index on to_char(created, 'YYYY') doesn't work

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The following does not work:

create index session_u_idx on session (to_char(created, 'YYYY'));
ERROR:  parser: parse error at or near "'YYYY'" at character 57

Can I make a function to do this and index using the result of that funtion?
Do anyone have an example of such a function?

- --
Andreas Joseph Krogh <andreak@officenet.no>There will always be someone who agrees with youbut is, inexplicably, a
moron.

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