以下对 Postgres 数据库的查询有时非常慢(4,000+ms):
EXPLAIN ANALYZE SELECT "sms".* FROM "sms" WHERE "sms"."From" = 'NUMBER1' AND "sms"."To" = 'NUMBER2' AND "sms"."SmsMessageSid" = 'UNIQUE_ID' ORDER BY "sms"."id" ASC LIMIT 1;
当我运行 psql 并分析查询时，结果如下:
Limit (cost=5045.12..5045.12 rows=1 width=609) (actual time=57.011..57.011 rows=0 loops=1)
-> Sort (cost=5045.12..5045.12 rows=1 width=609) (actual time=57.009..57.009 rows=0 loops=1)
Sort Key: id
Sort Method: quicksort Memory: 25kB
-> Bitmap Heap Scan on sms (cost=46.02..5045.11 rows=1 width=609) (actual time=56.993..56.993 rows=0 loops=1)
Recheck Cond: (("To")::text = 'NUMBER1'::text)
Filter: ((("From")::text = 'NUMBER2'::text) AND (("SmsMessageSid")::text = 'UNIQUE_ID'::text))
Rows Removed by Filter: 2501
Heap Blocks: exact=1230
-> Bitmap Index Scan on "index_sms_on_To" (cost=0.00..46.02 rows=2623 width=0) (actual time=0.345..0.345 rows=2566 loops=1)
Index Cond: (("To")::text = 'NUMBER1'::text)
我创建了一个索引如下:
add_index "sms", ["id", "From", "To", "SmsMessageSid"], name: "on_id_from_to_sms_message_sid"
但是分析方法没有达到索引。我是否包含了错误的列？
最佳答案
id 应该在最后。喜欢
add_index "sms", ["From", "To", "SmsMessageSid", "id"], name: "on_id_from_to_sms_message_sid"
通常:首先是相等，然后是范围。相关:
id 仍应附加，除非每个 ("From", "To", "SmsMessageSid") 组合的行很少。这样，Postgres 可以直接从索引的顶部选取第一行 - 在索引扫描中，而不是像您现在看到的位图索引扫描。
相关:
关于ruby-on-rails - 查询未命中索引 - 这些列是否适合索引？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/56158884/