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My application has a Product and a Purchased table.

The Purchased table includes a full copy of the Product row at time of purchase, so that, even if a product name or description was changed, the purchase table maintains a historical record of the product details.

Purchased.name and Purchased.description fields are taking up too much space. On average, 1 of 30 strings in these fields are unique, and 29 of 30 are duplicates.

I would like to avoid creating a HistoricalProduct table, as many facets of Product change on a regular basis, and that table would grow quite large, too.

Is there a way to get PostgreSQL to manage deduplication of these variable character fields automatically for me?

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    There is no mechanism that will do this for you behind the scenes. However, you can normalize these data into two separate tables (purchased_name and purchased_description, for example) and have a foreign key in the original table referencing these two. – dezso Nov 11 '14 at 9:41
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    How long are those strings? Postgres compresses values automatically that exceed a certain length – a_horse_with_no_name Nov 11 '14 at 9:47
  • Name is 88 bits on average. Description is 350 bits. – Chris Betti Nov 11 '14 at 17:16
  • You mean bytes? – dezso Nov 11 '14 at 17:28
  • I meant bits. Name is 11 bytes, description is 44 bytes. – Chris Betti Nov 11 '14 at 17:54
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PostgreSQL has functions for regular expressions. PostgreSQL string functions
So you can write triggers (function + dml event) to be done what you need. I don't believe it has some optimizer that compress strings across the records. Or maybe I didn't understand what you mean in the question.

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