如何阅读PostgreSQL EXPLAIN,顺序:自上而下或自下而上？

Question

这是explain.depesz.com上的示例计划:

Limit  (cost=65301.950..65301.950 rows=1 width=219) (actual time=886.074..886.074 rows=0 loops=1)
  ->  Sort  (cost=65258.840..65301.950 rows=17243 width=219) (actual time=879.683..885.211 rows=17589 loops=1)
          Sort Key: juliet.romeo
          Sort Method: external merge  Disk: 4664kB
        ->  Hash Join  (cost=30177.210..62214.980 rows=17243 width=219) (actual time=278.986..852.834 rows=17589 loops=1)
                Hash Cond: (whiskey_quebec.whiskey_five = juliet.quebec)
              ->  Bitmap Heap Scan on whiskey_quebec  (cost=326.060..21967.630 rows=17243 width=4) (actual time=7.494..65.956 rows=17589 loops=1)
                      Recheck Cond: (golf = 297)
                    ->  Bitmap Index Scan on kilo  (cost=0.000..321.750 rows=17243 width=0) (actual time=4.638..4.638 rows=17589 loops=1)
                            Index Cond: (golf = 297)
              ->  Hash  (cost=15750.510..15750.510 rows=329651 width=219) (actual time=267.388..267.388 rows=329651 loops=1)
                      Buckets: 1024  Batches: 128  Memory Usage: 679kB
                    ->  Seq Scan on juliet  (cost=0.000..15750.510 rows=329651 width=219) (actual time=0.003..65.704 rows=329651 loops=1)

这些操作的执行顺序是什么？自下而上？自顶向下？

Answer 1

两个方向都有道理.从上到下请求数据.从下到上的数据.你看到一棵树 - 每个节点都有一个孩子.在执行时,节点调用他的孩子 - "发送数据",这个调用被递归地重复到叶子 - seq扫描,索引扫描,设置返回的函数调用,......产生数据和逐行 - 是发送给父母的数据.