15

有两个通过 id 链接的表:

item_tbl (id)
link_tbl (item_id)

中的某些记录item_tbl没有匹配的行link_tbl。将计算其数量的选择将是:

SELECT COUNT(*)
FROM link_tbl lnk LEFT JOIN item_tbl itm ON lnk.item_id=itm.id
WHERE itm.id IS NULL

我想从中删除那些孤立记录(那些在另一个表中没有匹配的记录),link_tbl但我能想到的唯一方法是:

DELETE FROM link_tbl lnk
WHERE lnk.item_id NOT IN (SELECT itm.id FROM item_tbl itm)

中的16,844,347
孤儿记录中的link_tbl
3,033,811条中有item_tbl
262,086,253link_tbl条记录。
服务器有 4GB RAM 和 8 核 CPU。

EXPLAIN DELETE FROM link_tbl lnk
WHERE lnk.item_id NOT IN (SELECT itm.id FROM item_tbl itm)

回报:

Delete on link lnk  (cost=0.00..11395249378057.98 rows=131045918 width=6)
->  Seq Scan on link lnk  (cost=0.00..11395249378057.98 rows=131045918 width=6)
     Filter: (NOT (SubPlan 1))
     SubPlan 1
       ->  Materialize  (cost=0.00..79298.10 rows=3063207 width=4)
             ->  Seq Scan on item itm  (cost=0.00..52016.07 rows=3063207 width=4)

问题是:

  1. 有没有更好的方法来删除孤儿记录link_tbl
  2. 上面的解释有多准确,或者删除这些记录需要多长时间?

    • 编辑:根据 Erwin Brandstetter 的评论修复。
    • 编辑:PostgreSql 版本是 9.1
    • 编辑:postgresql.config 的某些部分
      1. 共享缓冲区 = 368MB
      2. 临时缓冲区 = 32MB
      3. 工作内存 = 32MB
      4. 维护工作内存 = 64MB
      5. 最大堆栈深度 = 6MB
      6. fsync = 关闭
      7. 同步提交 = 关闭
      8. full_page_writes = 关闭
      9. wal_buffers = 16MB
      10. wal_writer_delay = 5000 毫秒
      11. 提交延迟 = 10
      12. commit_siblings = 10
      13. 有效缓存大小 = 1600MB

解析度:

谢谢大家的建议,很有帮助。我终于使用了 Erwin Brandstetter https://stackoverflow.com/a/15959896/1331340建议的删除,但我稍微调整了一下:

DELETE FROM link_tbl lnk
WHERE lnk.item_id BETWEEN 0 AND 10000
  AND lnk.item_id NOT IN (SELECT itm.id FROM item itm
                          WHERE itm.id BETWEEN 0 AND 10000)

我比较了 NOT IN 和 NOT EXISTS 的结果,输出如下,尽管我使用 COUNT 而不是 DELETE,我认为应该是相同的(我的意思是为了进行相对比较):

EXPLAIN ANALYZE SELECT COUNT(*) 
FROM link_tbl lnk
WHERE lnk.item_id BETWEEN 0 AND 20000
  AND lnk.item_id NOT IN (SELECT itm.id
                          FROM item_tbl itm
                          WHERE itm.id BETWEEN 0 AND 20000);

QUERY PLAN
Aggregate  (cost=6002667.56..6002667.57 rows=1 width=0) (actual time=226817.086..226817.088 rows=1 loops=1)
->  Seq Scan on link_tbl lnk  (cost=1592.50..5747898.65 rows=101907564 width=0) (actual time=206.029..225289.570 rows=566625 loops=1)
     Filter: ((item_id >= 0) AND (item_id <= 20000) AND (NOT (hashed SubPlan 1)))
     SubPlan 1
       ->  Index Scan using item_tbl_pkey on item_tbl itm  (cost=0.00..1501.95 rows=36221 width=4) (actual time=0.056..99.266 rows=17560 loops=1)
             Index Cond: ((id >= 0) AND (id <= 20000))
Total runtime: 226817.211 ms


EXPLAIN ANALYZE SELECT COUNT(*)
FROM link_tbl lnk WHERE lnk.item_id>0 AND lnk.item_id<20000
  AND NOT EXISTS (SELECT 1 FROM item_tbl itm WHERE itm.id=lnk.item_id);

QUERY PLAN
Aggregate  (cost=8835772.00..8835772.01 rows=1 width=0)
   (actual time=1209235.133..1209235.135 rows=1 loops=1)
->  Hash Anti Join  (cost=102272.16..8835771.99 rows=1 width=0)
   (actual time=19315.170..1207900.612 rows=566534 loops=1)
     Hash Cond: (lnk.item_id = itm.id)
     ->  Seq Scan on link_tbl lnk  (cost=0.00..5091076.55 rows=203815128 width=4) (actual time=0.016..599147.604 rows=200301872 loops=1)
           Filter: ((item_id > 0) AND (item_id < 20000))
     ->  Hash  (cost=52016.07..52016.07 rows=3063207 width=4) (actual time=19313.976..19313.976 rows=3033811 loops=1)
           Buckets: 131072  Batches: 4  Memory Usage: 26672kB
           ->  Seq Scan on item_tbl itm  (cost=0.00..52016.07 rows=3063207 width=4) (actual time=0.013..9274.158 rows=3033811 loops=1)
Total runtime: 1209260.228 ms

NOT EXISTS 慢了 5 倍。

数据的实际删除并没有花很长时间,只要我担心,我就可以分5批删除(10000-20000、20000-100000、100000-200000、200000-1000000和1000000-1755441)。起初我发现了 max item_id,我只需要经过一半的桌子。

当我尝试 NOT IN 或 EXISTS without the range (with select count) 它甚至没有完成时,我让它在晚上运行,它仍然在早上运行。

我想我正在寻找使用 Wildplasser 的答案https://stackoverflow.com/a/15988033/1331340中的 DELETE,但为时已晚。

DELETE FROM one o
USING (
    SELECT o2.id
    FROM one o2
    LEFT JOIN two t ON t.one_id = o2.id
    WHERE t.one_id IS NULL
    ) sq
WHERE sq.id = o.id
    ;
4

3 回答 3

19

我对四个典型查询进行了基准测试,{work_mem, Effective_cache_size, random_page_cost} 的设置不同,这些设置对所选计划的影响最大。我首先使用默认设置进行了“运行”以预热缓存。注意:测试集足够小以允许所有需要的页面都存在于缓存中。

测试集

SET search_path=tmp;

/************************/
DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp ;
SET search_path=tmp;

CREATE TABLE one
        ( id SERIAL NOT NULL PRIMARY KEY
        , payload varchar
        );

CREATE TABLE two
        ( id SERIAL NOT NULL PRIMARY KEY
        , one_id INTEGER REFERENCES one
        , payload varchar
        );

INSERT INTO one (payload) SELECT 'Text_' || gs::text FROM generate_series(1,30000) gs;
INSERT INTO two (payload) SELECT 'Text_' || gs::text FROM generate_series(1,30000) gs;


UPDATE two t
SET one_id = o.id
FROM one o
WHERE o.id = t.id
AND random() < 0.1;

INSERT INTO two (one_id,payload) SELECT one_id,payload FROM two;
INSERT INTO two (one_id,payload) SELECT one_id,payload FROM two;
INSERT INTO two (one_id,payload) SELECT one_id,payload FROM two;

VACUUM ANALYZE one;
VACUUM ANALYZE two;
/***************/

查询:

\echo NOT EXISTS()
EXPLAIN ANALYZE
DELETE FROM one o
WHERE NOT EXISTS ( SELECT * FROM two t
        WHERE t.one_id = o.id
        );

\echo NOT IN()
EXPLAIN ANALYZE 
DELETE FROM one o
WHERE o.id NOT IN ( SELECT one_id FROM two t)
        ;

\echo USING (subquery self LEFT JOIN two where NULL)
EXPLAIN ANALYZE
DELETE FROM one o
USING (
        SELECT o2.id
        FROM one o2
        LEFT JOIN two t ON t.one_id = o2.id
        WHERE t.one_id IS NULL
        ) sq
WHERE sq.id = o.id
        ;

\echo USING (subquery self WHERE NOT EXISTS(two)))
EXPLAIN ANALYZE
DELETE FROM one o
USING (
        SELECT o2.id
        FROM one o2
        WHERE NOT EXISTS ( SELECT *
                FROM two t WHERE t.one_id = o2.id
                )
        ) sq
WHERE sq.id = o.id
        ;

结果(总结)

                        NOT EXISTS()    NOT IN()        USING(LEFT JOIN NULL)   USING(NOT EXISTS)
1) rpc=4.0.csz=1M wmm=64        80.358  14389.026       77.620                  72.917
2) rpc=4.0.csz=1M wmm=64000     60.527  69.104          51.851                  51.004
3) rpc=1.5.csz=1M wmm=64        69.804  10758.480       80.402                  77.356
4) rpc=1.5.csz=1M wmm=64000     50.872  69.366          50.763                  53.339
5) rpc=4.0.csz=1G wmm=64        84.117  7625.792        69.790                  69.627
6) rpc=4.0.csz=1G wmm=64000     49.964  67.018          49.968                  49.380
7) rpc=1.5.csz=1G wmm=64        68.567  3650.008        70.283                  69.933
8) rpc=1.5.csz=1G wmm=64000     49.800  67.298          50.116                  50.345

legend: 
rpc := "random_page_cost"
csz := "effective_cache_size"
wmm := "work_mem"

如您所见,该NOT IN()变体对work_mem. 同意,设置 64(KB) 非常低,但这个“或多或少”对应于大型数据集,它也不适合散列表。

EXTRA:在预热阶段,NOT EXISTS()查询遭受了极端的 FK 触发器争用。这似乎是与真空守护程序冲突的结果,该守护程序在表设置后仍处于活动状态。:

PostgreSQL 9.1.2 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1, 64-bit
NOT EXISTS()
                                                           QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------
 Delete on one o  (cost=6736.00..7623.94 rows=27962 width=12) (actual time=80.596..80.596 rows=0 loops=1)
   ->  Hash Anti Join  (cost=6736.00..7623.94 rows=27962 width=12) (actual time=49.174..61.327 rows=27050 loops=1)
         Hash Cond: (o.id = t.one_id)
         ->  Seq Scan on one o  (cost=0.00..463.00 rows=30000 width=10) (actual time=0.003..5.156 rows=30000 loops=1)
         ->  Hash  (cost=3736.00..3736.00 rows=240000 width=10) (actual time=49.121..49.121 rows=23600 loops=1)
               Buckets: 32768  Batches: 1  Memory Usage: 1015kB
               ->  Seq Scan on two t  (cost=0.00..3736.00 rows=240000 width=10) (actual time=0.006..33.790 rows=240000 loops=1)
 Trigger for constraint two_one_id_fkey: time=467720.117 calls=27050
 Total runtime: 467824.652 ms
(9 rows)
于 2013-04-13T12:56:54.513 回答
8

首先:你的文字说:

我想从item_tbl.

但是你的代码说:

DELETE FROM link_tbl lnk ...

更新: 在重读 QI 时,您更有可能想要删除link_tbl. 行数指向那个方向。@Lucas ) 查询在这种情况下是正确的。但恐怕,NOT EXISTS实际上比这种情况要慢。NOT IN

为了验证我运行了一个测试用例,这与您的设置很相似。不能让它变得更大,否则 SQLfiddle 会超时。

-> SQLfiddle

NOT EXISTS对于相反的情况会更快。(我也测试过。)EXISTS更适合测试“多”端。通常,获得的好处EXISTS多于获得的好处NOT EXISTS-无论如何,该表格必须检查整个表格。证明某事不存在比证明某事存在要难得多。这个普遍真理也适用于数据库。

分而治之

此操作适合拆分。特别是如果您有并发事务(但即使没有),我会考虑将其DELETE分成几个部分,以便事务可以COMMIT在相当长的时间后进行。

就像是:

DELETE FROM link_tbl l
WHERE  l.item_id < 1000000
AND    l.item_id NOT IN (SELECT i.id FROM item_tbl i)

然后l.item_id BETWEEN 100001 AND 200000,等等。

您不能使用函数自动执行此操作。这会将所有内容都包装到交易中并违背目的。因此,您必须从任何客户端编写脚本。
或者你可以使用 ..

数据库链接

这个附加模块允许您在任何数据库中运行单独的事务,包括正在运行的数据库。这可以通过持久连接来完成,这应该消除大部分连接开销。有关如何安装它的说明:
如何在 PostgreSQL 中使用(安装)dblink?

DO会完成这项工作(PostgreSQL 9.0 或更高版本)。DELETE一次为 50000运行 100 个命令item_id

DO
$$
DECLARE
   _sql text;
BEGIN

PERFORM dblink_connect('port=5432 dbname=mydb');  -- your connection parameters

FOR i IN 0 .. 100
LOOP
   _sql := format('
   DELETE FROM link_tbl l
   WHERE  l.item_id BETWEEN %s AND %s
   AND    l.item_id NOT IN (SELECT i.id FROM item_tbl i)'
   , (50000 * i)::text
   , (50000 * (i+1))::text);

   PERFORM  dblink_exec(_sql);
END LOOP;

PERFORM dblink_disconnect();

END
$$

如果脚本应该被中断:dblink_connect将它执行的内容写入数据库日志,这样您就可以看到已经完成的内容。

于 2013-04-11T22:21:13.230 回答
5

也许是这样:

DELETE FROM link_tbl lnk
WHERE NOT EXISTS
  ( SELECT 1 FROM item_tbl item WHERE item.id = lnk.item_id );

当处理大量记录时,创建一个临时表,执行INSERT INTO SELECT * FROM ...然后删除原始表,重命名临时表,然后添加索引会更有效......

于 2013-04-11T21:28:22.980 回答