Slow planning time for simple query

Hello -

We have an odd scenario on one of our OLTP systems, which behaves the same
way on a streamer, of a 700-1000ms planning time for a query like this:

SELECT *
FROM table1
WHERE source_id IN (SELECT id FROM table2 WHERE customer_id = $1);

The actual execution time is sub-ms.

We initially thought maybe catalog bloat? But we were able to reindex all
of the highly churned catalog tables, and I even did VACUUM FULL on
pg_attribute and pg_statistic, to no avail.

There are no custom settings for pg_attribute for the given tables either.

Interestingly, the problem goes away on a SAN snapshot of the target system.

Any ideas of what else we could try? A PL function that caches the query
plan works, but that is just a workaround.

Thanks!
Jeremy