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All Your GUCs in a Row: The geqo Family
PostgreSQL's Genetic Query Optimizer (GEQO) has seven configuration parameters (GUCs), but only one truly matters for day-to-day operations: geqo_threshold. GEQO kicks in when a query involves too many relations for the regular dynamic programming planner to handle exhaustively. The threshold defaults to 12 — calibrated for 1990s hardware — and raising it into the mid-to-high teens is usually safe on modern machines. The other six parameters (geqo, geqo_effort, geqo_pool_size, geqo_generations, geqo_selection_bias, geqo_seed) are either a master switch to leave on, or hyperparameters that are practically untunable without a proper benchmark workload. GEQO is widely considered de facto deprecated within the PostgreSQL project, with a 2021 pgsql-hackers thread explicitly calling it that, though no replacement has landed yet. Key practical advice: leave geqo on as a safety backstop, raise geqo_threshold carefully while monitoring planning time, keep it above collapse limits, and treat queries hitting GEQO as a signal to investigate the query structure rather than tune the algorithm.
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