add index to media_files to aid lookups (#1727)

_database/sql/migrations/2026-07-03-074740-0000_alter_media_files_add_user_created_at_index/down.sql

@@ -0,0 +1,7 @@
+-- noinspection SqlDialectInspectionForFile
+-- noinspection SqlNoDataSourceInspectionForFile
+-- noinspection SqlResolveForFile
+
+ALTER TABLE media_files
+  DROP INDEX idx_creator_created_at,
+  ALGORITHM=INPLACE, LOCK=NONE;

_database/sql/migrations/2026-07-03-074740-0000_alter_media_files_add_user_created_at_index/up.sql

@@ -0,0 +1,17 @@
+-- noinspection SqlDialectInspectionForFile
+-- noinspection SqlNoDataSourceInspectionForFile
+-- noinspection SqlResolveForFile
+
+-- Composite index for the library / user profile list queries:
+--   WHERE maybe_creator_user_token = ? ... ORDER BY created_at DESC LIMIT n
+--
+-- Without this, the optimizer picks an index-merge intersection against huge
+-- low-selectivity indexes (e.g. index_user_deleted_at, ~4.75M NULL entries),
+-- scanning millions of index records per page load.
+--
+-- NB: ADD INDEX is online DDL (ALGORITHM=INPLACE, LOCK=NONE) — reads and writes
+-- continue during the build. Expect a few minutes of elevated I/O on a table
+-- this size (~8.5M rows).
+ALTER TABLE media_files
+  ADD INDEX idx_creator_created_at (maybe_creator_user_token, created_at),
+  ALGORITHM=INPLACE, LOCK=NONE;

_database/sql/migrations_squashed/media_files.sql

@@ -364,6 +364,7 @@ CREATE TABLE media_files (
   KEY index_user_deleted_at (user_deleted_at),
   KEY index_mod_deleted_at (mod_deleted_at),
   KEY index_media_files_created_at (created_at),
-  KEY index_media_files_updated_at (updated_at)
+  KEY index_media_files_updated_at (updated_at),
+  KEY idx_creator_created_at (maybe_creator_user_token, created_at)
 
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;