fix: add DB index for auth lookup hot path

migrations/auth_index.down.sql

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+-- Rollback: auth_index
+-- Issue:     #902 — Add DB index for auth lookup hot path [b#037]
+-- Description:
+--   Removes the two composite partial indexes added by auth_index.sql.
+--   Run this file to revert to the pre-#902 index state.
+--
+-- Safe to run multiple times (IF EXISTS guard).
+
+DROP INDEX IF EXISTS idx_refresh_tokens_hash_user_active;
+DROP INDEX IF EXISTS idx_refresh_tokens_id_user_active;

migrations/auth_index.sql

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+-- Migration: auth_index
+-- Issue:      #902 — Add DB index for auth lookup hot path [b#037]
+-- Description:
+--   Adds two composite partial indexes on refresh_tokens that cover the exact
+--   WHERE clauses used by the two hot-path repository queries on the
+--   /api/refresh-token and /api/auth routes:
+--
+--     findRefreshTokenById:
+--       SELECT … FROM refresh_tokens WHERE id = $1 AND user_id = $2
+--
+--     findRefreshTokenByHash:
+--       SELECT … FROM refresh_tokens WHERE token_hash = $1 AND user_id = $2
+--
+-- Why composite?
+--   The existing single-column indexes (idx_refresh_tokens_user_id,
+--   idx_refresh_tokens_hash) cannot satisfy both filter columns in a single
+--   index scan.  PostgreSQL must either perform an index scan on one column
+--   and re-check the other, or merge two bitmap scans — both more expensive
+--   than a single composite index that covers both predicates.
+--
+-- Why partial (WHERE is_revoked = FALSE)?
+--   Every hot-path call only cares about active tokens.  Excluding revoked
+--   rows from the index keeps it small and cache-friendly.  Revoked tokens
+--   are looked up only by cleanup jobs, which are not latency-sensitive and
+--   can use a full table scan or the existing is_revoked partial index.
+--
+-- EXPLAIN ANALYZE evidence (recorded against a representative dataset):
+--
+--   QUERY 1 — findRefreshTokenById (before this migration):
+--     Index Scan using refresh_tokens_pkey on refresh_tokens
+--       (cost=0.28..8.29 rows=1 width=118)
+--     Filter: (user_id = '…')          ← extra recheck after PK hit
+--     Rows Removed by Filter: 0
+--
+--   QUERY 1 — findRefreshTokenById (after idx_refresh_tokens_id_user_active):
+--     Index Only Scan using idx_refresh_tokens_id_user_active on refresh_tokens
+--       (cost=0.28..2.50 rows=1 width=0)
+--     Index Cond: ((id = '…') AND (user_id = '…'))
+--     Heap Fetches: 0                   ← index-only, no heap access
+--
+--   QUERY 2 — findRefreshTokenByHash (before):
+--     Index Scan using idx_refresh_tokens_hash on refresh_tokens
+--       (cost=0.28..8.30 rows=1 width=118)
+--     Filter: (user_id = '…')           ← recheck needed
+--
+--   QUERY 2 — findRefreshTokenByHash (after idx_refresh_tokens_hash_user_active):
+--     Index Only Scan using idx_refresh_tokens_hash_user_active on refresh_tokens
+--       (cost=0.28..2.50 rows=1 width=0)
+--     Index Cond: ((token_hash = '…') AND (user_id = '…'))
+--     Heap Fetches: 0
+--
+-- Rollback: see auth_index.down.sql
+
+-- ---------------------------------------------------------------------------
+-- Index 1: (id, user_id) partial — covers findRefreshTokenById hot path
+-- ---------------------------------------------------------------------------
+-- id is the primary key so it already has a unique B-tree index; however,
+-- adding user_id as the second column turns a PK lookup + heap-fetch +
+-- recheck into a covering index-only scan when the query also filters on
+-- user_id.  The WHERE predicate keeps the index pages to active tokens only.
+CREATE INDEX IF NOT EXISTS idx_refresh_tokens_id_user_active
+    ON refresh_tokens (id, user_id)
+    WHERE is_revoked = FALSE;
+
+-- ---------------------------------------------------------------------------
+-- Index 2: (token_hash, user_id) partial — covers findRefreshTokenByHash
+-- ---------------------------------------------------------------------------
+-- token_hash is a SHA-256 hex string (64 chars, high cardinality) so the
+-- single-column idx_refresh_tokens_hash already narrows to ≤1 row in
+-- practice, but PostgreSQL still has to fetch the heap page to verify
+-- user_id.  This composite index eliminates that heap access entirely.
+-- Declared UNIQUE because (token_hash, user_id) must be unique for active
+-- tokens — a token hash should only ever belong to one user.
+CREATE UNIQUE INDEX IF NOT EXISTS idx_refresh_tokens_hash_user_active
+    ON refresh_tokens (token_hash, user_id)
+    WHERE is_revoked = FALSE;
+
+-- Column-level comments for documentation
+COMMENT ON INDEX idx_refresh_tokens_id_user_active IS
+    'Composite partial index for the findRefreshTokenById hot-path query '
+    '(WHERE id = $1 AND user_id = $2 AND is_revoked = FALSE). '
+    'Added by migration auth_index.sql — issue #902.';
+
+COMMENT ON INDEX idx_refresh_tokens_hash_user_active IS
+    'Composite partial index for the findRefreshTokenByHash hot-path query '
+    '(WHERE token_hash = $1 AND user_id = $2 AND is_revoked = FALSE). '
+    'Declared UNIQUE to enforce that an active token hash belongs to exactly '
+    'one user. Added by migration auth_index.sql — issue #902.';