Postgres慢速分组查询最大值

时间:2013-11-05 12:06:21

标签: postgresql group-by

我正在使用postgres 9.1,我有一个包含大约3.5M行eventtype(varchar)和eventtime(timestamp)的表 - 以及其他一些字段。只有大约20种不同的事件类型,事件时间跨度大约为4年。

我想获取每种事件类型的最后一个时间戳。如果我运行如下查询:

select eventtype, max(eventtime)
from allevents
group by eventtype

需要大约20秒。选择不同的eventtype同样慢。查询计划显示了表的完整顺序扫描 - 这并不奇怪它很慢。

解释以上查询的分析给出:

HashAggregate  (cost=84591.47..84591.68 rows=21 width=21) (actual time=20918.131..20918.141 rows=21 loops=1)
  ->  Seq Scan on allevents  (cost=0.00..66117.98 rows=3694698 width=21) (actual time=0.021..4831.793 rows=3694392 loops=1)
Total runtime: 20918.204 ms

如果我添加一个where子句来选择特定的事件类型,则需要40ms到150ms,这至少是不错的。

选择特定事件类型时的查询计划:

GroupAggregate  (cost=343.87..24942.71 rows=1 width=21) (actual time=98.397..98.397 rows=1 loops=1)
  ->  Bitmap Heap Scan on allevents  (cost=343.87..24871.07 rows=14325 width=21) (actual time=6.820..89.610 rows=19736 loops=1)
        Recheck Cond: ((eventtype)::text = 'TEST_EVENT'::text)
        ->  Bitmap Index Scan on allevents_idx2  (cost=0.00..340.28 rows=14325 width=0) (actual time=6.121..6.121 rows=19736 loops=1)
              Index Cond: ((eventtype)::text = 'TEST_EVENT'::text)
Total runtime: 98.482 ms

主键是(eventtype,eventtime)。我还有以下索引:

allevents_idx (event time desc, eventtype)
allevents_idx2 (eventtype).

如何加快查询速度?

以下@denis建议的相关子查询的查询播放结果包含14个手动输入值:

Function Scan on unnest val  (cost=0.00..185.40 rows=100 width=32) (actual time=0.121..8983.134 rows=14 loops=1)
   SubPlan 2
     ->  Result  (cost=1.83..1.84 rows=1 width=0) (actual time=641.644..641.645 rows=1 loops=14)
          InitPlan 1 (returns $1)
             ->  Limit  (cost=0.00..1.83 rows=1 width=8) (actual time=641.640..641.641 rows=1 loops=14)
                  ->  Index Scan using allevents_idx on allevents  (cost=0.00..322672.36 rows=175938 width=8) (actual time=641.638..641.638 rows=1 loops=14)
                         Index Cond: ((eventtime IS NOT NULL) AND ((eventtype)::text = val.val))
Total runtime: 8983.203 ms

使用@jjanes建议的递归查询,查询运行4到5秒,并采用以下计划:

CTE Scan on t  (cost=260.32..448.63 rows=101 width=32) (actual time=0.146..4325.598 rows=22 loops=1)
  CTE t
    ->  Recursive Union  (cost=2.52..260.32 rows=101 width=32) (actual time=0.075..1.449 rows=22 loops=1)
          ->  Result  (cost=2.52..2.53 rows=1 width=0) (actual time=0.074..0.074 rows=1 loops=1)
            InitPlan 1 (returns $1)
                  ->  Limit  (cost=0.00..2.52 rows=1 width=13) (actual time=0.070..0.071 rows=1 loops=1)
                        ->  Index Scan using allevents_idx2 on allevents  (cost=0.00..9315751.37 rows=3696851 width=13) (actual time=0.070..0.070 rows=1 loops=1)
                              Index Cond: ((eventtype)::text IS NOT NULL)
          ->  WorkTable Scan on t  (cost=0.00..25.58 rows=10 width=32) (actual time=0.059..0.060 rows=1 loops=22)
                Filter: (eventtype IS NOT NULL)
                SubPlan 3
                  ->  Result  (cost=2.53..2.54 rows=1 width=0) (actual time=0.059..0.059 rows=1 loops=21)
                        InitPlan 2 (returns $3)
                          ->  Limit  (cost=0.00..2.53 rows=1 width=13) (actual time=0.057..0.057 rows=1 loops=21)
                                ->  Index Scan using allevents_idx2 on allevents  (cost=0.00..3114852.66 rows=1232284 width=13) (actual time=0.055..0.055 rows=1 loops=21)
                                      Index Cond: (((eventtype)::text IS NOT NULL) AND ((eventtype)::text > t.eventtype))
  SubPlan 6
    ->  Result  (cost=1.83..1.84 rows=1 width=0) (actual time=196.549..196.549 rows=1 loops=22)
          InitPlan 5 (returns $6)
            ->  Limit  (cost=0.00..1.83 rows=1 width=8) (actual time=196.546..196.546 rows=1 loops=22)
                  ->  Index Scan using allevents_idx on allevents  (cost=0.00..322946.21 rows=176041 width=8) (actual time=196.544..196.544 rows=1 loops=22)
                        Index Cond: ((eventtime IS NOT NULL) AND ((eventtype)::text = t.eventtype))
Total runtime: 4325.694 ms

3 个答案:

答案 0 :(得分:6)

您需要的是“跳过扫描”或“loose index scan”。 PostgreSQL的规划器还没有自动实现它们,但你可以通过使用递归查询来欺骗它。

WITH RECURSIVE  t AS (
SELECT min(eventtype) AS eventtype FROM allevents
           UNION ALL
SELECT (SELECT min(eventtype) as eventtype FROM allevents WHERE eventtype > t.eventtype)
   FROM t where t.eventtype is not null
)
select eventtype, (select max(eventtime) from allevents where eventtype=t.eventtype) from t;

可能有一种方法可以将max(eventtime)折叠到递归查询中,而不是在查询之外进行,但如果是这样的话,我还没有碰到它。

这需要一个索引(eventtype,eventtime)才能有效。您可以在事件发生时将其设为DESC,但这不是必需的。只有当eventtype只有几个不同的值(在你的情况下为21个)时,这才有效。

答案 1 :(得分:1)

基于您已经拥有相关索引的问题。

如果升级到Postgres 9.3或(eventtype, eventtime desc)上的索引没有什么区别,那么如果您可以枚举所有事件类型,则重写查询以便它使用相关子查询的情况非常有效手动:

select val as eventtype,
       (select max(eventtime)
        from allevents
        where allevents.eventtype = val
        ) as eventtime
from unnest('{type1,type2,…}'::text[]) as val;

这是我在运行类似查询时得到的计划:

denis=# select version();
                                                              version                                                              
-----------------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 9.3.1 on x86_64-apple-darwin11.4.2, compiled by Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn), 64-bit
(1 row)

测试数据:

denis=# create table test (evttype int, evttime timestamp, primary key (evttype, evttime));
CREATE TABLE
denis=# insert into test (evttype, evttime) select i, now() + (i % 3) * interval '1 min' - j * interval '1 sec' from generate_series(1,10) i, generate_series(1,10000) j;
INSERT 0 100000
denis=# create index on test (evttime, evttype);
CREATE INDEX
denis=# vacuum analyze test;
VACUUM

首先查询:

denis=# explain analyze select evttype, max(evttime) from test group by evttype;                                                    QUERY PLAN                                                     
-------------------------------------------------------------------------------------------------------------------
 HashAggregate  (cost=2041.00..2041.10 rows=10 width=12) (actual time=54.983..54.987 rows=10 loops=1)
   ->  Seq Scan on test  (cost=0.00..1541.00 rows=100000 width=12) (actual time=0.009..15.954 rows=100000 loops=1)
 Total runtime: 55.045 ms
(3 rows)

第二次查询:

denis=# explain analyze select val as evttype, (select max(evttime) from test where test.evttype = val) as evttime from unnest('{1,2,3,4,5,6,7,8,9,10}'::int[]) val;
                                                                        QUERY PLAN                                                                         
-----------------------------------------------------------------------------------------------------------------------------------------------------------
 Function Scan on unnest val  (cost=0.00..48.39 rows=100 width=4) (actual time=0.086..0.292 rows=10 loops=1)
   SubPlan 2
     ->  Result  (cost=0.46..0.47 rows=1 width=0) (actual time=0.024..0.024 rows=1 loops=10)
           InitPlan 1 (returns $1)
             ->  Limit  (cost=0.42..0.46 rows=1 width=8) (actual time=0.021..0.021 rows=1 loops=10)
                   ->  Index Only Scan Backward using test_pkey on test  (cost=0.42..464.42 rows=10000 width=8) (actual time=0.019..0.019 rows=1 loops=10)
                         Index Cond: ((evttype = val.val) AND (evttime IS NOT NULL))
                         Heap Fetches: 0
 Total runtime: 0.370 ms
(9 rows)

答案 2 :(得分:0)

<{1}}上的

索引应该会有所帮助。主键索引上的(eventtype, eventtime desc)reindex。我还建议将事件类型的类型替换为枚举(如果类型的数量是固定的)或int / smallint。这将减少数据和索引的大小,以便查询运行得更快。