SQLite應用之路---SQL查詢優化

SQLite應用之路---SQL查詢優化

temp1: 2499條數據
temp6: 969596條數據
//注意時間單位ms和s

//其中temp1和temp2已經給eid加上索引

1.in和 exists

//外表大於子表的時候，使用in
//外表小於字表的時候，使用exists
select * from temp1 where eid in (select eid from temp6)//1.92s
select * from temp1 where exists (select eid from temp6 where eid = temp1.eid)//66.48ms

select * from temp6 where eid in (select eid from temp1)//98.25ms
select * from temp6 where exists (select eid from temp1 where eid = temp6.eid)//2.40s

2.limit

//offset 越大，執行時間越長。
select * from temp6 limit 900000,100//1.25s
select * from temp6 limit 1,100//7.12ms

//下面兩句並沒有網上說的效果
select * from temp6 where eid >= (select eid from temp6 order by eid limit 800000,1) limit 100//515.14ms
select * from temp6 limit 800000,100//474.13ms

3.union 和 union all

//在不考慮重複數據的時候，union all 比union效率高。
//union all 兩張表的時候，應該將大表放在左邊
select * from temp6 union select * from temp1//187.17s
select * from temp1 union select * from temp6//190.30s

select * from temp6 union all select * from temp1//8.39s
select * from temp1 union all select * from temp6//42.03s

4.join

//join兩張表的時候，應該把小表放在左邊
select * from temp1 a join temp6 b on a.eid = b.eid//54.21ms
select * from temp6 a join temp1 b on a.eid = b.eid//1.12s