db: add completed env_build_assignments tip-lookup index and harden its statistics

packages/db/migrations/20260727032500_env_build_assignments_tip_lookup_index.sql

@@ -0,0 +1,44 @@
+-- +goose Up
+-- +goose NO TRANSACTION
+
+-- CREATE INDEX CONCURRENTLY and ANALYZE cannot run inside a transaction;
+-- the migrator's session default (3h, scripts/migrator.go) bounds them.
+
+-- The current-tip lookup orders by (created_at DESC, build_id DESC) under an
+-- (env_id, tag) equality. That exact shape belongs to an internal
+-- first-party consumer of this table whose queries are not in this
+-- repository's tree (searching only this repo won't find it); the
+-- latest-build laterals here order by created_at alone and ride the same
+-- prefix unchanged. The existing index stops at created_at: the planner
+-- still needs a top-N sort, so its plan choice rides on per-column
+-- estimates that every autoanalyze re-samples. Completing the index to the
+-- full sort key turns the lookup into a descend-and-stop ordered index scan
+-- (LIMIT 1, no sort), which stays the cheapest plan under any statistics
+-- roll; newer-assignment probes ride the same prefix.
+-- This migration is deliberately ADDITIVE: the superseded 3-column index
+-- (idx_env_build_assignments_env_tag_created, identical ordering prefix) is
+-- dropped by a separate follow-up migration once the new index has soaked.
+-- A failed CONCURRENTLY build leaves an INVALID index that a plain retry
+-- would trip over; drop it first.
+DROP INDEX CONCURRENTLY IF EXISTS idx_env_build_assignments_env_tag_created_build;
+CREATE INDEX CONCURRENTLY idx_env_build_assignments_env_tag_created_build
+    ON public.env_build_assignments (env_id, tag, created_at DESC, build_id DESC);
+
+-- Stabilize the estimate inputs themselves: larger per-column samples cut the
+-- roll-to-roll variance of n_distinct/MCV on the lookup's key columns (same
+-- treatment as env_builds.status_group).
+ALTER TABLE public.env_build_assignments ALTER COLUMN env_id SET STATISTICS 2000;
+ALTER TABLE public.env_build_assignments ALTER COLUMN tag SET STATISTICS 2000;
+
+-- Rebuild stats immediately rather than waiting for the next autoanalyze.
+ANALYZE public.env_build_assignments;
+
+-- +goose Down
+-- +goose NO TRANSACTION
+
+DROP INDEX CONCURRENTLY IF EXISTS idx_env_build_assignments_env_tag_created_build;
+
+ALTER TABLE public.env_build_assignments ALTER COLUMN env_id SET STATISTICS -1;
+ALTER TABLE public.env_build_assignments ALTER COLUMN tag SET STATISTICS -1;
+
+ANALYZE public.env_build_assignments;