This may be irrational but it's something that worries me about using postgres in production. Sure as a developer I love all the features, but the fact that the query planner can suddenly decide on a radically different (and incredibly inefficient) query plan makes it hard to trust. In some ways, a dumber query planner that needs coercing into the right query plan is more reassuring, in that you know it'll keep using that query plan unless explicitly told otherwise.
But that dumber query planner will bite you when your data changes. If the relative size of multiple tables change the query might have to change to be still efficient. Postgres query planner handles that just fine. I've used Postgres for years multi TB databases and I've experienced a problem with Postgres suddenly changing plans.
Is there a “never” missing from your last sentence?
Yes :( And it is too late for editing it.
haha, this example is actually from Mongo, but it's pretty rare.