SQLite `ifnull()` vs `coalesce()` 性能答案

【问题标题】：SQLite `ifnull()` vs `coalesce()` performanceSQLite `ifnull()` vs `coalesce()` 性能
【发布时间】：2013-07-23 07:53:17
【问题描述】：

来自SQLite site，

ifnull() 函数等效于有两个参数的 coalesce()。

那么，如果我有两个参数，哪个更快？

【问题讨论】：

如果它们是等效的，那么它们应该具有相同的性能。换句话说，唯一的区别在于编译阶段。

标签： sql performance sqlite coalesce ifnull

【解决方案1】：

我已经通过一些实验证实了 Gordon Linoff 的说法。设置：Windows 8、i5 上的 SQLiteSpy 和 RAM 中的整个表。

两个查询都花费了大约 1.64 秒，波动非常小（小于 0.1 秒）。大多数情况下，他们在 1.64 秒时是正确的。

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;

【讨论】：