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Displaying posts with tag: Great-Circle Formula (reset)
Finding things within some distance in SQL

One of the query optimization scenarios I’ve seen a lot over the years is finding something within some distance from a point. For example, finding people within some distance of yourself, apartments in a radius from a postal code, and so on.

These queries usually use the great-circle formula. That might be because Google finds lots of pages claiming that this is the right way to do a radius search. “The earth is not flat!”, they all say. That’s true, but it doesn’t mean that the great-circle formula is a good approach. It’s usually a really bad approach, in fact. It’s needlessly precise for most things, not precise enough for others, and it’s an expensive query to execute; all the trig functions tend to eat a bunch of CPU, and make it impossible to use ordinary indexes. This is true for all of the databases I’ve used — MySQL, Postgres, and SQL Server.

The great-circle formula is needlessly precise for a few …

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