A blockchain table is a tamper-proof, insert-only table with an associated table-level and row-level retention period. Rows are organised into chains, with each row containing a hash of the data contained in the row, and the hash of the previous rows data.

This feature has been backported to Oracle 19.10, but also requires patch 32431413 and the COMPATIBLE parameter set to 19.10.0 or later. From 19.11 the additional patch is not required. There have been significant changes introduced to blockchain tables in Oracle 23ai, which you can read about

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