As I understand it, the current patch performs immediate IVM using AFTER STATEMENT trigger transition tables.
However, multiple tables can be modified *before* AFTER STATEMENT triggers are fired.
CREATE TABLE example1 (a int);
CREATE TABLE example2 (a int);
CREATE INCREMENTAL MATERIALIZED VIEW mv AS
SELECT example1.a, example2.a
FROM example1 JOIN example2 ON a;
WITH
insert1 AS (INSERT INTO example1 VALUES (1)),
insert2 AS (INSERT INTO example2 VALUES (1))
SELECT NULL;
Changes to example1 are visible in an AFTER STATEMENT trigger on example2, and vice versa. Would this not result in the (1, 1) tuple being "double-counted"?
IVM needs to either:
(1) Evaluate deltas "serially' (e.g. EACH ROW triggers)
(2) Have simultaneous access to multiple deltas:
delta_mv = example1 x delta_example2 + example2 x delta_example1 - delta_example1 x delta_example2
This latter method is the "logged" approach that has been discussed for deferred evaluation.
tl;dr It seems that AFTER STATEMENT triggers required a deferred-like implementation anyway.