PostgreSQL 9.5.0
我有一个名为message_attachments
的表,它有1931964
行。
我在该表中搜索的一个键是message_id
。
我也总是包含deleted_at
是NULL语句(例如软删除)。
创建了一个索引:
CREATE INDEX message_attachments_message_id_idx
ON message_attachments (message_id)
WHERE deleted_at IS NULL;
所以它应该直接匹配此查询:
EXPLAIN ANALYZE
select *
from "message_attachments"
where "deleted_at" is null
and "message_id" = 33998052;
但最终的查询计划如下所示:
Seq Scan on message_attachments (cost=0.00..69239.91 rows=4 width=149) (actual time=1667.850..1667.850 rows=0 loops=1)
Filter: ((deleted_at IS NULL) AND (message_id = 33998052))
Rows Removed by Filter: 1931896
Planning time: 0.114 ms
Execution time: 1667.885 ms
我在我的数据库中使用了这些索引,但似乎它在某个特定的表上似乎并不喜欢它。
关于基数,最多5列具有相同的值。
还在该表上运行了ANALYZE和VACUUM ANALYZE。
修改1
SET enable_seqscan to off
SET enable_seqscan to off; EXPLAIN ANALYZE select * from "message_attachments" where "deleted_at" is null and "message_id" = 33998052;
SET
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on message_attachments (cost=36111.83..105378.49 rows=4 width=149) (actual time=2343.361..2343.361 rows=0 loops=1)
Recheck Cond: (deleted_at IS NULL)
Filter: (message_id = 33998052)
Rows Removed by Filter: 1932233
Heap Blocks: exact=45086
-> Bitmap Index Scan on message_attachments_deleted_at_index (cost=0.00..36111.82 rows=1934453 width=0) (actual time=789.836..789.836 rows=1933784 loops=1)
Index Cond: (deleted_at IS NULL)
Planning time: 0.098 ms
Execution time: 2343.425 ms
这将在该表的第二个索引上运行,看起来像这样:(绝对不应该使用)
CREATE INDEX message_attachments_deleted_at_index ON message_attachments USING btree (deleted_at)
修改2
\d+ message_attachments
Table "public.message_attachments"
Column | Type | Modifiers | Storage | Stats target | Description
------------+-----------------------------+------------------------------------------------------------------+----------+--------------+-------------
id | bigint | not null default nextval('message_attachments_id_seq'::regclass) | plain | |
created_at | timestamp without time zone | not null | plain | |
updated_at | timestamp without time zone | not null | plain | |
deleted_at | timestamp without time zone | | plain | |
name | character varying(255) | not null | extended | |
filename | character varying(255) | not null | extended | |
content | bytea | | extended | |
hash | character varying(255) | not null | extended | |
mime | character varying(255) | not null | extended | |
size | bigint | not null | plain | |
message_id | bigint | not null | plain | |
Indexes:
"message_attachments_pkey" PRIMARY KEY, btree (id)
"message_attachments_deleted_at_index" btree (deleted_at)
"message_attachments_message_id_idx" btree (message_id) WHERE deleted_at IS NULL
Foreign-key constraints:
"message_attachments_message_id_foreign" FOREIGN KEY (message_id) REFERENCES messages(id)
EDIT3
热备用主机上的行为完全相同。 (这是up2date)
Edit4
select seq_scan,seq_tup_read,idx_scan,idx_tup_fetch,n_live_tup,pg_stat_all_tables.n_dead_tup,last_analyze,pg_stat_all_tables.analyze_count,pg_stat_all_tables.last_autoanalyze from pg_stat_all_tables where relname = 'message_attachments';
seq_scan | seq_tup_read | idx_scan | idx_tup_fetch | n_live_tup | n_dead_tup | last_analyze | analyze_count | last_autoanalyze
----------+----------------+----------+---------------+------------+------------+-------------------------------+---------------+-------------------------------
18728036 | 26379554229720 | 1475541 | 808566894 | 1934435 | 28052 | 2017-04-12 09:48:34.638184+02 | 68 | 2017-02-02 18:41:05.902214+01
select * from pg_stat_all_indexes where relname = 'message_attachments';
relid | indexrelid | schemaname | relname | indexrelname | idx_scan | idx_tup_read | idx_tup_fetch
--------+------------+------------+---------------------+--------------------------------------+----------+--------------+---------------
113645 | 113652 | public | message_attachments | message_attachments_pkey | 1475563 | 804751648 | 802770401
113645 | 113659 | public | message_attachments | message_attachments_deleted_at_index | 3 | 5801165 | 0
113645 | 20954507 | public | message_attachments | message_attachments_message_id_idx | 0 | 0 | 0
答案 0 :(得分:0)
好的,我刚刚解决了这个问题。
对于在php中被杀死的查询,我们以某种方式挂了LOCK,但几天前从未退出postgres的进程。
因此,对于遇到相同问题的每个人,请检查LOCKS:
SELECT relation::regclass, * FROM pg_locks WHERE NOT GRANTED;
而且,如果几天前有任何连接打开:
select * from pg_stat_activity order by query_start limit 10;