Optionally record a plan_id in PlannedStmt to identify plan shape (v4) #15
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This can be useful either for jumbling expressions in other contexts (e.g. to calculate a plan jumble), or to allow extensions to use a modified jumbling logic more easily. This intentionally supports the use case where a separate jumbling logic does not care about recording constants, as the query jumble does.
When enabled via the new compute_plan_id GUC (default off), this utilizes the existing treewalk in setrefs.c after planning to calculate a hash (the "plan_id", or plan identifier) that can be used to identify which plan was chosen. The plan_id generally intends to be the same if a given EXPLAIN (without ANALYZE) output is the same. The plan_id includes both the top-level plan as well as all subplans. Execution statistics are excluded. If enabled, the plan_id is shown for currently running queries in pg_stat_activity, as well as recorded in EXPLAIN and auto_explain output. Other in core users or extensions can use this facility to show or accumulate statistics about the plans used by queries, to help identify plan regressions, or drive plan management decisions. Note that this commit intentionally does not include a facility to map a given plan_id to the EXPLAIN text output - it is a assumed that users can utilize the auto_explain extension to establish this mapping as needed, or extensions can record this via the existing planner hook.
This extension allows tracking per-plan call counts and execution time, as well as capturing the plan text, aka EXPLAIN (COSTS OFF), for the first execution of a given plan. This utilize the compute_plan_id functionality for tracking different plans.
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