[GENERAL] indexing date_part

[DEV]

Hello all,   I am interested in setting up an index with a date_part as part of it… From what I have been able to find I need to write a function that will return the date_part that I want I have tried CREATE FUNCTION month_idxable(date) returns date AS 'SELECT date_part('month', date) from mchistw' LANGUAGE 'SQL' WITH (iscachable); And I get: ERROR:  parser: parse error at or near "month" at character 72   So what am I doing wrong and how would I include the function into an index?   Thank you all for all of your help!