sql – 在另一个表中没有匹配的DELETE记录

发布时间：2020-12-12 16:30:41 所属栏目：MsSql教程来源：网络整理

导读：有两个表链接到一个id： item_tbl (id)link_tbl (item_id) 在item_tbl中有一些记录在link_tbl中没有匹配的行.将会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.idWHERE itm.id IS NULL 我想从link_tb

有两个表链接到一个id：

item_tbl (id)
link_tbl (item_id)

在item_tbl中有一些记录在link_tbl中没有匹配的行.将会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link_tbl中删除那些孤立的记录(那些没有匹配的记录),但我想到的唯一方法是：

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

有
link_tbl中有262,086,253条记录
item_tbl中的3,033,811
link_tbl中的16,844,347个孤立记录.
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)

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：

>有没有更好的方法如何从link_tbl删除孤立记录？
>上面的解释有多准确,删除这些记录可能需要多长时间？

>编辑：根据Erwin Brandstetter评论修正.
>编辑：PostgreSql版本是9.1
>编辑：postgresql.config的一些部分

> shared_buffers = 368MB
> temp_buffers = 32MB
> work_mem = 32MB
> maintenance_work_mem = 64MB
> max_stack_depth = 6MB
> fsync = off
> synchronization_commit = off
> full_page_writes =关闭
> wal_buffers = 16MB
> wal_writer_delay = 5000ms
> commit_delay = 10
> commit_siblings = 10
> effective_cache_size = 1600MB

解析度：

感谢大家的意见,这是非常有帮助的.我终于使用了Erwin Brandstetter https://stackoverflow.com/a/15959896/1331340建议的删除,但我调整了一点：

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,输出结果如下,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5倍.

实际删除的数据并不需要,只要我担心,我能够以5批(10000-20000,20000-100000,100000-200000,200000-1000000和1000000-1755441)删除它.起初我发现最大的item_id,我只需要经过一半的桌子.

当我尝试没有IN或EXISTS没有范围(选择计数)它甚至没有完成,我让它运行在夜间,它仍然在早上运行.

我想我正在用野猫寻求https://stackoverflow.com/a/15988033/1331340的DELETE,但是来得太晚了.

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
    ;

解决方法

我对四个典型查询进行了基准测试,对于{work_mem,effective_cache_size,random_page_cost}的不同设置,这些设置对所选计划的影响最大.我首先使用我的默认设置进行“运行”以加快缓存.
注意：测试集足够小以允许所有需要的页面存在于缓存中.

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 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"

如你所见,NOT IN()变体对于work_mem的不足非常敏感.同意,设置64(KB)非常低,但这个“或多或少”对应于大型数据集,也不适用于哈希表.

EXTRA：在暖身阶段,NOT EXISTS()查询遭受极端的FK触发争用.这是因为与真空脱水机的冲突的结果,在桌面设置后仍然活跃.

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)

（编辑：李大同）

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