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ISO 8601:1988 means that if the week containing January 1 has four or more days in the new year, then it is week 1; otherwise it is the last week of the previous year, and the next week is week 1. The ISO-8601 week starts on Monday.
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SELECT EXTRACT(WEEK FROM TIMESTAMP '2010-01-25 12:44:06');
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SELECT EXTRACT(YEAR FROM TIMESTAMP '2009-02-16 20:38:40');
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SELECT date_part('day', TIMESTAMP '2010-07-16 10:12:05');
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SELECT date_part('hour', INTERVAL '5 hours 12 minutes');