo...Visitor` extract info from... ，including involved_fields and else
5. `ExecExprVisitor` Generate bitmask according to expression
6. `ExecPlanNodeVistor` physical plan executor, for now only support ANNS node.