Optimizations have been made for UPDATE requests that allow the affected join index rows to be located using direct access. For example, if an UPDATE request specifies a search condition on the primary or secondary of a join index, the affected join index rows are not reproduced. Instead, the join index may be directly searched for the qualifying rows and modified accordingly.
Preconditions for Update Optimization
To use this optimized method (that is, the direct update approach), the following conditions must be present:
- A primary or secondary access path to the join index.
- If join_index_column_2 is defined, no modifications to join_index_column_1 columns.
- No modifications to the join condition columns appearing in the join index definition.
- No modifications to the primary index columns of the join index.
Example: Optimal Method for Maintaining a Join Index during an UPDATE Request
The following is an example of an optimized method for maintaining join index during an UPDATE request:
EXPLAIN UPDATE lineitem SET l_quantity=l_quantity - 5 WHERE l_orderkey=10;
*** Help information returned. 11 rows.
*** Total elapsed time was 1 second.
Explanation
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1) First, we execute the following steps in parallel.
1) We do a single-AMP UPDATE from join index table
df2.OrderJoinLine by way of the primary index
"df2.OrderJoinLine.l_orderkey = 10" with a residual
condition of ("df2.OrderJoinLine.l_orderkey = 10").
2) We do a single-AMP UPDATE from df2.lineitem by way of the
primary index "df2.lineitem.l_orderkey = 10" with no
residual conditions.
2) Finally, we send out an END TRANSACTION step to all AMPs involved
in processing the request.
-> No rows are returned to the user as the result of statement 1.