CASSANDRA-21178 add created_at column to system_distributed.compression_dictionaries

[smiklosovic]

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[smiklosovic]

I removed this here because when I was testing import / export with created_at, I realized that we can not import dictionary bigger than 1MiB BUT WE CAN TRAIN IT.

So we train > 1MiB but we can not import after export.

It is possible to override the configuration via nodetool or cql, there we do not check max size, we check that only on import ...

I can revert this change and treat is more robustly in a completely different ticket, hardening sizes on all levels (cql, nodetool ...), can go in even after 6.0-alpha1. If I remove it here, we will at least not see the discrepancy I described above.

[yifan-c]

Thanks for the background! It helps to understand.

Dictionaries are attached to every SSTable. The size limit is added with this context. The size of the dictionaries are typically 64~100 KiB. That said, the underlying zstd trainer do allow train large dictionaries. The questions, do we want to train dictionaries larger than 1 MiB? The added dictionary size might outweighs the compression gains (64 KiB vs. 1 MiB dictionaries)