dbms_comparison was introduced in 11g as a powerful tool to compare and synchronize table data in different tables.
Even though it’s quite flexible it has some limitations:
1. The tables are supposed to be in different databases. Therefore if you want to compare tables in the same database you will need to create fake db link.
2. The tables must have unique indexes.

Another consequence from the flexibility is that synchronizing is being applied in two steps (merge & delete) even though it could have been done using single merge statement.
Moreover those statements are based not only on tables that are synchronized but also on auxiliary data populated after comparison.

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