Create index on the year of a date column

Hi,

I am trying to create an index on the year of a date field, to speed up
some queries. Table structure is as follows

------------------------------------------------------------------------
----
CREATE SEQUENCE "sm_item_id_seq" start 1 increment 1 maxvalue
9223372036854775807 minvalue 1 cache 1;
CREATE TABLE "sm_item" (
"item_id" int4 DEFAULT nextval('"sm_item_id_seq"'::text) NOT
NULL,
"item_created_date_start" date,
CONSTRAINT "sm_item_pkey" PRIMARY KEY ("item_id")
) WITHOUT OIDS;
------------------------------------------------------------------------
----

And I have tried the following to create the actual index

------------------------------------------------------------------------
----
create index sm_item_cdates_idx ON sm_item (extract(year from
item_created_date_start));
------------------------------------------------------------------------
----

The response I get from psql is

------------------------------------------------------------------------
----
sm_live=# create index sm_item_cdates_idx ON sm_item (extract(year from
item_created_date_start));
ERROR: parser: parse error at or near "(" at character 52
------------------------------------------------------------------------
----

This relates to the brackets surrounding the "year from
item_created_date_start" bit.

Am I doing anything blatantly wrong? Can I actually use the extract
function for an index? Would I still get a speed improvement if I were
to just index the whole of the field, rather than just the year?

An example query that I have been running is:

------------------------------------------------------------------------
----SELECT item_id, item_created_date_start FROM sm_item WHERE
extract(year FROM item_created_date_start) = 1685;
------------------------------------------------------------------------
----

Which of course has been using a seq scan, as there is absolutely no
index on this column as yet.

Kind Regards,

Nick Barr
WebBased Ltd.

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