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`collection_name` is the name of colletion the collection to delete entities from.
`expr` is an expression indicated whether an entity should be deleted in the collection. Only the `in` operator is supported in the Delete API. Document of expression: https://github.com/milvus-io/milvus/blob/master/docs/design_docs/query_boolean_expr.md
`partition_name` is the name of the partition to delete entities from, `None` means all partition.
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Same as Insert API, Milvus only guarantee the visibility of operations with a one client. This means that, within the sequence of the operations "delete(), search()", the result of the search will not contains the entities deleted. Since different clients connect to different Proxy, the time between different Proxy is not exactly th the same. So even if you manually call the delete method and the search method on two clients sequentially, it is uncertain whether the search request returns the deleted entities.
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