每次在PostgreSQL中的查询上运行EXPLAIN ANALYZE时，Execution time都会减少。为什么？
我需要在表上做一些索引，这样我就不能确定我的动作是否会提高性能。你向我推荐什么？
示例的连续执行结果说明（分析，缓冲区）：
my_db=# explain (analyze, buffers)
SELECT COUNT(*) AS count
FROM my_view
WHERE creation_time >= '2019-01-18 00:00:00'
AND creation_time <= '2019-01-18 09:43:36'
ORDER BY count DESC
LIMIT 10000;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
--------------------------------------------------------------------
Limit (cost=2568854.23..2568854.23 rows=1 width=8) (actual time=24380.613..24380.614 rows=1 loops=1)
Buffers: shared hit=14915 read=2052993, temp read=562 written=563
-> Sort (cost=2568854.23..2568854.23 rows=1 width=8) (actual time=24380.611..24380.611 rows=1 loops=1)
Sort Key: (count(*)) DESC
Sort Method: quicksort Memory: 25kB
Buffers: shared hit=14915 read=2052993, temp read=562 written=563
-> Aggregate (cost=2568854.21..2568854.22 rows=1 width=8) (actual time=24380.599..24380.599 rows=1 loops=1)
Buffers: shared hit=14915 read=2052993, temp read=562 written=563
-> GroupAggregate (cost=2568854.18..2568854.20 rows=1 width=28) (actual time=24339.455..24380.589 rows=40 loops=1)
Group Key: my_table.creation_time, my_table.some_field
Buffers: shared hit=14915 read=2052993, temp read=562 written=563
-> Sort (cost=2568854.18..2568854.18 rows=1 width=12) (actual time=24338.361..24357.171 rows=199309 loops=1)
Sort Key: my_table.creation_time, my_table.some_field
Sort Method: external merge Disk: 4496kB
Buffers: shared hit=14915 read=2052993, temp read=562 written=563
-> Gather (cost=1000.00..2568854.17 rows=1 width=12) (actual time=23799.237..24142.217 rows=199309 loops=1)
Workers Planned: 2
Workers Launched: 2
Buffers: shared hit=14915 read=2052993
-> Parallel Seq Scan on my_table (cost=0.00..2567854.07 rows=1 width=12) (actual time=23796.695..24087.574 rows=66436 loops=3)
Filter: (creation_time >= '2019-01-18 00:00:00+00'::timestamp with time zone) AND (creation_time <= '2019-01-18 09:43:36+00'::timestamp with time zone)
Rows Removed by Filter: 21818095
Buffers: shared hit=14915 read=2052993
Planning time: 10.982 ms
Execution time: 24381.544 ms
(25 rows)
my_db=# explain (analyze, buffers)
SELECT COUNT(*) AS count
FROM my_view
WHERE creation_time >= '2019-01-18 00:00:00'
AND creation_time <= '2019-01-18 09:43:36'
ORDER BY count DESC
LIMIT 10000;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
--------------------------------------------------------------------
Limit (cost=2568854.23..2568854.23 rows=1 width=8) (actual time=6836.247..6836.248 rows=1 loops=1)
Buffers: shared hit=15181 read=2052727, temp read=562 written=563
-> Sort (cost=2568854.23..2568854.23 rows=1 width=8) (actual time=6836.245..6836.246 rows=1 loops=1)
Sort Key: (count(*)) DESC
Sort Method: quicksort Memory: 25kB
Buffers: shared hit=15181 read=2052727, temp read=562 written=563
-> Aggregate (cost=2568854.21..2568854.22 rows=1 width=8) (actual time=6836.232..6836.232 rows=1 loops=1)
Buffers: shared hit=15181 read=2052727, temp read=562 written=563
-> GroupAggregate (cost=2568854.18..2568854.20 rows=1 width=28) (actual time=6792.036..6836.221 rows=40 loops=1)
Group Key: my_table.creation_time, my_table.some_field
Buffers: shared hit=15181 read=2052727, temp read=562 written=563
-> Sort (cost=2568854.18..2568854.18 rows=1 width=12) (actual time=6790.807..6811.469 rows=199309 loops=1)
Sort Key: my_table.creation_time, my_table.some_field
Sort Method: external merge Disk: 4496kB
Buffers: shared hit=15181 read=2052727, temp read=562 written=563
-> Gather (cost=1000.00..2568854.17 rows=1 width=12) (actual time=6271.571..6604.946 rows=199309 loops=1)
Workers Planned: 2
Workers Launched: 2
Buffers: shared hit=15181 read=2052727
-> Parallel Seq Scan on my_table (cost=0.00..2567854.07 rows=1 width=12) (actual time=6268.383..6529.416 rows=66436 loops=3)
Filter: (creation_time >= '2019-01-18 00:00:00+00'::timestamp with time zone) AND (creation_time <= '2019-01-18 09:43:36+00'::timestamp with time zone)
Rows Removed by Filter: 21818095
Buffers: shared hit=15181 read=2052727
Planning time: 0.570 ms
Execution time: 6837.137 ms
(25 rows)
谢谢。
答案 0 :(得分：0)
这是什么版本的PostgreSQL？
有两个问题：
缺少索引：
<div class="form-group">
<label for="reservation_id" class="col-md-4 control-label">Reservation</label>
<div class="col-md-8">
<select name="reservation_id" class="form-control">
<option value="">select reservation</option>
<?php
foreach($all_reservations as $reservation)
{
$selected = ($reservation['id'] == $this->input->post('reservation_id')) ? ' selected="selected"' : "";
echo '<option value="'.$reservation['id'].'" '.$selected.'>'.$reservation['id'].'</option>';
}
?>
</select>
</div>
</div>
CREATE INDEX ON my_table (creation_time);
时间的值分布统计信息不正确，这导致PostgreSQL严重低估了结果行的数量。
这不是问题的直接原因，但是您应该调查那里发生的事情：
如果creation改进了估算值，请参见该表的自动分析运行频率更高。
如果ANALYZE my_table没有帮助，请收集更好的统计信息：
ANALYZE my_table使用ALTER TABLE my_table ALTER creation_time SET STATISTICS 1000;
ANALYZE my_table;
观察到的执行时间减少可能是由于缓存效应所致。