一般在查詢數據的時候,咱們都會用到limit
分頁,由於這樣避免了全表查詢,會提升查詢效率。可是在一個表的數據量多了以後,分頁查詢會明細的變慢mysql
建立測試表card 2000萬數據
mysql> select count(*) from card; +----------+ | count(*) | +----------+ | 20000000 | +----------+ 1 row in set (0.00 sec)
-首先測試前1000行查詢速度sql
mysql> select * from card limit 1000,10; +---------+--------------------------------------+ | card_id | card_number | +---------+--------------------------------------+ | 1001 | 13fc90a6-2e3b-11e8-ae62-9c5c8e6e37cf | | 1002 | 13fc923e-2e3b-11e8-ae62-9c5c8e6e37cf | | 1003 | 13fc93d5-2e3b-11e8-ae62-9c5c8e6e37cf | | 1004 | 13fc956a-2e3b-11e8-ae62-9c5c8e6e37cf | | 1005 | 13fc9702-2e3b-11e8-ae62-9c5c8e6e37cf | | 1006 | 13fc9899-2e3b-11e8-ae62-9c5c8e6e37cf | | 1007 | 13fc9a31-2e3b-11e8-ae62-9c5c8e6e37cf | | 1008 | 13fc9bc6-2e3b-11e8-ae62-9c5c8e6e37cf | | 1009 | 13fc9d5e-2e3b-11e8-ae62-9c5c8e6e37cf | | 1010 | 13fc9ef5-2e3b-11e8-ae62-9c5c8e6e37cf | +---------+--------------------------------------+ 10 rows in set (0.00 sec)
-測試100萬以後的查詢性能
mysql> select * from card limit 1000000,10; +---------+--------------------------------------+ | card_id | card_number | +---------+--------------------------------------+ | 1000001 | 2d87021a-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000002 | 2d8703ac-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000003 | 2d87053b-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000004 | 2d8706cd-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000005 | 2d87085f-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000006 | 2d8709f1-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000007 | 2d870b83-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000008 | 2d870d18-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000009 | 2d870eaa-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000010 | 2d871039-2e3b-11e8-ae62-9c5c8e6e37cf | +---------+--------------------------------------+ 10 rows in set (0.18 sec)
-測試1000萬以後的查詢測試
mysql> select * from card limit 10000000,10; +----------+--------------------------------------+ | card_id | card_number | +----------+--------------------------------------+ | 10000001 | b11ad76c-2e49-11e8-ae62-9c5c8e6e37cf | | 10000002 | b11aefd5-2e49-11e8-ae62-9c5c8e6e37cf | | 10000003 | b11af868-2e49-11e8-ae62-9c5c8e6e37cf | | 10000004 | b11b0031-2e49-11e8-ae62-9c5c8e6e37cf | | 10000005 | b11b07ad-2e49-11e8-ae62-9c5c8e6e37cf | | 10000006 | b11b0f0f-2e49-11e8-ae62-9c5c8e6e37cf | | 10000007 | b11b1669-2e49-11e8-ae62-9c5c8e6e37cf | | 10000008 | b11b1db2-2e49-11e8-ae62-9c5c8e6e37cf | | 10000009 | b11b24fa-2e49-11e8-ae62-9c5c8e6e37cf | | 10000010 | b11b2c37-2e49-11e8-ae62-9c5c8e6e37cf | +----------+--------------------------------------+ 10 rows in set (1.29 sec)
能夠看到越到後面查詢效率會越低。由於在查詢100萬以後的數據的時候,mysql會首先查詢100萬零10條數據,而後截取後面的十條數據。這些就形成的性能的下降。
那麼怎麼去避免這個掃描100萬條數據呢。咱們能夠明確的知道,100萬以後的主鍵是大於100萬的。因此咱們能夠將sql改寫,讓其用到索引,下降掃描的行數3d
mysql> select * from card where card_id>=1000000 limit 10; +---------+--------------------------------------+ | card_id | card_number | +---------+--------------------------------------+ | 1000000 | 2d870088-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000001 | 2d87021a-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000002 | 2d8703ac-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000003 | 2d87053b-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000004 | 2d8706cd-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000005 | 2d87085f-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000006 | 2d8709f1-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000007 | 2d870b83-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000008 | 2d870d18-2e3b-11e8-ae62-9c5c8e6e37cf | | 1000009 | 2d870eaa-2e3b-11e8-ae62-9c5c8e6e37cf | +---------+--------------------------------------+ 10 rows in set (0.00 sec)
這樣就能夠很大的提升查詢效率code