[PERFORM] create index with substr function

[Ray]

Hi All,   I have a table in my postgres: Table: doc      Column     |            Type             | Modifiers      ---------------+-----------------------------+-----------  doc_id          | bigint                      | not null  comp_grp_id | bigint                      | not null  doc_type      | character varying(10)| not null  doc_urn        | character varying(20)| not null I want to create an index on doc_urn column with using substr function like this: CREATE INDEX idx_doc_substr_doc_urn ON doc USING btree (SUBSTR(doc_urn,10));   but there is an error: ERROR:  parser: parse error at or near "10" at character 68   what's wrong for this SQL? As I have found some reference on the internet, I can't find anything wrong in this SQL.   Thanks Ray