So the next question is, what pg version is the original poster using? because 8.1.x doesn't report trigger execution times, and 8.2.x would use a single bitmap index scan with an = ANY condition, not a BitmapOr.
I have tried 8.1.0 and 8.1.3 for this query.
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TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
