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points by senderista 3 days ago | 0 comments

The time-split B-tree is designed to make time-travel queries efficient in both time and space. That is not a trivial problem.

I don’t know if Flashback uses this technique, but the SQL Server architect who taught me time-split B-trees now works at Oracle :)

[–] mike_hearn 2 days ago

Hah, interesting. I need to investigate that data structure. I don't know if Flashback uses it either - if you enable the time travel feature then undo logs are processed into internal tables partitioned by SCN, but I suppose it's possible those tables are stored in a different way to regular heap-organized or index-organized tables.

[–] senderista 1 day ago

The nodes in a time-split B-tree tile the 2-D time/primary-key space in an arbitrary way except they must cover it fully with no overlaps (think of each node as an arbitrary rectangle with primary key as one axis and time as the other). That gives you a lot of freedom in how you partition the space, allowing you to exploit local sparsity in either the time distribution or the primary key distribution.

[–] mike_hearn 23 hours ago

Thanks, that's interesting!