[英]Update using join on big table - performance tips?
一直在為這個更新而苦苦掙扎,永遠不會完成
update votings v
set voter_id = (select pv.number from voters pv WHERE pv.person_id = v.person_id);
當前表有 96M 條記錄
select count(0) from votings;
count
----------
96575239
(1 registro)
更新顯然正在使用索引
explain update votings v
set voter_id = (select pv.number from voters pv WHERE pv.rl_person_id = v.person_id);
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------
Update on votings v (cost=0.00..788637465.40 rows=91339856 width=1671)
-> Seq Scan on votings v (cost=0.00..788637465.40 rows=91339856 width=1671)
SubPlan 1
-> Index Scan using idx_voter_rl_person_id on voters pv (cost=0.56..8.58 rows=1 width=9)
Index Cond: (rl_person_id = v.person_id)
(5 registros)
這是我的投票索引
Índices:
"votings_pkey" PRIMARY KEY, btree (id)
"votings_election_id_voter_id_key" UNIQUE CONSTRAINT, btree (election_id, person_id)
"votings_external_id_external_source_key" UNIQUE CONSTRAINT, btree (external_id, external_source)
"idx_votings_updated_at" btree (updated_at DESC)
"idx_votings_vote_party" btree (vote_party)
"idx_votings_vote_state_vote_party" btree (vote_state, vote_party)
"idx_votings_voter_id" btree (person_id)
Restrições de chave estrangeira:
"votings_election_id_fkey" FOREIGN KEY (election_id) REFERENCES elections(id)
"votings_voter_id_fkey" FOREIGN KEY (person_id) REFERENCES people_all(id)
伙計們,誰在更新運行緩慢方面發揮最大作用? 行數或正在使用的連接?
我可以在這里提出的一個建議是對子查詢查找使用覆蓋索引:
CREATE INDEX idx_cover ON voters (person_id, number);
雖然在 select 的上下文中,這可能不會比您當前在person_id
上的索引有太大優勢,但在更新的上下文中,它可能更重要。 原因是對於更新,此索引可能會減輕 Postgres 在更新之前必須在其 state 中創建和維護原始表的副本。
如果您在voting
中實際有 91339856 行,那么對voters
的 91339856 次索引掃描肯定是主要的成本因素。 順序掃描會更快。
如果您不強制 PostgreSQL 進行嵌套循環連接,您可能會提高性能:
UPDATE votings
SET voter_id = voters.number
FROM voters
WHERE votings.person_id = voters.person_id;
更新表中的所有行將非常昂貴。 我建議重新創建表:
create temp_votings as
select v.*, vv.vote_id
from votings v join
voters vv
on vv.person_id = v.person_id;
對於此查詢,您需要一個關於votes(person_id, vote_id)
的索引。 我猜person_id
可能已經是主鍵了; 如果是這樣,則不需要額外的索引。
然后,您可以替換現有表——但首先要備份它:
truncate table votings;
insert into votings ( . . . ) -- list columns here
select . . . -- and the same columns here
from temp_votings;
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