postgresql - 添加 limit 子句后，Insert-select 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)

我从编写选择开始(ext.t_event 和 ext.t_event_data 是 oracle_fdw(1.1 版)从远程 oracle 数据库获取的两个外部表)

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。

但后来我把选择变成了插入选择

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;

我被迫终止查询，它已经运行了 30 多分钟!

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;

并且......在 20 秒内出乎意料，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))

选择限制

 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))

所以它基本上向 Oracle 发送相同的查询，并在获取完成后立即在本地应用 FILTER。

INSER-SELECT 计划看起来一样吗？不!

INSERT_SELECT 和 LIMIT

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))

INSERT-SELECT 没有 LIMIT 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

这就解释了为什么它比另一个花费 waaaaaay 更长的时间。它从一个外部表中检索日期过滤的记录，从第二个外部表中检索完整集并在本地进行连接。这将需要很长时间!这是几百万条记录与几千条记录。

最后是我的两个问题

1) 我想要第一个计划，但要去掉 LIMIT 子句(让我脊背发凉 :-))。你会怎么做？除了 join 子句之外，我没有办法对 ext.t_event_data 应用过滤器。

2) 为什么两个 INSERT-SELECT 计划看起来如此不同，即使两个 SELECT 计划看起来如此相似？

感谢阅读，祝您有愉快的一天

Answer 1

规划器似乎认为无论哪种方式它都只会得到几千行，这显然是不对的，通过运行“ANALYZE ext.t_event”确保外部表的统计数据是最新的，ext 也是如此。 t_event_data 因为:

https://github.com/laurenz/oracle_fdw

PostgreSQL will not automatically gather statistics for foreign tables with the autovacuum daemon.

Keep in mind that analyzing an Oracle foreign table will result in a full sequential table scan. You can use the table option sample_percent to speed this up by using only a sample of the Oracle table.

连接是在选择情况下下推到Oracle，如果使用限制，则在插入情况下，所以我能看到不在无限制插入中使用它的唯一原因是缺少精确的表统计。您可以尝试将插入查询重写为 CTE(出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

您也可以尝试将查询重写为显式内部联接，而不是在 where 子句中加入联接条件 (te.id_data=ted.id_data)。