[strlen]

Amazon Aurora uses Z-order curves. Here's a post about it, including a comparison of Aurora's Z-order curve implementation of spatial indexing with (stock MySQL 5.7) Rtrees: https://aws.amazon.com/blogs/database/amazon-aurora-under-th... (Disclaimer: I worked on this project.)

I believe Microsoft SQL server uses Hilbert curves: https://docs.microsoft.com/en-us/sql/relational-databases/sp...