On 16/07/10 12:26, Boxuan Zhai wrote:
For the EXPLAIN MERGE command, I expect it to return a result similar to
that of a SELECT command.
I think the EXPLAIN command is to show how the tables in a query is scaned
and joined. In my design, the merge command will generate a top-level query
(and plan) as the main query. It is in fact a left join select query over
the source and target tables. This main query (plan) decides how the tables
are scanned. The merge actions will not effect this process. So when we
explain the merge command, a similar result will be returned.
For example the command
EXPLAIN
MERGE INTO Stock USING Sale ON Stock.stock_id = Sale.sale_id
WHEN MATCHED THEN UPDATE SET balance = balance + sale.vol;
WHEN ....
.....
Will return a result just like that of the following command:
EXPLAIN
SELECT * FROM Sale LEFT JOIN Stock ON stock_id = sale_id;
You really need to look at the changes in 9.0 in this area, you now have
a Update/Delete/Insert node (implemented in
src/backend/executor/nodeModifyTable.c) at the top of the plan for
update/insert/delete commands:
postgres=# explain UPDATE foo SET id = 456 WHERE id = 123;
QUERY PLAN
-----------------------------------------------------------
Update (cost=0.00..40.00 rows=12 width=6)
-> Seq Scan on foo (cost=0.00..40.00 rows=12 width=6)
Filter: (id = 123)
(3 rows)
I would expect there to be a Merge node similar to that, with
Update/Insert/Delete subnodes for each action.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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