我正在使用这些结果来分析一个简单的查询，该查询不会从少于200条记录的表中返回150条以上的记录，因为我有一个存储最新值的表，而其他字段是数据的FK。
更新：稍后将在同一查询中查看新结果。该网站不是公开的，并且/或者现在应该没有用户，因为它正在开发中。
explain analyze
SELECT lv.station_id,
s.name AS station_name,
s.latitude,
s.longitude,
s.elevation,
lv.element_id,
e.symbol AS element_symbol,
u.symbol,
e.name AS element_name,
lv.last_datetime AS datetime,
lv.last_value AS valor,
s.basin_id,
s.municipality_id
FROM (((element_station lv /*350 records*/
JOIN stations s ON ((lv.station_id = s.id))) /*40 records*/
JOIN elements e ON ((lv.element_id = e.id))) /*103 records*/
JOIN units u ON ((e.unit_id = u.id))) /* 32 records */
WHERE s.id = lv.station_id AND e.id = lv.element_id AND lv.interval_id = 6 and
lv.last_datetime >= ((now() - '06:00:00'::interval) - '01:00:00'::interval)
我已经尝试过了VACUUM，然后保存了一些，但是过一段时间后又恢复了。我已经在字段上实现了索引。
Nested Loop (cost=0.29..2654.66 rows=1 width=92) (actual time=1219.390..35296.253 rows=157 loops=1)
Join Filter: (e.unit_id = u.id)
Rows Removed by Join Filter: 4867
-> Nested Loop (cost=0.29..2652.93 rows=1 width=92) (actual time=1219.383..35294.083 rows=157 loops=1)
Join Filter: (lv.element_id = e.id)
Rows Removed by Join Filter: 16014
-> Nested Loop (cost=0.29..2648.62 rows=1 width=61) (actual time=1219.301..35132.373 rows=157 loops=1)
-> Seq Scan on element_station lv (cost=0.00..2640.30 rows=1 width=20) (actual time=1219.248..1385.517 rows=157 loops=1)
Filter: ((interval_id = 6) AND (last_datetime >= ((now() - '06:00:00'::interval) - '01:00:00'::interval)))
Rows Removed by Filter: 168
-> Index Scan using stations_pkey on stations s (cost=0.29..8.31 rows=1 width=45) (actual time=3.471..214.941 rows=1 loops=157)
Index Cond: (id = lv.station_id)
-> Seq Scan on elements e (cost=0.00..3.03 rows=103 width=35) (actual time=0.003..0.999 rows=103 loops=157)
-> Seq Scan on units u (cost=0.00..1.32 rows=32 width=8) (actual time=0.002..0.005 rows=32 loops=157)
Planning time: 8.312 ms
Execution time: 35296.427 ms
更新，今晚运行相同的查询；没有变化：
Sort (cost=601.74..601.88 rows=55 width=92) (actual time=1.822..1.841 rows=172 loops=1)
Sort Key: lv.last_datetime DESC
Sort Method: quicksort Memory: 52kB
-> Nested Loop (cost=11.60..600.15 rows=55 width=92) (actual time=0.287..1.680 rows=172 loops=1)
-> Hash Join (cost=11.31..248.15 rows=55 width=51) (actual time=0.263..0.616 rows=172 loops=1)
Hash Cond: (e.unit_id = u.id)
-> Hash Join (cost=9.59..245.60 rows=75 width=51) (actual time=0.225..0.528 rows=172 loops=1)
Hash Cond: (lv.element_id = e.id)
-> Bitmap Heap Scan on element_station lv (cost=5.27..240.25 rows=75 width=20) (actual time=0.150..0.359 rows=172 loops=1)
Recheck Cond: ((last_datetime >= ((now() - '06:00:00'::interval) - '01:00:00'::interval)) AND (interval_id = 6))
Heap Blocks: exact=22
-> Bitmap Index Scan on element_station_latest (cost=0.00..5.25 rows=75 width=0) (actual time=0.136..0.136 rows=226 loops=1)
Index Cond: ((last_datetime >= ((now() - '06:00:00'::interval) - '01:00:00'::interval)) AND (interval_id = 6))
-> Hash (cost=3.03..3.03 rows=103 width=35) (actual time=0.062..0.062 rows=103 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 15kB
-> Seq Scan on elements e (cost=0.00..3.03 rows=103 width=35) (actual time=0.006..0.031 rows=103 loops=1)
-> Hash (cost=1.32..1.32 rows=32 width=8) (actual time=0.019..0.019 rows=32 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 10kB
-> Seq Scan on units u (cost=0.00..1.32 rows=32 width=8) (actual time=0.003..0.005 rows=32 loops=1)
-> Index Scan using stations_pkey on stations s (cost=0.29..6.39 rows=1 width=45) (actual time=0.005..0.006 rows=1 loops=172)
Index Cond: (id = lv.station_id)
Planning time: 2.390 ms
Execution time: 2.009 ms
问题是在上连续扫描时错误估计了行数element_station。自动分析已开始并为该表计算了新的统计信息，或者数据已更改。
问题可能是PostgreSQL不知道
((now() - '06:00:00'::interval) - '01:00:00'::interval)
在查询计划时。
如果可能，请分两个步骤进行操作：首先，计算上述表达式（在PostgreSQL或客户端）。然后以结果作为常量运行查询。这将使PostgreSQL更容易估计结果计数。
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