:) CREATE TABLE test.nested (EventDate Date, UserID UInt64, Attrs Nested(Key String, Value String)) ENGINE = MergeTree(EventDate, UserID, 8192)

CREATE TABLE test.nested
(
    EventDate Date, 
    UserID UInt64, 
    Attrs Nested(
    Key String, 
    Value String)
) ENGINE = MergeTree(EventDate, UserID, 8192)

Ok.

0 rows in set. Elapsed: 0.003 sec. 

:) INSERT INTO test.nested VALUES ('2016-01-01', 123, ['price', 'color'], ['high', 'red'])

INSERT INTO test.nested VALUES

Ok.

1 rows in set. Elapsed: 0.003 sec. 

:) SELECT * FROM test.nested

SELECT *
FROM test.nested

┌──EventDate─┬─UserID─┬─Attrs.Key─────────┬─Attrs.Value────┐
│ 2016-01-01 │    123 │ ['price','color'] │ ['high','red'] │
└────────────┴────────┴───────────────────┴────────────────┘

1 rows in set. Elapsed: 0.004 sec. 

:) SELECT * FROM test.nested ARRAY JOIN Attrs

SELECT *
FROM test.nested
ARRAY JOIN Attrs

┌──EventDate─┬─UserID─┬─Attrs.Key─┬─Attrs.Value─┐
│ 2016-01-01 │    123 │ price     │ high        │
│ 2016-01-01 │    123 │ color     │ red         │
└────────────┴────────┴───────────┴─────────────┘

2 rows in set. Elapsed: 0.002 sec. 

:) SELECT * FROM test.nested WHERE Attrs.Value[indexOf(Attrs.Key, 'color')] = 'red'

SELECT *
FROM test.nested
WHERE Attrs.Value[indexOf(Attrs.Key, 'color')] = 'red'

┌──EventDate─┬─UserID─┬─Attrs.Key─────────┬─Attrs.Value────┐
│ 2016-01-01 │    123 │ ['price','color'] │ ['high','red'] │
└────────────┴────────┴───────────────────┴────────────────┘

1 rows in set. Elapsed: 0.008 sec.