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MERGE 2 INTO target_table tgt 3 USING source_table src 4 ON ( src.object_id = tgt.object_id ) 5 WHEN MATCHED 6 THEN 7 UPDATE 8 SET tgt.object_type = src.object_type 9 , tgt.object_name = src.object_name 10 WHEN NOT MATCHED 11 THEN 12 INSERT ( tgt.object_id 13 , tgt.object_name 14 , tgt.object_type ) 15 VALUES ( src.object_id 16 , src.object_name 17 , src.object_type ); This simply means that a single target row might be updated multiple times, which could give rise to some data quality issues (for example, if compound arithmetic such as summing is included in the update of some columns).

MERGE 2 INTO target_table tgt 3 USING source_table src 4 ON ( src.object_id = tgt.object_id ) 5 WHEN MATCHED 6 THEN 7 UPDATE 8 SET tgt.object_name = src.object_name 9 , tgt.object_type = src.object_type 10 WHEN NOT MATCHED 11 THEN 12 INSERT ( tgt.object_id 13 , tgt.object_name 14 , tgt.object_type ) 15 VALUES ( src.object_id 16 , src.object_name 17 , src.object_type ); Note that sqlplus reports the number of rows merged. Oracle treats MERGE as a MERGE and not an UPDATE INSERT statement. As a rough sanity check, we can report the record count in the target table following the MERGE.We could speed up the latter by using bulk processing, but we wouldn't be able to achieve a reduction of two-thirds required to match the MERGE. This means that for each source row, Oracle needs to be able to identify a single target record for update.The simplest method of ensuring that the MERGE is key-preserved is to join source and target according to the primary key of the target.Without the outer join, Oracle would need to implement a "two-pass" solution such as we might code ourselves with a separate INSERT and UPDATE statement.Major performance gains will be achieved by tuning the source-target join (for example, using indexes, hints, partitioning etc) or by tuning the USING clause if it is a complex in-line view.