多列优化Postgresql内部联接（特别是自联接）

我正在运行PostgresSQL 9.6.2并且有一个包含7列大约2,900,000行的表。该表是临时的，它是主题重复数据删除过程的一部分，它旨在根据不同的规则集将新的id（s_id_new）分配给相同的主题。总的来说，我执行的内部连接大约10-12次，每次都是类似的，但稍有不同的数据子集/不同的WHERE条件/不同的连接列。多列优化Postgresql内部联接（特别是自联接）

现在，查询效率很低，没有完成（必须在2小时后取消）。

为了优化的目的，我创建了一个数据子集（50000行）。

\d subject_subset; 
    Column  |   Type   | Modifiers 
----------------+------------------------+----------- 
s_id   | text     | 
surname_clean | character varying(20) | 
name_clean  | character varying(20) | 
fullname_clean | character varying(100) | 
id1   | character varying(20) | 
id2   | character varying(20) | 
id3   | character varying(20) | 
s_id_new  | character varying(20) | 
Indexes: 
    "subject_subset_s_id_new_idx" btree (s_id_new)

我想查询优化

select s_id_new, max(I_s_id) as s_id_deduplicated 
from (select a.*, b.s_id_new as I_s_id 
       from public.subject_subset a 
       inner join public.subject_subset b on a.surname_clean=b.surname_clean 
       and a.id2=b.id2 
       where 
        a.id1 is null 
        and a.id2 is not null 
        and a.surname_clean is not null) h 
group by s_id_new; 



The result of the EXPLAIN ANALYZE: 
https://explain.depesz.com/s/7knH 

"GroupAggregate (cost=5616.65..5620.39 rows=142 width=90) (actual time=32542.127..46938.858 rows=2889 loops=1)" 
" Group Key: a.s_id_new" 
" -> Sort (cost=5616.65..5617.42 rows=310 width=116) (actual time=32542.116..43194.626 rows=18356220 loops=1)" 
"  Sort Key: a.s_id_new" 
"  Sort Method: external merge Disk: 531760kB" 
"  -> Hash Join (cost=1114.72..5603.82 rows=310 width=116) (actual time=13.159..4892.011 rows=18356220 loops=1)" 
"    Hash Cond: (((b.surname_clean)::text = (a.surname_clean)::text) AND ((b.id2)::text = (a.id2)::text))" 
"    -> Seq Scan on subject_subset b (cost=0.00..1111.00 rows=50000 width=174) (actual time=0.011..10.775 rows=50000 loops=1)" 
"    -> Hash (cost=1111.00..1111.00 rows=248 width=174) (actual time=13.137..13.137 rows=15044 loops=1)" 
"     Buckets: 16384 (originally 1024) Batches: 1 (originally 1) Memory Usage: 1151kB" 
"     -> Seq Scan on subject_subset a (cost=0.00..1111.00 rows=248 width=174) (actual time=0.005..9.330 rows=15044 loops=1)" 
"       Filter: ((id1 IS NULL) AND (id2 IS NOT NULL) AND (surname_clean IS NOT NULL))" 
"       Rows Removed by Filter: 34956" 
"Planning time: 0.236 ms" 
"Execution time: 47013.839 ms"

至于我可以看到它的子查询的是造成的问题，当全表进行排序消耗的超大空间排序，但我无法弄清楚如何优化它。

性能略有提高的唯一原因是分配新的整数ID与dense_rank，但它是不够的。

来源

2017-06-02 Dominix

如果你用文字解释这个特定查询试图完成的目标，这将有所帮助。否则，我们必须尝试根据查询来猜测任务。 – 2017-06-02 12:49:20

该查询旨在重复删除主体 - 公司和自然人 - 为其分配相同的ID。两个具有相同文档ID的Jonh Smiths在数据库中具有不同的ID（s_id） - > Code为他们分配一个新的ID =他现在拥有的s_id的最大值。有时辅助数据用于重复数据删除（地址，电话等），但想法保持不变。 – Dominix

大排序正在杀死你。

我有三个建议：

运行ANALYZE subject_subset来收集表表统计信息。不会为临时表自动收集统计信息，您的情况下估算值相当不重要。

也许这足以让它变得更好！

尝试索引(id2, surname_clean, s_id_new)，这将有助于嵌套循环连接（不知道这是否更快）。

你可以尝试横向加入像

SELECT a.s_id_new, 
     max(b.i_s_id) AS s_id_deduplicated 
FROM subject_subset a 
    CROSS JOIN LATERAL (SELECT s_id_new AS i_s_id 
         FROM subject_subset 
         WHERE a.surname_clean = surname_clean 
         AND a.id2 = id2 
         ORDER BY s_id_new DESC 
         LIMIT 1 
        ) b 
GROUP BY a.s_id_new;

嵌套循环连接将是昂贵的，但那种要快。

坚持一个哈希联接，但减少的行数：

SELECT a.s_id_new, 
     max(b.i_s_id) AS s_id_deduplicated 
FROM subject_subset a 
    JOIN (SELECT surname_clean, id2, 
       max(s_id_new) AS i_s_id 
     FROM subject_subset 
     GROUP BY surname_clean, id2 
     ) b 
     USING (surname_clean, id2) 
WHERE a.id1 IS NULL 
    AND a.id2 IS NOT NULL 
    AND a.surname_clean IS NOT NULL 
GROUP BY a.s_id_new;

也许在(surname_clean, id2)索引可以帮助，不知道。

来源

2017-06-02 14:26:30

多列优化Postgresql内部联接（特别是自联接）

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