Can a trigger catch updates done by a select for update + lob.write()?
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You can't do that , at least up to 12c : "Using OCI functions or the DBMS_LOB package to update LOB values or LOB attributes of object columns does not fire triggers defined on the table that contains the columns or attributes." (docs.oracle.com/database/121/ADDCI/lobs.htm#ADDCI4383 )