PostgreSQL and Sequential Data

Question

I have a dataset that contains:

Table { date itemName }

The date for the most part is sequential. There are no duplicates of the date [as it is the primary key].

The question is split up into multiple parts (all with respect to using SQL):

1. Is it possible to find gaps in the date series listed in the table? For example: Dates 1/2/09-1/3/09 are missing
2. Is it possible to find sections of dates that are missing from the table, that has a range greater than n (this is a number determined at run time)? For example: For n = 2 Dates 1/2/09-1/3/09 are not returned but Dates 5/6/09-6/1/09 are.

Answer 1

If you can use PostgreSQL 8.4 then window functions will help:

SELECT *
    FROM (SELECT itemName, date, date - lag(date) OVER w AS gap
              FROM someTable WINDOW w AS (ORDER BY date)
         ) AS pairs
    WHERE pairs.gap > '1 day'::interval;