Warning: Using PARTITION expression that returns a Numeric value

https://my.vertica.com/docs/7.1.x/HTML/index.htm#Authoring/AdministratorsGuide/Partitions/DefiningPartitions.htm%3FTocPath%3DAdministrator's%2520Guide%7CUsing%2520Table%2520Partitions%7C_____2

CREATE TABLE trade (
    tdate DATE NOT NULL, 
    tsymbol VARCHAR(8) NOT NULL,
    ttime TIME) 
PARTITION BY EXTRACT (year FROM tdate);

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Warnings: --->

   W (1): Using PARTITION expression that returns a Numeric value

Hint: This PARTITION expression may cause too many data partitions.  Use of an expression that returns a more accurate value, such as a regular VARCHAR or INT, is encouraged

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I am following the directions from the official documentation, should I not expect that to have examples with no errors?