fix: added missing index (#3950)

Signed-off-by: Uroš Marolt <uros@marolt.me>

Author: themarolt

backend/src/database/migrations/U1774363114__add-btree-index-for-member-identities-email-lookup.sql

@@ -0,0 +1 @@
+DROP INDEX CONCURRENTLY IF EXISTS "idx_memberIdentities_verified_email_lower_value";

backend/src/database/migrations/V1774363114__add-btree-index-for-member-identities-email-lookup.sql

@@ -0,0 +1,20 @@
+-- The existing idx_memberIdentities_email_verified_trgm covers the right partial
+-- conditions (verified = true, type = 'email', deletedAt IS NULL) but is a GIN
+-- trigram index -- PostgreSQL will not use it for equality joins on
+-- lower(mi.value) against input emails, for example:
+--
+--   JOIN ON lower(mi.value) = input_email
+--   WHERE mi.verified = true AND mi.type = 'email' AND mi."deletedAt" IS NULL
+--
+-- The only usable B-tree index (idx_memberIdentities_lower_value) only carries
+-- the partial condition deletedAt IS NULL, so PG must heap-fetch every matched
+-- row to re-check verified and type, which degrades as the table grows.
+--
+-- This B-tree partial index lets the planner do a direct nested-loop index scan
+-- (one lookup per input email) with no extra heap fetches for verified/type.
+
+CREATE INDEX CONCURRENTLY IF NOT EXISTS "idx_memberIdentities_verified_email_lower_value"
+    ON "memberIdentities" (lower(value))
+    WHERE verified = true
+      AND type = 'email'
+      AND "deletedAt" IS NULL;