sql - 如何读取PostgreSQL EXPLAIN, order : top down or bottom up?

Question

这是关于 explain.depesz.com 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)

这些操作以什么顺序执行？自下而上？自上而下？

Answer 1

两个方向都有意义。从上到下是数据请求。从下到上是数据。你看到一棵树——每个节点都有最小的一个 child 。在执行时，节点调用他的子节点 - “向我发送数据”并且此调用递归地重复到叶子 - seq 扫描，索引扫描，设置返回的函数调用，...逐行生成数据 - 将数据发送给父节点.