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    <p>You could use an update trigger on <code>Product</code> but it sounds like the <code>Product</code> table is not actually being updated, it is being replaced; if this is the case then there is no update to trigger the trigger so you'll have to do it the hard way by caching the price in <code>Wishlist</code> (as user247245 notes) and scanning <code>Product</code> for price changes.</p> <p>If <code>Products</code> is being updated rather than replaced wholesale, then an update trigger could be used to note when a price changes and arrange to have the interested parties notified. The trigger would probably queue up the notifications by inserting them into a separate table (to avoid locking up the <code>Products</code> update) called, say <code>Product_price_changes</code>; then, when the <code>Products</code> update is done, a separate task can compare <code>Product_price_changes</code> to <code>Wishlist</code>, notify the interested users, and finish off by deleting everything in <code>Product_price_changes</code>.</p> <p>Note that recent versions of PostgreSQL you can restrict an update trigger to only fire when a column changes rather than firing on every row change.</p>
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    1. POMethod to track price drops for a product on a user's wishlist?
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    1. COIt seems like an update trigger would be the right way to go if the table wasn't being replaced wholesale, but it is. Unfortunately the `Products` table is huge and importing the data takes a really long time. Replacing the table (rather than UPSERT-ing rows) speeds it up significantly but I lose the ability to use an update trigger. I guess I'll have to evaluate which is more important: total import speed or the ability to have an update trigger. I might be able to mimic the same functionality with a ruby observer that creates `product_price_changes` after the import is finished I suppose.
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    2. COHow often do the values in `Product` really change? If the daily product data dump comes from an outside source you might be able to work out a diff/patch deal (which might even be a simple diff(1) on text files) system to trim it down to a small set of changes, then apply just those changes to the `Product` table. The trick is to trim down the work load or spread it out into small chunks to avoid locking up `Product` and getting in the online system's way. Hopefully the "big database" crew will show up tomorrow with some fresh ideas.
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    3. COWell, they might not change that often, but the problem is the sheer volume of records and data I'm working with... it's several million records total, so if even a smallish percentage of them are updated, it's still quite a lot of data. (I'm importing other tables as well, this is just one piece in a somewhat large data set.) You're right though, I may need to rethink replacing the tables entirely on import...
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