Senior LevelPostgreSQL

How do Custom Planner Hooks (planner_hook) intercept and rewrite query execution trees?

Q: How do Custom Planner Hooks (planner_hook) intercept and rewrite query execution trees?

PostgreSQL exposes C function pointers like **'planner_hook'** that allow extensions to replace or augment the standard Query Planner: ### Extension Capabilities: Extensions intercept incoming query tree parse trees, injecting custom optimization heuristics, cost evaluations, or rewriting execution paths (used by extensions like 'pg_hint_plan'!). Key Takeaway: Planner hooks allow C extensions to inject custom optimizer heuristics and query plan rewriting.

❓ Interview Prompt: Explain `planner_hook` C function pointer, intercepting query planning, query tree mutators, and custom optimizer extensions.

PostgreSQL exposes C function pointers like 'planner_hook' that allow extensions to replace or augment the standard Query Planner:

Extension Capabilities:

Extensions intercept incoming query tree parse trees, injecting custom optimization heuristics, cost evaluations, or rewriting execution paths (used by extensions like 'pg_hint_plan'!).

Code Implementation Example

// C Planner Hook Interception
static planner_hook_type prev_planner_hook = NULL;

PlannedStmt *custom_planner(Query *parse, const char *query_string, int cursorOptions, ParamListInfo boundParams) {
    // Intercept and inject custom query planning logic!
    if (prev_planner_hook)
        return prev_planner_hook(parse, query_string, cursorOptions, boundParams);
    return standard_planner(parse, query_string, cursorOptions, boundParams);
}

💡Intercepting query tree planning using C planner_hook.

🎯 Interview Key Takeaway

Planner hooks allow C extensions to inject custom optimizer heuristics and query plan rewriting.

#planner_hook#Query Planner#C Extensions#Senior

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