How to merge 2 tables in Oracle
You can fully achieve this with MERGE:

MERGE INTO big_table B
USING (SELECT key_col, col1, col2 FROM small_table) S
ON (B.key_col = S.key_col)
WHEN MATCHED THEN UPDATE SET B.col1=S.col1, B.col2=S.col2
WHEN NOT MATCHED THEN INSERT (B.col1, B.col2)
VALUES (S.col2, S.col2)
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