我遇到一个问题，即表上的相同查询有时需要几秒钟，大多数时间在几毫秒内完成。 Postgres的重新启动似乎解决了这个问题并将其保留了几天。
在上述条件持续一段时间后，查询开始需要5秒以上才能完成。
只需重新启动客户端，Postgres通过将其降低到几毫秒来解决这个问题。
explain analyze显示此表的顺序扫描时间。但是，当Postgres刚刚启动时，它大约为毫秒级，之后会增加到几秒钟。
此表中有大约25000条记录。
有关seq扫描速度慢时如何调试的任何建议和想法？它可能是shared_buffer中的碎片或导致这种情况的表的锁定。我怀疑这是一个磁盘或IO问题，因为无论Postgres何时启动它们都会引起问题。
在发布问题时，我错过了在分析中使用缓冲区。输出如下：
explain analyze SELECT s.severity, s.severityid from SEVERITY s where s.severityid =(select MIN(severityid) from ALARM a, (SELECT * FROM RESOURCEDETAILS) as RESOURCEDETAILS WHERE RESOURCEDETAILS.resourcegroupid = 132 AND RESOURCEDETAILS.resourceid = a.resourceid AND a.severityid 1 AND a.severityid 2 AND a.severityid 7);
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------
Seq Scan on severity s (cost=4701.39..4702.47 rows=1 width=13) (actual time=7057.939..7057.940 rows=1 loops=1)
Filter: (severityid = $0)
Rows Removed by Filter: 6
InitPlan 1 (returns $0)
-> Aggregate (cost=4701.38..4701.39 rows=1 width=8) (actual time=7057.918..7057.918 rows=1 loops=1)
-> Hash Join (cost=533.69..4701.25 rows=51 width=8) (actual time=1849.630..7057.783 rows=223 loops=1)
Hash Cond: (a.resourceid = resourcedetails.resourceid)
-> Seq Scan on alarm a (cost=0.00..4157.70 rows=2491 width=16) (actual time=45.792..6608.120 rows=852 loops=1)
Filter: ((severityid 1) AND (severityid 2) AND (severityid 7))
Rows Removed by Filter: 24376
-> Hash (cost=528.27..528.27 rows=434 width=8) (actual time=448.540..448.540 rows=19 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 1kB
-> Bitmap Heap Scan on resourcedetails (cost=7.65..528.27 rows=434 width=8) (actual time=0.128..448.495 rows=19 loops=1)
Recheck Cond: (resourcegroupid = 132)
-> Bitmap Index Scan on resourcedetails_rg_idx (cost=0.00..7.54 rows=434 width=0) (actual time=0.103..0.103 rows=63 loops=1)
Index Cond: (resourcegroupid = 132)
Total runtime: 7058.156 ms
(17 rows)
After postgres restart, the problem was resolved and here is the output:
(This time with all options of explain)
<pre>
explain (analyze,buffers,costs,timing,verbose) SELECT s.severity, s.severityid from SEVERITY s where s.severityid =(select MIN(severityid) from ALARM a, (SELECT * FROM RESOURCEDETAILS) as RESOURCEDETAILS WHERE RESOURCEDETAILS.resourcegroupid = 132 AND RESOURCEDETAILS.resourceid = a.resourceid AND a.severityid <> 1 AND a.severityid <> 2 AND a.severityid <> 7);
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
-----------------------------------------
Seq Scan on public.severity s (cost=4516.93..4518.02 rows=1 width=13) (actual time=15.881..15.882 rows=1 loops=1)
Output: s.severity, s.severityid
Filter: (s.severityid = $0)
Rows Removed by Filter: 6
Buffers: shared hit=3167
InitPlan 1 (returns $0)
-> Aggregate (cost=4516.92..4516.93 rows=1 width=8) (actual time=15.874..15.875 rows=1 loops=1)
Output: min(a.severityid)
Buffers: shared hit=3166
-> Hash Join (cost=520.61..4516.91 rows=5 width=8) (actual time=7.043..15.869 rows=2 loops=1)
Output: a.severityid
Hash Cond: (a.resourceid = resourcedetails.resourceid)
Buffers: shared hit=3166
-> Seq Scan on public.alarm a (cost=0.00..3995.37 rows=232 width=16) (actual time=0.142..15.492 rows=108 loops=1)
Output: a.alarmid, a.eventtypeid, a.severityid, a.source, a.resourceid, a.componentid, a.category, a.createdtime, a.updatedtime, a.owner, a.ackstatus, a.acktime, a.clearedtime, a.messa
ge, a.remedy, a.optionalfield, a.version
Filter: ((a.severityid <> 1) AND (a.severityid <> 2) AND (a.severityid <> 7))
Rows Removed by Filter: 26130
Buffers: shared hit=3108
-> Hash (cost=515.39..515.39 rows=418 width=8) (actual time=0.338..0.338 rows=19 loops=1)
Output: resourcedetails.resourceid
Buckets: 1024 Batches: 1 Memory Usage: 1kB
Buffers: shared hit=58
-> Bitmap Heap Scan on public.resourcedetails (cost=7.53..515.39 rows=418 width=8) (actual time=0.033..0.330 rows=19 loops=1)
Output: resourcedetails.resourceid
Recheck Cond: (resourcedetails.resourcegroupid = 132)
Buffers: shared hit=58
-> Bitmap Index Scan on resourcedetails_rg_idx (cost=0.00..7.42 rows=418 width=0) (actual time=0.023..0.023 rows=61 loops=1)
Index Cond: (resourcedetails.resourcegroupid = 132)
Buffers: shared hit=3
Total runtime: 15.937 ms
(30 rows)
postgresql重新启动后，查询似乎稳定恶化......