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A comment on Comparison of Joins: MongoDB vs. PostgreSQL. Parent comment

Re:Comparison of Joins: MongoDB vs. PostgreSQL by d_watt on Friday April 10, 2020 @01:00PM (#22834927)
Is this actually an objective truth? "relational" just refers to the relation within the table, it's less about having a database system with good inter-table relationship handling. It happens to be that a lot of the things we think about as relational are OLTP and have good fkeys, etc.

Is there any real reason why a document store can't have good joins between collections? If you index the lookup keys, I don't understand what about the underlying document model would make joining impossible to optimize as well as a relational model.

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Re:Comparison of Joins: MongoDB vs. PostgreSQL by alexpetralia on Friday April 10, 2020 @01:08PM (#22834991)
I don't think most document stores enforce foreign key relationships (and hence consistency) like RDBMS do, so in practice that would make joins difficult.
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Re:Comparison of Joins: MongoDB vs. PostgreSQL by d_watt on Friday April 10, 2020 @01:53PM (#22835472)
Is joining on an fkey any more performant than without it assuming the same indexing is applied?

When you say difficult do you mean from a developer ergonomics standpoint (knowing there's data integrity), or from a database optimizability standpoint?

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Re:Comparison of Joins: MongoDB vs. PostgreSQL by alexpetralia on Friday April 10, 2020 @03:18PM (#22836409)
The former. You have a guarantee of consistency, whereas otherwise you must adjust for this in your application code. Database performance wise I agree they are equivalent.
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