milvus/docs/design_docs/segcore/visitor.md

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 search:
   1. FloatVectorANNS FloatVector search execution node
   2. BinaryVectorANNS BinaryVector search execution node
2. ${core}/src/query/Expr.h contains physical plan for scalar 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 prints PlanNode in json
2. ShowExprVisitor Expr -> json
3. Verify...Visitor validates ...
4. ExtractInfo...Visitor extracts info from..., including involved_fields and else
5. ExecExprVisitor generates bitmask according to expression
6. ExecPlanNodeVistor physical plan executor only supports ANNS node for now