MongoDB table engine

MongoDB engine is read-only table engine which allows to read data from a remote collection.

MongoDB engine is read-only table engine which allows to read data from a remote MongoDB collection.

Only MongoDB v3.6+ servers are supported. Seed list(mongodb+srv) is not yet supported.

Creating a table

CREATE TABLE [IF NOT EXISTS] [db.]table_name
(
    name1 [type1],
    name2 [type2],
    ...
) ENGINE = MongoDB(host:port, database, collection, user, password[, options[, oid_columns]]);

Engine Parameters

Parameter	Description
host:port	MongoDB server address.
database	Remote database name.
collection	Remote collection name.
user	MongoDB user.
password	User password.
options	Optional. MongoDB connection string options as a URL formatted string. e.g. 'authSource=admin&ssl=true'
oid_columns	Comma-separated list of columns that should be treated as oid in the WHERE clause. _id by default.

If you are using the MongoDB Atlas cloud offering connection url can be obtained from ‘Atlas SQL’ option. Seed list(mongodb**+srv**) is not yet supported, but will be added in future releases.

Alternatively, you can pass a URI:

ENGINE = MongoDB(uri, collection[, oid_columns]);

Engine Parameters

Parameter	Description
uri	MongoDB server’s connection URI.
collection	Remote collection name.
oid_columns	Comma-separated list of columns that should be treated as oid in the WHERE clause. _id by default.

Types mappings

MongoDB	ClickHouse
bool, int32, int64	any numeric type except Decimals, Boolean, String
double	Float64, String
date	Date, Date32, DateTime, DateTime64, String
string	String, any numeric type(except Decimals) if formatted correctly
document	String(as JSON)
array	Array, String(as JSON)
oid	String
binary	String if in column, base64 encoded string if in an array or document
uuid (binary subtype 4)	UUID
any other	String

If key is not found in MongoDB document (for example, column name doesn’t match), default value or NULL (if the column is nullable) will be inserted.

OID

If you want a String to be treated as oid in the WHERE clause, just put the column’s name in the last argument of the table engine. This may be necessary when querying a record by the _id column, which by default has oid type in MongoDB. If the _id field in the table has other type, for example uuid, you need to specify empty oid_columns, otherwise the default value for this parameter _id is used.

db.sample_oid.insertMany([
    {"another_oid_column": ObjectId()},
]);

db.sample_oid.find();
[
    {
        "_id": {"$oid": "67bf6cc44ebc466d33d42fb2"},
        "another_oid_column": {"$oid": "67bf6cc40000000000ea41b1"}
    }
]

By default, only _id is treated as oid column.

CREATE TABLE sample_oid
(
    _id String,
    another_oid_column String
) ENGINE = MongoDB('mongodb://user:pass@host/db', 'sample_oid');

SELECT count() FROM sample_oid WHERE _id = '67bf6cc44ebc466d33d42fb2'; --will output 1.
SELECT count() FROM sample_oid WHERE another_oid_column = '67bf6cc40000000000ea41b1'; --will output 0

In this case the output will be 0, because ClickHouse doesn’t know that another_oid_column has oid type, so let’s fix it:

CREATE TABLE sample_oid
(
    _id String,
    another_oid_column String
) ENGINE = MongoDB('mongodb://user:pass@host/db', 'sample_oid', '_id,another_oid_column');

-- or

CREATE TABLE sample_oid
(
    _id String,
    another_oid_column String
) ENGINE = MongoDB('host', 'db', 'sample_oid', 'user', 'pass', '', '_id,another_oid_column');

SELECT count() FROM sample_oid WHERE another_oid_column = '67bf6cc40000000000ea41b1'; -- will output 1 now

Supported clauses

Only queries with simple expressions are supported (for example, WHERE field = <constant> ORDER BY field2 LIMIT <constant>). Such expressions are translated to MongoDB query language and executed on the server side. You can disable all these restriction, using mongodb_throw_on_unsupported_query. In that case ClickHouse tries to convert query on best effort basis, but it can lead to full table scan and processing on ClickHouse side.

It’s always better to explicitly set type of literal because Mongo requires strict typed filters.
For example you want to filter by Date:

SELECT * FROM mongo_table WHERE date = '2024-01-01'

This will not work because Mongo will not cast string to Date, so you need to cast it manually:

SELECT * FROM mongo_table WHERE date = '2024-01-01'::Date OR date = toDate('2024-01-01')

This applied for Date, Date32, DateTime, Bool, UUID.

Usage example

Assuming MongoDB has sample_mflix dataset loaded

Create a table in ClickHouse which allows to read data from MongoDB collection:

Query

CREATE TABLE sample_mflix_table
(
    _id String,
    title String,
    plot String,
    genres Array(String),
    directors Array(String),
    writers Array(String),
    released Date,
    imdb String,
    year String
) ENGINE = MongoDB('mongodb://<USERNAME>:<PASSWORD>@atlas-sql-6634be87cefd3876070caf96-98lxs.a.query.mongodb.net/sample_mflix?ssl=true&authSource=admin', 'movies');

Query

SELECT count() FROM sample_mflix_table

Response

   ┌─count()─┐
1. │   21349 │
   └─────────┘

Query

-- JSONExtractString cannot be pushed down to MongoDB
SET mongodb_throw_on_unsupported_query = 0;

-- Find all 'Back to the Future' sequels with rating > 7.5
SELECT title, plot, genres, directors, released FROM sample_mflix_table
WHERE title IN ('Back to the Future', 'Back to the Future Part II', 'Back to the Future Part III')
    AND toFloat32(JSONExtractString(imdb, 'rating')) > 7.5
ORDER BY year
FORMAT Vertical;

Response

Row 1:
──────
title:     Back to the Future
plot:      A young man is accidentally sent 30 years into the past in a time-traveling DeLorean invented by his friend, Dr. Emmett Brown, and must make sure his high-school-age parents unite in order to save his own existence.
genres:    ['Adventure','Comedy','Sci-Fi']
directors: ['Robert Zemeckis']
released:  1985-07-03

Row 2:
──────
title:     Back to the Future Part II
plot:      After visiting 2015, Marty McFly must repeat his visit to 1955 to prevent disastrous changes to 1985... without interfering with his first trip.
genres:    ['Action','Adventure','Comedy']
directors: ['Robert Zemeckis']
released:  1989-11-22

Query

-- Find top 3 movies based on Cormac McCarthy's books
SELECT title, toFloat32(JSONExtractString(imdb, 'rating')) AS rating
FROM sample_mflix_table
WHERE arrayExists(x -> x LIKE 'Cormac McCarthy%', writers)
ORDER BY rating DESC
LIMIT 3;

Response

   ┌─title──────────────────┬─rating─┐
1. │ No Country for Old Men │    8.1 │
2. │ The Sunset Limited     │    7.4 │
3. │ The Road               │    7.3 │
   └────────────────────────┴────────┘

Troubleshooting

You can see the generated MongoDB query in DEBUG level logs.

Implementation details can be found in mongocxx and mongoc documentations.