Is it possible to map MERGE onto a query with CTEs that does the the
various DMLs, with all but the last RETURNING? Here's a sketch:
WITH matched_rows AS ( SELECT FROM <target> t WHERE <condition> ), updated_rows AS ( UPDATE <target>
t SET ... WHERE ... AND t in (SELECT j FROM matched_rows j) RETURNING t ), inserted_rows AS (
INSERT INTO <target> t SELECT ... WHERE ... AND t NOT IN (SELECT j FROM matched_rows j) RETURNING t
),
DELETE FROM <target> t
WHERE ...;
Now, one issue is that in PG CTEs are basically like temp tables, and
also like optimizer barriers, so this construction is not online, and if
matched_rows is very large, that would be a problem.
As an aside, I'd like to be able to control which CTEs are view-like and
which are table-like. In SQLite3, for example, they are all view-like,
and the optimizer will act accordingly, whereas in PG they are all
table-like, and thus optimizer barriers.
Nico
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