explain(analyze) select name from t_vm where name = 'test';
QUERY PLAN
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Index Only Scan using idx_name_vm on t_vm (cost=0.29..4.30 rows=1 width=11) (actual time=0.025..0.215 rows=1000 loops=1)
Index Cond: (name = 'test'::text)
Heap Fetches: 1000
Planning Time: 0.072 ms
Execution Time: 0.285 ms

其中Heap Fetches的指标怎么理解？