Exclude null values from image studio id index

pkg/sqlite/database.go

@@ -34,7 +34,7 @@ const (
 	cacheSizeEnv = "STASH_SQLITE_CACHE_SIZE"
 )
 
-var appSchemaVersion uint = 67
+var appSchemaVersion uint = 68
 
 //go:embed migrations/*.sql
 var migrationsBox embed.FS

pkg/sqlite/migrations/68_image_studio_index.up.sql

@@ -0,0 +1,7 @@
+-- with the existing index, if no images have a studio id, then the index is 
+-- not used when filtering by studio id. The assumption with this change is that
+-- most images don't have a studio id, so filtering by non-null studio id should
+-- be faster with this index. This is a tradeoff, as filtering by null studio id
+-- will be slower.
+DROP INDEX index_images_on_studio_id;
+CREATE INDEX `index_images_on_studio_id` on `images` (`studio_id`) WHERE `studio_id` IS NOT NULL;