sql - 为什么 Postgres 查询规划器受 LIMIT 影响？

Question

EXPLAIN ANALYZE SELECT     "alerts"."id", 
            "alerts"."created_at", 
            't1'::text AS src_table 
 FROM       "alerts" 
 INNER JOIN "devices" 
 ON         "devices"."id" = "alerts"."device_id" 
 INNER JOIN "sites" 
 ON         "sites"."id" = "devices"."site_id" 
 WHERE      "sites"."cloud_id" = 111
 AND        "alerts"."created_at" >= '2019-08-30'
 ORDER BY   "created_at" DESC limit 9;

 Limit  (cost=1.15..36021.60 rows=9 width=16) (actual time=30.505..29495.765 rows=9 loops=1)
  ->  Nested Loop  (cost=1.15..232132.92 rows=58 width=16) (actual time=30.504..29495.755 rows=9 loops=1)
        ->  Nested Loop  (cost=0.86..213766.42 rows=57231 width=24) (actual time=0.029..29086.323 rows=88858 loops=1)
              ->  Index Scan Backward using alerts_created_at_index on alerts  (cost=0.43..85542.16 rows=57231 width=24) (actual time=0.014..88.137 rows=88858 loops=1)
                    Index Cond: (created_at >= '2019-08-30 00:00:00'::timestamp without time zone)
              ->  Index Scan using devices_pkey on devices  (cost=0.43..2.23 rows=1 width=16) (actual time=0.016..0.325 rows=1 loops=88858)
                    Index Cond: (id = alerts.device_id)
        ->  Index Scan using sites_pkey on sites  (cost=0.29..0.31 rows=1 width=8) (actual time=0.004..0.004 rows=0 loops=88858)
              Index Cond: (id = devices.site_id)
              Filter: (cloud_id = 7231)
              Rows Removed by Filter: 1
Total runtime: 29495.816 ms

现在我们改为 LIMIT 10:

 EXPLAIN ANALYZE SELECT     "alerts"."id", 
            "alerts"."created_at", 
            't1'::text AS src_table 
 FROM       "alerts" 
 INNER JOIN "devices" 
 ON         "devices"."id" = "alerts"."device_id" 
 INNER JOIN "sites" 
 ON         "sites"."id" = "devices"."site_id" 
 WHERE      "sites"."cloud_id" = 111
 AND        "alerts"."created_at" >= '2019-08-30'
 ORDER BY   "created_at" DESC limit 10;

Limit  (cost=39521.79..39521.81 rows=10 width=16) (actual time=1.557..1.559 rows=10 loops=1)
  ->  Sort  (cost=39521.79..39521.93 rows=58 width=16) (actual time=1.555..1.555 rows=10 loops=1)
        Sort Key: alerts.created_at
        Sort Method: quicksort  Memory: 25kB
        ->  Nested Loop  (cost=5.24..39520.53 rows=58 width=16) (actual time=0.150..1.543 rows=11 loops=1)
              ->  Nested Loop  (cost=4.81..16030.12 rows=2212 width=8) (actual time=0.137..0.643 rows=31 loops=1)
                    ->  Index Scan using sites_cloud_id_index on sites  (cost=0.29..64.53 rows=31 width=8) (actual time=0.014..0.057 rows=23 loops=1)
                          Index Cond: (cloud_id = 7231)
                    ->  Bitmap Heap Scan on devices  (cost=4.52..512.32 rows=270 width=16) (actual time=0.020..0.025 rows=1 loops=23)
                          Recheck Cond: (site_id = sites.id)
                          ->  Bitmap Index Scan on devices_site_id_index  (cost=0.00..4.46 rows=270 width=0) (actual time=0.006..0.006 rows=9 loops=23)
                                Index Cond: (site_id = sites.id)
              ->  Index Scan using alerts_device_id_index on alerts  (cost=0.43..10.59 rows=3 width=24) (actual time=0.024..0.028 rows=0 loops=31)
                    Index Cond: (device_id = devices.id)
                    Filter: (created_at >= '2019-08-30 00:00:00'::timestamp without time zone)
                    Rows Removed by Filter: 12
Total runtime: 1.603 ms

alerts表有百万条记录，其他表以千计。

我已经可以通过简单地不使用 limit < 10 来优化查询。我不明白的是为什么 LIMIT 会影响性能。也许有比硬编码这个神奇数字“10”更好的方法。

Answer 1

结果行数会影响 PostgreSQL 优化器，因为快速返回前几行的计划不一定是尽快返回整个结果的计划。

在您的情况下，PostgreSQL 认为对于 LIMIT 的小值, 通过扫描 alerts 会更快表格顺序为ORDER BY使用索引的子句，然后使用嵌套循环连接其他表，直到找到 9 行。

这种策略的好处是它不必计算连接的完整结果，然后对其进行排序并丢弃除前几行以外的所有结果。 危险在于找到 9 个匹配行所花费的时间比预期的要长，这就是打击你的地方:

Index Scan Backward using alerts_created_at_index on alerts (cost=0.43..85542.16 rows=57231 width=24) (actual time=0.014..88.137 <strong>rows=88858</strong> loops=1)

因此 PostgreSQL 必须处理 88858 行并使用嵌套循环连接(如果必须经常循环，效率很低)，直到找到 9 个结果行。这可能是因为它低估了条件的选择性，或者因为许多匹配行恰好具有低 created_at。 .

数字 10 恰好是 PostgreSQL 认为使用该策略不再更有效的分界点，它是一个会随着数据库中数据的变化而变化的值。

您可以通过使用 ORDER BY 来完全避免使用该计划。与索引不匹配的子句:

ORDER BY (created_at + INTERVAL '0 days') DESC