Implement n_distinct()

[thisisnic]

No description provided.

[thisisnic]

In DuckDB, this appears to be implemented thus:

> duckdb_get_substrait("SELECT COUNT(DISTINCT hp) AS cd FROM mtcars", list(mtcars = mtcars))
message of type 'substrait.Plan' with 2 fields set
extensions {
  extension_function {
    function_anchor: 1
    name: "count"
  }
}
relations {
  root {
    input {
      project {
        input {
          aggregate {
            input {
              read {
                base_schema {
                  names: "mpg"
                  names: "cyl"
                  names: "disp"
                  names: "hp"
                  names: "drat"
                  names: "wt"
                  names: "qsec"
                  names: "vs"
                  names: "am"
                  names: "gear"
                  names: "carb"
                  struct {
                    types {
                      fp64 {
                        nullability: NULLABILITY_NULLABLE
                      }
                    }
                    types {
                      fp64 {
                        nullability: NULLABILITY_NULLABLE
                      }
                    }
                    types {
                      fp64 {
                        nullability: NULLABILITY_NULLABLE
                      }
                    }
                    types {
                      fp64 {
                        nullability: NULLABILITY_NULLABLE
                      }
                    }
                    types {
                      fp64 {
                        nullability: NULLABILITY_NULLABLE
                      }
                    }
                    types {
                      fp64 {
                        nullability: NULLABILITY_NULLABLE
                      }
                    }
                    types {
                      fp64 {
                        nullability: NULLABILITY_NULLABLE
                      }
                    }
                    types {
                      fp64 {
                        nullability: NULLABILITY_NULLABLE
                      }
                    }
                    types {
                      fp64 {
                        nullability: NULLABILITY_NULLABLE
                      }
                    }
                    types {
                      fp64 {
                        nullability: NULLABILITY_NULLABLE
                      }
                    }
                    types {
                      fp64 {
                        nullability: NULLABILITY_NULLABLE
                      }
                    }
                    nullability: NULLABILITY_REQUIRED
                  }
                }
                projection {
                  select {
                    struct_items {
                      field: 3
                    }
                  }
                  maintain_singular_struct: true
                }
                named_table {
                  names: "mtcars"
                }
              }
            }
            groupings {
            }
            measures {
              measure {
                function_reference: 1
                output_type {
                  i64 {
                    nullability: NULLABILITY_NULLABLE
                  }
                }
                arguments {
                  value {
                    selection {
                      direct_reference {
                        struct_field {
                        }
                      }
                      root_reference {
                      }
                    }
                  }
                }
              }
            }
          }
        }
        expressions {
          selection {
            direct_reference {
              struct_field {
              }
            }
            root_reference {
            }
          }
        }
      }
    }
    names: "cd"
  }
}