Everything from 1.4.1 through 1.4.9 happened inside a single function, standard_planner(). Building paths, costing them, searching for a join order, estimating cardinality from statistics: all of it runs inside that one function. Yet PostgreSQL does not call standard_planner() directly. It puts another function, planner(), one step ahead of it, and has planner() call standard_planner(). And planner() can be made to call some other function instead of standard_planner().
That replacement is what the planner hook enables. When pg_stat_statements measures per-query planning time, or pg_hint_plan rewrites a plan according to hints, it all goes through this hook. Let's look at how PostgreSQL provides a way to observe or change planning behavior without touching a single line of the core, and how external code plugs into it.
All planner() does is check the hook
The body of planner() is essentially this.
if (planner_hook)
