Re: FK triggers misused?

I have investigated a bit now and found the following:

When I perform the update the *first* time, the triggers are actually
not evaluated. But from the second update they are.

Also notice that the number of rows changes. Shouldn't that number of
rows always be 2 as question_id is primary key?

Example:

=> explain analyze update questions set cancelled_time = now() where
question_id in (10,11);
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on questions (cost=4.01..12.04 rows=2 width=112)
(actual time=0.193..0.205 rows=2 loops=1)
Recheck Cond: ((question_id = 10) OR (question_id = 11))
-> BitmapOr (cost=4.01..4.01 rows=2 width=0) (actual
time=0.046..0.046 rows=0 loops=1)
-> Bitmap Index Scan on questions_pkey (cost=0.00..2.00
rows=1 width=0) (actual time=0.037..0.037 rows=1 loops=1)
Index Cond: (question_id = 10)
-> Bitmap Index Scan on questions_pkey (cost=0.00..2.00
rows=1 width=0) (actual time=0.005..0.005 rows=1 loops=1)
Index Cond: (question_id = 11)
Trigger for constraint questions_repost_of_fkey: time=0.023 calls=2
Total runtime: 0.734 ms
(9 rows)

=> explain analyze update questions set cancelled_time = now() where
question_id in (10,11);
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on questions (cost=4.01..12.04 rows=2 width=112)
(actual time=0.085..0.097 rows=2 loops=1)
Recheck Cond: ((question_id = 10) OR (question_id = 11))
-> BitmapOr (cost=4.01..4.01 rows=2 width=0) (actual
time=0.047..0.047 rows=0 loops=1)
-> Bitmap Index Scan on questions_pkey (cost=0.00..2.00
rows=1 width=0) (actual time=0.036..0.036 rows=2 loops=1)
Index Cond: (question_id = 10)
-> Bitmap Index Scan on questions_pkey (cost=0.00..2.00
rows=1 width=0) (actual time=0.007..0.007 rows=2 loops=1)
Index Cond: (question_id = 11)
Trigger for constraint questions_repost_of_fkey: time=0.025 calls=2
Trigger for constraint questions_author_id_fkey: time=0.167 calls=2
Trigger for constraint questions_category_id_fkey: time=0.196 calls=2
Trigger for constraint questions_lock_user_id_fkey: time=0.116 calls=2
Total runtime: 1.023 ms
(12 rows)