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about foreign key

I have impression that foreign key definition helps Vertica optimizer. In the scenario that I have a slowly changing dimension and fact table has the dimension key without version, create table product_dim ( product_key int, version int, date_from date, date_to date, price decimal(6,2), primary key (product_key, version) ) create table sales_fact ( date_key int, product_key int, sales_amount decimal(8,2) ) alter table sales_fact add constraint fk_product foreign key (product_key) references product_dim (product_key); If I define foreign key like above, I got error "ROLLBACK 4550: Referenced primary key constraint does not exist" since version is part of primary key but not foreign key. How can I address this or just forget about foreign key? How much benefit foreign key can help with the query performance? Thanks

Comments

  • Hi Mike, The performance advantage of defining the PK/FK relationship can be quite substantial or it can be quite small; it depends on your data and on your queries, and on whether Vertica gets things like the join order correct without the added information. It looks like your relationship is not a PK/FK relationship. The primary key is not "product_key", it is "(product_key, version)". The reason for the performance advantage is because, with a PK/FK relationship, Vertica is told that the primary key is both unique and the join key from that table. If both of these facts are true against the product_key field alone, then you should set that field as the primary key. If they are not both always true against just the product_key field, then Vertica can't make that assumption; you are asking for something that is fundamentally more complicated (a many-to-many join), so it may not perform as well. If there are relatively few distinct values of the product_key field, but the field is not necessarily unique, then you should instead analyze statistics on the two tables. PK/FK is a simple hard constraint, perfectly unique; statistics tell Vertica what to do if that's not quite the case but the data still has interesting relevant properties. Adam

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