[GENERAL] can't create index with 'dowcast' row

[Louis-David Mitterrand]

Hi,

To constraint unique'ness of my visitors to a 24h periode I tried 
created a index including the 'date' part of the created_on timestamp:

        CREATE UNIQUE INDEX visit_idx ON visit_buffer USING btree (id_session, 
        id_story, created_on::date);

        psql:visit_pkey.sql:5: ERROR:  syntax error at or near "::"
        LINE 1: ...buffer USING btree (id_session, id_story, created_on::date);

and this:

        CREATE UNIQUE INDEX visit_idx ON visit_buffer USING btree (id_session, 
id_story, extract(date from created_on));
        psql:visit_pkey.sql:4: ERROR:  functions in index expression must be 
marked IMMUTABLE

How can I achieve what I am trying?

Thanks,

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