我正在运行Postgressql 9.2并且具有12列关系,大约6,700,000行.它包含3D空间中的节点,每个节点都引用一个用户(创建它的用户).要查询哪个用户创建了多少个节点,我将执行以下操作(添加了解释分析以获取更多信息)：

EXPLAIN ANALYZE SELECT user_id,count(user_id) FROM treenode WHERE project_id=1 GROUP BY user_id;
                                                    QUERY PLAN                                                         
---------------------------------------------------------------------------------------------------------------------------
 HashAggregate  (cost=253668.70..253669.07 rows=37 width=8) (actual time=1747.620..1747.623 rows=38 loops=1)
   ->  Seq Scan on treenode  (cost=0.00..220278.79 rows=6677983 width=8) (actual time=0.019..886.803 rows=6677983 loops=1)
         Filter: (project_id = 1)
 Total runtime: 1747.653 ms

如您所见,这需要大约1.7秒.考虑到数据量,这不是太糟糕,但我想知道这是否可以改进.我试图在用户列上添加BTree索引,但这没有任何帮助.

你有其他建议吗？

为了完整起见,这是包含所有索引的完整表定义(没有外键约束,引用和触发器)：

Column     |           Type           |                      Modifiers                    
---------------+--------------------------+------------------------------------------------------
 id            | bigint                   | not null default nextval('concept_id_seq'::regclass)
 user_id       | bigint                   | not null
 creation_time | timestamp with time zone | not null default now()
 edition_time  | timestamp with time zone | not null default now()
 project_id    | bigint                   | not null
 location      | double3d                 | not null
 reviewer_id   | integer                  | not null default (-1)
 review_time   | timestamp with time zone |
 editor_id     | integer                  |
 parent_id     | bigint                   |
 radius        | double precision         | not null default 0
 confidence    | integer                  | not null default 5
 skeleton_id   | bigint                   |
Indexes:
    "treenode_pkey" PRIMARY KEY,btree (id)
    "treenode_id_key" UNIQUE CONSTRAINT,btree (id)
    "skeleton_id_treenode_index" btree (skeleton_id)
    "treenode_editor_index" btree (editor_id)
    "treenode_location_x_index" btree (((location).x))
    "treenode_location_y_index" btree (((location).y))
    "treenode_location_z_index" btree (((location).z))
    "treenode_parent_id" btree (parent_id)
    "treenode_user_index" btree (user_id)

编辑：这是结果,当我使用@ypercube提出的查询(和索引)时(查询大约需要5.3秒而没有EXPLAIN ANALYZE)：

EXPLAIN ANALYZE SELECT u.id,( SELECT COUNT(*) FROM treenode AS t WHERE t.project_id=1 AND t.user_id = u.id ) AS number_of_nodes FROM auth_user As u;
                                                                        QUERY PLAN                                                                     
----------------------------------------------------------------------------------------------------------------------------------------------------------
 Seq Scan on auth_user u  (cost=0.00..6987937.85 rows=46 width=4) (actual time=29.934..5556.147 rows=46 loops=1)
   SubPlan 1
     ->  Aggregate  (cost=151911.65..151911.66 rows=1 width=0) (actual time=120.780..120.780 rows=1 loops=46)
           ->  Bitmap Heap Scan on treenode t  (cost=4634.41..151460.44 rows=180486 width=0) (actual time=13.785..114.021 rows=145174 loops=46)
                 Recheck Cond: ((project_id = 1) AND (user_id = u.id))
                 Rows Removed by Index Recheck: 461076
                 ->  Bitmap Index Scan on treenode_user_index  (cost=0.00..4589.29 rows=180486 width=0) (actual time=13.082..13.082 rows=145174 loops=46)
                       Index Cond: ((project_id = 1) AND (user_id = u.id))
 Total runtime: 5556.190 ms
(9 rows)

Time: 5556.804 ms

编辑2：这是结果,当我使用project_id上的索引,user_id(但尚未进行模式优化)@ erwin-brandstetter建议时(查询以与原始查询相同的速度运行1.5秒)：

EXPLAIN ANALYZE SELECT user_id,count(user_id) as ct FROM treenode WHERE project_id=1 GROUP BY user_id;
                                                        QUERY PLAN                                                      
---------------------------------------------------------------------------------------------------------------------------
 HashAggregate  (cost=253670.88..253671.24 rows=37 width=8) (actual time=1807.334..1807.339 rows=38 loops=1)
   ->  Seq Scan on treenode  (cost=0.00..220280.62 rows=6678050 width=8) (actual time=0.183..893.491 rows=6678050 loops=1)
         Filter: (project_id = 1)
 Total runtime: 1807.368 ms
(4 rows)

SELECT user_id,count(*) AS ct
FROM   treenode
WHERE  project_id = 1
GROUP  BY user_id;

CREATE INDEX treenode_project_id_user_id_index ON treenode (project_id,user_id);

"treenode_pkey" PRIMARY KEY,btree (id)

  

  
   
  "treenode_id_key" UNIQUE CONSTRAINT,btree (id)