性状:
PostgeSQL 9.1,PostGIS 2.0。
1。查询:
SELECT ST_AsText(geo_point)
FROM lightnings
ORDER BY stroke_when DESC, stroke_when_second DESC
LIMIT 1
总运行时间: 31100.911 ms !
EXPLAIN(解析,关闭VERBOSE,开启费用,BUFFERS):
Limit (cost=169529.67..169529.67 rows=1 width=144) (actual time=31100.869..31100.869 rows=1 loops=1)
Buffers: shared hit=3343 read=120342
-> Sort (cost=169529.67..176079.48 rows=2619924 width=144) (actual time=31100.865..31100.865 rows=1 loops=1)
Sort Key: stroke_when, stroke_when_second
Sort Method: top-N heapsort Memory: 17kB
Buffers: shared hit=3343 read=120342
-> Seq Scan on lightnings (cost=0.00..156430.05 rows=2619924 width=144) (actual time=1.589..29983.410 rows=2619924 loops=1)
Buffers: shared hit=3339 read=120342
2。选择其他字段:
SELECT id
FROM lightnings
ORDER BY stroke_when DESC, stroke_when_second DESC
LIMIT 1
总运行时间: 2144.057 ms 。
EXPLAIN(解析,关闭VERBOSE,开启费用,BUFFERS):
Limit (cost=162979.86..162979.86 rows=1 width=24) (actual time=2144.013..2144.014 rows=1 loops=1)
Buffers: shared hit=3513 read=120172
-> Sort (cost=162979.86..169529.67 rows=2619924 width=24) (actual time=2144.011..2144.011 rows=1 loops=1)
Sort Key: stroke_when, stroke_when_second
Sort Method: top-N heapsort Memory: 17kB
Buffers: shared hit=3513 read=120172
-> Seq Scan on lightnings (cost=0.00..149880.24 rows=2619924 width=24) (actual time=0.056..1464.904 rows=2619924 loops=1)
Buffers: shared hit=3509 read=120172
3。正确优化:
SELECT id
FROM lightnings
ORDER BY stroke_when DESC
LIMIT 1
总运行时间: 0.044 ms
EXPLAIN(解析,关闭VERBOSE,开启费用,BUFFERS):
Limit (cost=0.00..3.52 rows=1 width=16) (actual time=0.020..0.020 rows=1 loops=1)
Buffers: shared hit=5
-> Index Scan Backward using lightnings_idx on lightnings (cost=0.00..9233232.80 rows=2619924 width=16) (actual time=0.018..0.018 rows=1 loops=1)
Buffers: shared hit=5
正如您所看到的,当SQL优化器使用索引时,查询是非常原始的,但有两个不同且非常不同的冲突:
当然,上面是一个简化的查询,从更复杂的结构中提取。通常我会按日期范围选择行,应用复杂的过滤器。
答案 0 :(得分:2)
PostgreSQL无法使用您的索引按前两个查询所需的顺序生成值。当两行或多行具有相同的store_when
相同时,它们将以任意顺序从索引扫描返回。要确定行的正确顺序,需要进行二次排序。因为PostgreSQL执行程序没有执行该次要排序的工具,所以它回退到完全排序方法。
如果您经常需要使用该订单查询表,请将当前索引替换为包含两列的复合索引。
您可以将当前查询转换为仅在store_when
的最大值上显式指定辅助排序的表单:
SELECT ST_AsText(geo_point) FROM lightnings
WHERE store_when = (SELECT max(store_when) FROM lightnings)
ORDER BY stroke_when_second DESC LIMIT 1
答案 1 :(得分:1)
第一步可能是:在{stroke_when,stroke_when_second}上创建一个复合索引