Postgresql string_agg和崩溃问题

Question

我们有一个视图，它在一个包含5400万条记录的表上执行 string_agg ，其中string_agg需要处理每组2个字符串。使用如下查询的视图定义导致postgresql服务器进程因内存不足而死亡。

SELECT id, string_agg(msg, “,”) FROM msgs GROUP BY id

当视图查询被修改如下时，它正常工作。

SELECT id, string_agg(msg, ',') as msgs
FROM ( SELECT id, msg, row_number() over (partition by id) as row_num 
       FROM msgs) as limited_alerts
WHERE row_num < 5
GROUP BY id

是什么原因？是因为postgres能够在第二种情况下使用临时文件吗？是否有任何链接/文章解释详细信息？

表格消息

• id bigint PRIMARY KEY
• 服务文字
• msg text

  

总行数：5400万

     

每个id的最大消息数：2

看起来崩溃的是使用HashAggregate和其他一个GroupAggregate

explain select count(*) from (SELECT msgs.id, string_agg(msgs.alerts, ','::text) AS alerts FROM msgs GROUP BY msgs.id) as a;
                                              QUERY PLAN                                              
------------------------------------------------------------------------------------------------------
 Aggregate  (cost=3032602.58..3032602.59 rows=1 width=0)
   ->  HashAggregate  (cost=3032597.58..3032600.08 rows=200 width=108)
         Group Key: msgs.id
         ->  Append  (cost=0.00..2587883.05 rows=88942906 width=108)
               ->  Seq Scan on msgs  (cost=0.00..0.00 rows=1 width=548)
               ->  Seq Scan on msgs_2016_01_24  (cost=0.00..506305.40 rows=17394640 width=107)
               ->  Seq Scan on msgs_2016_01_31  (cost=0.00..509979.80 rows=17512480 width=107)
               ->  Seq Scan on msgs_2016_02_07  (cost=0.00..491910.32 rows=16883332 width=108)
               ->  Seq Scan on msgs_2016_02_14  (cost=0.00..496443.84 rows=17071384 width=108)
               ->  Seq Scan on msgs_2016_02_21  (cost=0.00..552162.84 rows=19026084 width=108)
               ->  Seq Scan on msgs_2016_02_28  (cost=0.00..31038.05 rows=1054705 width=111)
               ->  Seq Scan on msgs_2016_03_06  (cost=0.00..10.70 rows=70 width=548)
               ->  Seq Scan on msgs_2016_03_13  (cost=0.00..10.70 rows=70 width=548)
               ->  Seq Scan on msgs_2016_03_20  (cost=0.00..10.70 rows=70 width=548)
               ->  Seq Scan on msgs_2016_03_27  (cost=0.00..10.70 rows=70 width=548)