>> For example, let's add another filter to get all the shipments with  
>> release code '5' that are 7 days old or newer.
>> >= current_date - 7

>       It's the order by + limit which makes the query behaves badly, and 
> which  
> forces use of kludges to use the index. If you add another condition like  
> that, it should be a breeze.

Actually, that date condition has its own problem, namely that the
compared-to value isn't a constant.  The 8.0 planner is able to realize
that this is a pretty selective condition, but prior releases fall back
on a very pessimistic default estimate.  I'm sure that has something to
do with Ken not being able to get it to use an index on date.

                        regards, tom lane

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TIP 8: explain analyze is your friend

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