It might be helpful to point out that the "suppress_redundant_updates"
trigger will cause trouble with the normal approach to doing an UPSERT, for
example that specified by the "merge_db" function in example 38-2 here:
If you happen to submit a redundant update against a table with the
suppress_redundant_updates trigger, using the merge_db function, you'll end
up in an infinite loop.
It'll similarly break any code where a result of "UPDATE 0" is assumed to
indicate that the record does not exist.
1. Add to http://www.postgresql.org/docs/8.4/static/functions-trigger.htmlthe
following copy, or something nicer :)
Note that the suppress_redundant_updates trigger invalidates any logic that
assumes that the number of rows affected by an UPDATE indicates the
existence or non-existence of the associated rows. Specifically, in the
case of the merge_db<http://www.postgresql.org/docs/current/static/plpgsql-control-structures.html#PLPGSQL-UPSERT-EXAMPLE>function,
any redundant updates performed by this function will cause an
2. Add to
Warning: Using this function or similar logic is incompatible with the
since it is assumed that if zero rows are updated, the row does not exist.
Alternatively, the example merge_db function could be reworked to be
compatible with the trigger by attempting the insert first, for example:
CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
-- first try to insert the row
INSERT INTO db(a,b) VALUES (key, data);
EXCEPTION WHEN unique_violation THEN
-- key already existed, try updating the row
UPDATE db SET b = data WHERE a = key;
The above is not as robust as the original, since it doesn't retry, and it's
a bit of a foot-gun if you don't actually have a unique constraint... Maybe
someone else can come up with a version that's strictly better than the old
one, and still plays nice with suppress_redundant_updates.
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