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If we explain a MERGE statement, we can see the mechanism Oracle uses to determine whether source and target rows match.

The following output is an Autotrace explain plan for our original MERGE statement from above.

In the following example, we'll remove the primary key from the source table and duplicate some source data.

We will revert to the key-preserved join and this time our MERGE should be successful.

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The update is written as an updateable in-line view which is often the fastest technique for bulk updating one table from another.

Execution Plan ---------------------------------------------------------- 0 MERGE STATEMENT Optimizer=CHOOSE (Cost=194 Card=86889 Bytes=9557790) 1 0 MERGE OF 'TARGET_TABLE' 2 1 VIEW 3 2 HASH JOIN (OUTER) (Cost=194 Card=86889 Bytes=7038009) 4 3 TABLE ACCESS (FULL) OF 'SOURCE_TABLE' (Cost=46 Card=86889 Bytes=2867337) 5 3 TABLE ACCESS (FULL) OF 'TARGET_TABLE' (Cost=24 Card=18950 Bytes=909600) We can see that Oracle performs an outer join between the source dataset and target table (in our case we happen to have a hash join).

This creates a non-mergeable view (this is an unfortunate coincidence in terminology) that is applied back to the target table.

This article introduces the new MERGE SQL command (sometimes referred to as "UPSERT").

MERGE is a DML command that enables us to optionally update or insert data into a target table, depending on whether matching records already exist.

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