perf(db): scope the workspace covering index's claim to created_at windows

Author: waleedlatif1

packages/db/migrations/0446_usage_log_workspace_created_at_cost_idx.sql

@@ -1,14 +1,16 @@
 -- Replaces usage_log_workspace_created_at_idx with a covering index under the same two leading
 -- columns.
 --
--- Every Workspaces drill-down read filters on the billing entity, the workspace and a created_at
--- window, and reads cost with user_id, source or workflow_id. The old index held only
+-- A Workspaces drill-down over a created_at window (a range, or a reporting period) filters on the
+-- billing entity, the workspace and that window, and reads cost with user_id, source or
+-- workflow_id. The old index held only
 -- (workspace_id, created_at), so a drill-down matched the window there or on
 -- usage_log_billing_entity_created_at_cost_idx and fetched every matched row from the heap to
 -- check the rest: the workspace's rows on one, the whole organization's on the other. Dropping
 -- usage_log_billing_entity_period_idx in 0441 took away the bitmap that used to narrow the second.
 -- With the entity and every column those reads return in the key, each one is an index-only scan
--- bounded by the workspace's rows in the window.
+-- bounded by the workspace's rows in the window. A drill-down over a Stripe period matches the
+-- period stamps instead, which this index does not carry, so it keeps the plans it had.
 --
 -- The new key extends the old one, so it serves every read the old index served in the same
 -- order: the workspace_id foreign key's ON DELETE SET NULL, and the ledger list filtered to a

packages/db/schema.ts

@@ -6116,10 +6116,11 @@ export const usageLog = pgTable(
       )`
     ),
     /**
-     * Serves the Workspaces drill-down index-only: every analytics read narrowed to
-     * one workspace (entity + `workspace_id` + a `created_at` window) reads its
-     * totals, series, and member, source and workflow rankings from here, and skips
-     * the workspace's rows billed to another entity without a heap fetch.
+     * Serves the Workspaces drill-down index-only: an analytics read narrowed to one
+     * workspace over a `created_at` window (entity + `workspace_id` + the window)
+     * reads its totals, series, and member, source and workflow rankings from here,
+     * and skips the workspace's rows billed to another entity without a heap fetch.
+     * A Stripe-period drill-down matches the period stamps, which are not here.
      *
      * `workspace_id` and `created_at` lead so the scan is bounded by the
      * workspace's rows in the window rather than its organization's, and so the