SQLite`ifnull（）`vs`coalesce（）`性能

Question

来自SQLite site，

  

ifnull（）函数等效于带有两个参数的coalesce（）。

然后，如果我有两个参数，哪一个更快？

Answer 1

我已经通过一些实验证实了戈登·林诺夫的声明。设置：Windows 8上的SQLiteSpy，i5和RAM中的整个表。

任何一次查询都需要大约1.64秒的波动（小于0.1秒）。大部分时间它们都是1.64秒。

查询：

    create table t (
      a int, b int);

    insert into t values (null,null);
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;
    insert into t select case when random() < .5 then 1 end, case when random() < .5 then 1 end from t;

    select sum(ifnull(a, b)) from t;
    select sum(coalesce(a, b)) from t;
    select count(*) from t;