tidb limit执行计划疑惑

CREATE TABLE tb_model (
entity_id bigint(20) unsigned NOT NULL,
entity_type char(8) NOT NULL,
tag_id bigint(20) unsigned NOT NULL,
region char(4) NOT NULL,
status tinyint(3) unsigned NOT NULL DEFAULT ‘1’,
expiry_time bigint(20) unsigned NOT NULL DEFAULT ‘0’,
create_time bigint(20) unsigned NOT NULL,
update_time bigint(20) unsigned NOT NULL,
id bigint(20) NOT NULL,
PRIMARY KEY (id) /*T![clustered_index] CLUSTERED */,
KEY idx_tag_entity (tag_id,entity_id,entity_type)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;