Add schema inference to MongoDB engine by evillique · Pull Request #54638 · ClickHouse/ClickHouse

evillique · 2023-09-14T15:59:22Z

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New Feature

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Add schema inference to MongoDB engine. Closes #51933
Add support for nested structures as Tuples. Closes #10919, reference #51929

Documentation entry for user-facing changes

Documentation is written (mandatory for new features)

robot-clickhouse-ci-2 · 2023-09-14T16:06:15Z

This is an automated comment for commit 78bc5f4 with description of existing statuses. It's updated for the latest CI running

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src/Processors/Sources/MongoDBSource.cpp

src/Storages/StorageMongoDB.cpp

Avogar · 2023-09-19T18:40:58Z

src/Storages/StorageMongoDB.cpp

+                    Poco::MongoDB::Element::Ptr child = parent->get(static_cast<int>(idx));
+                    types.push_back(getDataTypeMongoDB(*child));
+                }
+                return std::make_shared<DataTypeTuple>(types);


As I can see we don't support reading Array with different types as Tuple from Mongo. It's a good idea to implement it later, but now we should check that all nested types are the same or have common type (using getLeastSuperType) and throw an exception if there is no common type or create an Array(comon_type) otherwise.

Maybe I miss somethingm but I still don't see support for reading Array with different types as Tuple from Mongo

src/Storages/StorageMongoDB.cpp

src/Storages/StorageMongoDB.h

…ference

Avogar · 2023-12-12T13:41:11Z

src/Processors/Sources/MongoDBSource.cpp

+                Field field = Array();
+                Array & arr = field.get<Array &>();
+                arr.reserve(size);

-                while (!arrays.empty())
+                for (size_t child_idx = 0; child_idx < size; ++child_idx)
                {
-                    size_t dimension_idx = arrays.size() - 1;
+                    Poco::MongoDB::Element::Ptr child = parent->get(static_cast<int>(child_idx));
+                    arr.push_back(getValueField(nested_type, *child, name));
+                }

-                    if (dimension_idx + 1 > expected_dimensions)
-                        throw Exception(ErrorCodes::BAD_ARGUMENTS, "Got more dimensions than expected");
+                assert_cast<ColumnArray &>(column).insert(field);
+                break;


Working with Fields during insert is really suboptimal. Why don't just use insertValue function recursively here on nested column and Mongo array elements?

Avogar · 2023-12-12T13:42:53Z

src/Processors/Sources/MongoDBSource.cpp


-                        dimensions[dimension_idx].emplace_back(Array(dimensions[dimension_idx + 1].begin(), dimensions[dimension_idx + 1].end()));
-                        dimensions[dimension_idx + 1].clear();
+                for (const auto col_num : collections::range(0, column_tuple.tupleSize()))


Just curious why not just for (size_t i = 0; i != column_tuple.tupleSize(); ++i) ?

Avogar · 2023-12-12T13:50:01Z

src/Processors/Sources/MongoDBSource.cpp

    void insertValue(
        IColumn & column,
+        const IDataType & data_type,
        const ValueType type,
        const Poco::MongoDB::Element & value,
        const std::string & name,


Really interesting why do we need this ValueType at all. I would just remove it completely and use only our IDataType

Avogar · 2023-12-12T13:51:39Z

src/Processors/Sources/MongoDBSource.cpp

-                        arrays.emplace_back(child.get(), 0);
-                    }
-                    else if (child->type() == Poco::MongoDB::ElementTraits<Poco::MongoDB::NullValue>::TypeId)
+                    if (nested_value.isNull() || nested_value->type() == Poco::MongoDB::ElementTraits<Poco::MongoDB::NullValue>::TypeId)


This logic with Null value and Nullable column below repeats logic from generate() method. Let's just do it in the begining of the insertValue function as we can call it recursively now

Apidcloud · 2024-03-04T15:02:04Z

Could we merge this? This would make the clickhouse-mongodb connection a lot better

Slach · 2024-05-21T06:32:37Z

@Apidcloud could you resolve conflicts?

mdavidsonST · 2024-05-22T14:46:28Z

Would love if this PR could get across the finish line. It's a big limitation not being able to work with Object and Array fields.

clickhouse-gh · 2024-06-25T16:53:11Z

Dear @Avogar, this PR hasn't been updated for a while. You will be unassigned. Will you continue working on it? If so, please feel free to reassign yourself.

HitendraSinghRathore · 2024-07-01T10:20:21Z

Any updates on this?

clickhouse-gh · 2024-08-06T13:13:28Z

Dear @Avogar, this PR hasn't been updated for a while. You will be unassigned. Will you continue working on it? If so, please feel free to reassign yourself.

clickhouse-gh · 2024-09-10T13:11:48Z

Dear @Avogar, this PR hasn't been updated for a while. You will be unassigned. Will you continue working on it? If so, please feel free to reassign yourself.

alexey-milovidov · 2024-09-22T20:46:44Z

Good feature, let's continue!

alexey-milovidov · 2025-10-26T12:03:35Z

Obsolete.

Add schema inference to MongoDB engine

44c890d

evillique marked this pull request as draft September 14, 2023 16:00

robot-clickhouse-ci-2 added pr-improvement Pull request with some product improvements pr-status-⏳ labels Sep 14, 2023

robot-ch-test-poll1 added pr-status-❌ and removed pr-status-⏳ labels Sep 14, 2023

Avogar self-assigned this Sep 19, 2023