selecting rows older than X, ensuring index is used

Hi,

I want to select records that haven't had an error
(logged to last_error) in the last 24 hours. My query
is:

select * from table1
where last_error is null
or extract(epoch from now()-last_error) > 86400;

I've created an index on last_error (timestamp with
time zone - can be NULL), then used EXPLAIN:

Seq Scan on table1  (cost=0.00..20.86 rows=217
width=72)
  Filter: ((last_error IS NULL) OR
(date_part('epoch'::text, (now() - last_error)) >
86400::double precision))

There are over 550 rows in table1, so it doesn't look
the index is being used. Is there a way to rewrite
this query so the index is used?

Thanks,
CSN


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