Insert-select在添加限制子句时得到更好的计划

Question

这是我正在运行的服务器

select version();
                                                 version
---------------------------------------------------------------------------    
PostgreSQL 10.6 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-36), 64-bit
(1 row)

我从编写select开始(ext.t_event和ext.t_event_data是oracle_fdw (Version1.1)从远程oracle获取的两个外表)。

select 
  te.id_data, 
  te.id_device, 
  te.date_write, 
  te.date_event, 
  ted.i_inout, 
  ted.value
from ext.t_event te, ext.t_event_data ted 
where te.id_device =2749651 
  and te.date_event >= '2019-01-16'and te.date_event < '2019-01-17' 
  and te.id_data=ted.id_data;

获取整个记录集(3600条记录)大约需要10秒。

但随后我将select转换为insert select。

insert into stg_data
select 
  te.id_data, 
  te.id_device, 
  te.date_write, 
  te.date_event, 
  ted.i_inout, 
  ted.value
from ext.t_event te, ext.t_event_data ted 
where te.id_device =2749651 
  and te.date_event >= '2019-01-16'and te.date_event < '2019-01-17' 
  and te.id_data=ted.id_data;

我被迫杀死了这个查询，它已经运行了30多分钟了！

经过几个小时的挣扎和绝望的尝试，我决定试试这个。

insert into stg_data
select 
  te.id_data, 
  te.id_device, 
  te.date_write, 
  te.date_event, 
  ted.i_inout, 
  ted.value
from ext.t_event te, ext.t_event_data ted 
where te.id_device =2749651 
  and te.date_event >= '2019-01-16'and te.date_event < '2019-01-17' 
  and te.id_data=ted.id_data
  limit 5000;

and...surprise惊喜在20秒内，我将整个记录集存储在stg_data中。

为了更好地理解差异，我决定分析这些计划。

选择无限制

 Foreign Scan  (cost=10000.00..20000.00 rows=1000 width=548)
   Oracle query: SELECT /*eb01c463a72c3b6350f86f5db25e1353*/ r1."ID_DATA",
   r1."ID_DEVICE", r1."DATE_WRITE", r1."DATE_EVENT", r2."I_INOUT",
   r2."VALUE" FROM ("DISPATCH"."T_EVENT" r1 INNER JOIN
   "DISPATCH"."T_EVENT_DATA" r2 ON (r1."ID_DATA" = r2."ID_DATA") AND
  (r1."DATE_EVENT" >= (CAST ('2019-01-16 00:00:00.000000 AD' AS
  TIMESTAMP))) AND (r1."DATE_EVENT" < 
  (CAST ('2019-01-17 00:00:00.000000 AD' AS TIMESTAMP))) 
  AND (r1."ID_DEVICE" = 2749651))

有限制的选择

 Limit  (cost=10000.00..20000.00 rows=1000 width=548)
   ->  Foreign Scan  (cost=10000.00..20000.00 rows=1000 width=548)
      Oracle query: SELECT /*eb01c463a72c3b6350f86f5db25e1353*/
      r1."ID_DATA", r1."ID_DEVICE", r1."DATE_WRITE", r1."DATE_EVENT", 
      r2."I_INOUT", r2."VALUE" FROM ("DISPATCH"."T_EVENT" r1 INNER 
      JOIN "DISPATCH"."T_EVENT_DATA" r2 ON (r1."ID_DATA" = r2."ID_DATA")
      AND (r1."DATE_EVENT" >= (CAST ('2019-01-16 00:00:00.000000 AD' AS 
      TIMESTAMP))) AND (r1."DATE_EVENT" < (CAST ('2019-01-17
      00:00:00.000000 AD' AS TIMESTAMP))) AND (r1."ID_DEVICE" = 2749651))

因此，它基本上向Oracle发送相同的查询，并在获取完成后立即在本地应用筛选器。

INSER-SELECT计划看起来一样吗？不！

有极限的INSERT_SELECT

Insert on stg_data_hist  (cost=10000.00..20010.00 rows=1000 width=548)
   ->  Limit  (cost=10000.00..20000.00 rows=1000 width=548)
         ->  Foreign Scan  (cost=10000.00..20000.00 rows=1000 width=548)
               Oracle query: SELECT /*eb01c463a72c3b6350f86f5db25e1353*/ 
               r1."ID_DATA", r1."ID_DEVICE", r1."DATE_WRITE", 
               r1."DATE_EVENT", r2."I_INOUT", r2."VALUE" FROM 
               ("DISPATCH"."T_EVENT" r1 INNER JOIN 
               "DISPATCH"."T_EVENT_DATA" r2 ON (r1."ID_DATA" = 
               r2."ID_DATA") AND (r1."DATE_EVENT" >= (CAST ('2019-01-16 
               00:00:00.000000 AD' AS TIMESTAMP))) AND (r1."DATE_EVENT" < 
               (CAST('2019-01-17 00:00:00.000000 AD' AS TIMESTAMP))) AND 
               (r1."ID_DEVICE" = 2749651))

插入-选择无限制子句

Insert on stg_data_hist  (cost=30012.50..40190.00 rows=5000 width=548)
 ->  Hash Join  (cost=30012.50..40190.00 rows=5000 width=548)
       Hash Cond: (te.id_data = ted.id_data)
     ->  Foreign Scan on t_event te  (cost=10000.00..20000.00 rows=1000 width=28)
           Oracle query: SELECT /*93379c271b3f1bc08a1dbb94fb89f739*/ 
           r3."ID_DATA", r3."ID_DEVICE", r3."DATE_WRITE", r3."DATE_EVENT" 
           FROM "DISPATCH"."T_EVENT" r3 WHERE (r3."DATE_EVENT" >= 
           (CAST ('2019-01-16 00:00:00.000000 AD' AS TIMESTAMP))) AND 
           (r3."DATE_EVENT" < (CAST ('2019-01-17 00:00:00.000000 AD' AS 
           TIMESTAMP))) AND (r3."ID_DEVICE" = 2749651)
       ->  Hash  (cost=20000.00..20000.00 rows=1000 width=528)
           ->  Foreign Scan on t_event_data ted  
                  (cost=10000.00..20000.00 rows=1000 width=528)
                 Oracle query: SELECT /*21c8741f2fa8a8d13d037c3191e8ac96*/ 
                    r4."ID_DATA", r4."I_INOUT", r4."VALUE" FROM 
                    "DISPATCH"."T_EVENT_DATA" r4

这就解释了为什么花的时间比其他的长。它从一个外部表中检索经过日期过滤的记录，从第二个外部表中检索完整的集合，并在本地进行连接。那要花很长时间！！这是数以百万计的记录和几千张。

最后我的两个问题

1)我想有第一个计划，但去掉限制条款(让我的脊背颤抖:-)。你会怎么做？除了join子句之外，我不打算在ext.t_event_data上应用过滤器。

2)为什么两个INSERT-SELECT计划看起来如此不同，尽管两个选择计划看起来如此相似？

谢谢你的阅读，祝你今天愉快

Answer 1

规划师似乎认为，无论哪种方式，它都只会得到几千行，这显然是远远不够的，请通过运行“ANALYZE ext.t_event”来确保外部表的统计数据是最新的，而对于ext.t_event_data则是一样的，因为：

fdw

PostgreSQL不会自动收集具有自动真空守护进程的外部表的统计信息。 请记住，分析Oracle外部表将导致对表进行完整的顺序扫描。只需使用Oracle的示例，就可以使用表选项sample_percent来加快速度。

在select情况下，joins被向下推到Oracle，在insert情况下，如果使用限度，那么我可以看到没有在insert中无限制地使用它的唯一原因是缺乏精确的表统计信息。您可以尝试将insert查询重写为CTE (由于明显的原因尚未测试此查询)：

WITH foreign_data AS (
select 
  te.id_data, 
  te.id_device, 
  te.date_write, 
  te.date_event, 
  ted.i_inout, 
  ted.value
from ext.t_event te, ext.t_event_data ted 
where te.id_device =2749651 
  and te.date_event >= '2019-01-16'and te.date_event < '2019-01-17' 
  and te.id_data=ted.id_data
)

insert into stg_data from foreign_data

您还可以尝试将查询重写为显式的内部连接，而不是在where子句(te.id_data=ted.id_data)中具有联接条件。