Generic vs custom plans for prepared statements
Simple terms
A prepared statement can be planned fresh for every set of arguments, or planned once and reused forever. Postgres does both. It plans the first five executions individually, then compares, and if one reusable plan doesn't look more expensive on average, it switches to that and stops re-planning for good. Usually a fine trade. But now a single plan has to serve every value you throw at it, and on skewed data the plan that suits the common case can be badly wrong for the rare one.
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Explain custom vs generic plans for a prepared statement: when does PostgreSQL switch, how is the decision made, how do you observe which plan was used, and what is the failure mode on skewed data?
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