You can define a column as an key-value map by ending the column name in curly brackets ({}), followed by an object enclosed in curly brackets ({}). In addition, you can specify the data type for the key in the curly brackets in the column name. For date and time data types, if necessary, you can also specify a format after the key data type and separated by a comma.

A column as an key-value map takes the following form for a key-value map with two nested columns. You can define one or more nested columns in a key-map entry. 
"<column_map>{<key_data_type>,<format>}":{"<column_a>:<data_type>","<column_b>":"<data_type>"
column_map
is the column name that contains the key-value map.
key_data_type
specifies the data type mapping for the map key.
format
(optional) specifies the Java SimpleDateFormat, if the data type is of the data, time, timestamp type. This value is not required if the values use ISO format. See "Date, time, and timestamp formats" for details.
column
specifies the name of columns within the key-value map.
data_type
specifies the data type mapping for the column within the key-map.
The following example demonstrates defining a column as a key-value map with the key data type and format specified: 
"balancesheet{Date,MMddyyyy}":{"assets":"decimal(18.2)", "liabilities":"decimal(18.2)"}