performance - PostgreSQL Simple JOIN 很慢

Question

我有一个简单的查询和两个表:

向下钻取

CREATE SEQUENCE drilldown_id_seq;

CREATE TABLE drilldown (
    transactionid bigint NOT NULL DEFAULT nextval('drilldown_id_seq'),
    userid bigint NOT NULL default 0 REFERENCES users(id),
    pathid bigint NOT NULL default 0,
    reqms bigint NOT NULL default 0,
    quems bigint NOT NULL default 0,
    clicktime timestamp default current_timestamp,
    PRIMARY KEY(transactionid)
);

ALTER SEQUENCE drilldown_id_seq OWNED BY drilldown.transactionid;

CREATE INDEX drilldown_idx1 ON drilldown (clicktime);

querystats

CREATE SEQUENCE querystats_id_seq;

CREATE TABLE querystats (
    id bigint NOT NULL DEFAULT nextval('querystats_id_seq'),
    transactionid bigint NOT NULL default 0 REFERENCES drilldown(transactionid),
    querynameid bigint NOT NULL default 0 REFERENCES queryname(id),
    queryms bigint NOT NULL default 0,
    PRIMARY KEY(id)
);

ALTER SEQUENCE querystats_id_seq OWNED BY querystats.id;

CREATE INDEX querystats_idx1 ON querystats (transactionid);
CREATE INDEX querystats_idx2 ON querystats (querynameid);

drilldown 有 150 万条记录，querystats 有 1000 万条记录；当我在两者之间加入时，问题就发生了。

查询

explain analyse
select avg(qs.queryms)
  from querystats qs
  join drilldown d on (qs.transactionid=d.transactionid)
  where querynameid=1;

查询计划

Aggregate  (cost=528596.96..528596.97 rows=1 width=8) (actual time=5213.154..5213.154 rows=1 loops=1)   ->  Hash Join  (cost=274072.53..518367.59 rows=4091746 width=8) (actual time=844.087..3528.788 rows=4117717 loops=1)         Hash Cond: (qs.transactionid = d.transactionid)         ->  Bitmap Heap Scan on querystats qs  (cost=88732.62..210990.44 rows=4091746 width=16) (actual time=309.502..1321.029 rows=4117717 loops=1)               Recheck Cond: (querynameid = 1)               ->  Bitmap Index Scan on querystats_idx2  (cost=0.00..87709.68 rows=4091746 width=0) (actual time=307.916..307.916 rows=4117718 loops=1)                     Index Cond: (querynameid = 1)         ->  Hash  (cost=162842.29..162842.29 rows=1371250 width=8) (actual time=534.065..534.065 rows=1372574 loops=1)               Buckets: 4096  Batches: 64  Memory Usage: 850kB               ->  Index Scan using drilldown_pkey on drilldown d  (cost=0.00..162842.29 rows=1371250 width=8) (actual time=0.015..364.657 rows=1372574 loops=1) Total runtime: 5213.205 ms(11 rows)

I know there are some tuning parameters I can adjust for PostgreSQL, but what I want to know is the query I am doing the most optimal way of joing the two tables?

Or maybe some sort of INNER JOIN? I'm just not sure.

Any pointers are appreciated!

EDIT

database#\d drilldown
                                       Table "public.drilldown"
    Column     |            Type             |                       Modifiers                        
---------------+-----------------------------+--------------------------------------------------------
 transactionid | bigint                      | not null default nextval('drilldown_id_seq'::regclass)
 userid        | bigint                      | not null default 0
 pathid        | bigint                      | not null default 0
 reqms         | bigint                      | not null default 0
 quems         | bigint                      | not null default 0
 clicktime     | timestamp without time zone | default now()
Indexes:
    "drilldown_pkey" PRIMARY KEY, btree (transactionid)
    "drilldown_idx1" btree (clicktime)
Foreign-key constraints:
    "drilldown_userid_fkey" FOREIGN KEY (userid) REFERENCES users(id)
Referenced by:
    TABLE "querystats" CONSTRAINT "querystats_transactionid_fkey" FOREIGN KEY (transactionid) REFERENCES drilldown(transactionid)

database=# \d querystats
                            Table "public.querystats"
    Column     |  Type  |                        Modifiers                        
---------------+--------+---------------------------------------------------------
 id            | bigint | not null default nextval('querystats_id_seq'::regclass)
 transactionid | bigint | not null default 0
 querynameid   | bigint | not null default 0
 queryms       | bigint | not null default 0
Indexes:
    "querystats_pkey" PRIMARY KEY, btree (id)
    "querystats_idx1" btree (transactionid)
    "querystats_idx2" btree (querynameid)
Foreign-key constraints:
    "querystats_querynameid_fkey" FOREIGN KEY (querynameid) REFERENCES queryname(id)
    "querystats_transactionid_fkey" FOREIGN KEY (transactionid) REFERENCES drilldown(transactionid)

所以这是请求的两个表和版本

PostgreSQL 9.1.7 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3, 64-bit

所以这个查询所做的是从每个查询类型 (querynameid) 的 queryms 的所有行值中获取平均值

            name            |         current_setting          |        source        
----------------------------+----------------------------------+----------------------
 application_name           | psql                             | client
 client_encoding            | UTF8                             | client
 DateStyle                  | ISO, MDY                         | configuration file
 default_text_search_config | pg_catalog.english               | configuration file
 enable_seqscan             | off                              | session
 external_pid_file          | /var/run/postgresql/9.1-main.pid | configuration file
 lc_messages                | en_US.UTF-8                      | configuration file
 lc_monetary                | en_US.UTF-8                      | configuration file
 lc_numeric                 | en_US.UTF-8                      | configuration file
 lc_time                    | en_US.UTF-8                      | configuration file
 log_line_prefix            | %t                               | configuration file
 log_timezone               | localtime                        | environment variable
 max_connections            | 100                              | configuration file
 max_stack_depth            | 2MB                              | environment variable
 port                       | 5432                             | configuration file
 shared_buffers             | 24MB                             | configuration file
 ssl                        | on                               | configuration file
 TimeZone                   | localtime                        | environment variable
 unix_socket_directory      | /var/run/postgresql              | configuration file
(19 rows)

我看到 enable_seqscan=off，我没有触及任何设置，这是一个完全默认的安装。

更新

我根据以下评论做了一些修改，结果如下。

explain analyse SELECT (SELECT avg(queryms) AS total FROM querystats WHERE querynameid=3) as total FROM querystats qs JOIN drilldown d ON (qs.transactionid=d.transactionid) WHERE qs.querynameid=3 limit 1;
                                                                       QUERY PLAN                                                                        
---------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=196775.99..196776.37 rows=1 width=0) (actual time=2320.876..2320.876 rows=1 loops=1)
   InitPlan 1 (returns $0)
     ->  Aggregate  (cost=196775.94..196775.99 rows=1 width=8) (actual time=2320.815..2320.815 rows=1 loops=1)
           ->  Bitmap Heap Scan on querystats  (cost=24354.25..189291.69 rows=2993698 width=8) (actual time=226.516..1144.690 rows=2999798 loops=1)
                 Recheck Cond: (querynameid = 3)
                 ->  Bitmap Index Scan on querystats_idx  (cost=0.00..23605.83 rows=2993698 width=0) (actual time=225.119..225.119 rows=2999798 loops=1)
                       Index Cond: (querynameid = 3)
   ->  Nested Loop  (cost=0.00..1127817.12 rows=2993698 width=0) (actual time=2320.876..2320.876 rows=1 loops=1)
         ->  Seq Scan on drilldown d  (cost=0.00..76745.10 rows=1498798 width=8) (actual time=0.009..0.009 rows=1 loops=1)
         ->  Index Scan using querystats_idx on querystats qs  (cost=0.00..0.60 rows=2 width=8) (actual time=0.045..0.045 rows=1 loops=1)
               Index Cond: ((querynameid = 3) AND (transactionid = d.transactionid))
 Total runtime: 2320.940 ms
(12 rows)

Answer 1

它的行为就像您设置了 enable_seqscan = off 一样，因为它使用索引扫描来填充哈希表。除非作为诊断步骤，否则切勿关闭任何计划程序选项，如果您要显示计划，请显示使用的任何选项。这可以运行以显示很多有用的信息:

SELECT version();
SELECT name, current_setting(name), source
  FROM pg_settings
  WHERE source NOT IN ('default', 'override');

如果您告诉我们有关运行时环境的信息，尤其是机器上的 RAM 容量、存储系统的外观以及数据库的大小(或者更好，事件数据集 数据库中经常引用的数据)。

作为粗略的分解，5.2 秒分解为:

1. 1.3 秒找到符合您的选择标准的 4,117,717 querystats 行。
2. 2.3 秒随机匹配那些与 drilldown 记录。
3. 1.6 秒通过 4,117,717 行并计算平均值。

因此，即使您似乎削弱了它使用最快计划的能力，它只需要 1.26 微秒(百万分之一秒)来定位每一行，将其连接到另一行，并将其用于计算平均的。这在绝对基础上并不算太糟糕，但您几乎可以肯定会得到一个稍微更快的计划。

首先，如果您使用的是 9.2.x，其中 x 小于 3，请立即升级到 9.2.3。某些类型的计划存在性能退化，已在最近的版本中修复，这可能会影响此查询。一般来说，try to stay up-to-date on minor releases (版本号更改超过第二个点)。

您可以在单个 session 中测试不同的计划，方法是仅在该连接上设置计划因素并运行您的查询(或对其执行 EXPLAIN)。尝试这样的事情:

SET seq_page_cost = 0.1;
SET random_page_cost = 0.1;
SET cpu_tuple_cost = 0.05;
SET effective_cache_size = '3GB'; -- actually use shared_buffers plus OS cache

确保所有enable_ 设置都on。