alt Hacker NewsI don't understand the argument why `AsSplitQuery` could be more performant than a single round trip involving a multi join query. People mention data duplication and increased memory usage, but I would assume that `duplication` is just a matter of an extra pointer, not a bit-for-bit duplication of every reference to a single row.
Please enlighten me.
Replies
azibi • today at 8:28 AM
There is a name for this problem, Cartesian explosion: https://en.wikipedia.org/wiki/Cartesian_explosion
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fuzzy2 • today at 7:47 AM
If you join multiple/many tables, you could end up with a large volume of data. And yes, this is bit-for-bit duplication—on the network. The query result is (typically) a single table. This table will get serialized as-is, with all duplicate data.
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Assume you fetch a single customer entity with their 100 order entities as includes. With single query this will join both tables and produce 100 rows that contain the order data but also each one contains the customer data redundantly. Now Imagine you had two includes there, that will multiply the number of rows again.
AsSingleQuery is as dangerous as this makes it sound. This works surprisingly well if you know that the number of included entities is low, but only then.
You can get much better queries here if you write a Select() and let EF Core translate that into SQL. That will probably do roughly want you are imagining here, usually with subqueries fetching the data from related entities.