> The caching appears to disappear overnight.

You've probably got cron jobs that run late at night and blow out your
kernel disk cache by accessing a whole lot of non-Postgres stuff.
(A nightly disk backup is one obvious candidate.)  The most likely
solution is to run some cron job a little later to exercise your
database and thereby repopulate the cache with Postgres files before
you get to work ;-)

                        regards, tom lane

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TIP 9: the planner will ignore your desire to choose an index scan if your
      joining column's datatypes do not match

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