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 表有数百万条记录,其他表有数千条记录。
我已经可以通过简单地不使用限制来优化查询< 10. What I don't understand is why the LIMIT affects the performance. Perhaps there's a better way than hardcoding this magic number "10".
结果行数影响 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。
在你的例子中,PostgreSQL认为对于
LIMIT
的小值,通过使用索引按照alerts
子句的顺序扫描ORDER BY
表会更快,然后使用嵌套循环连接其他表,直到它已找到 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个结果行之外的所有结果行。 危险在于找到 9 个匹配行所需的时间比预期要长,这就是你遇到的问题:
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)
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都恰好具有低
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的数据变化而变化的值。
您可以通过使用与索引不匹配的
ORDER BY
子句来完全避免使用该计划:
ORDER BY created_at DESC NULLS LAST