sql - SQLite `ifnull()` vs `coalesce()` 性能

Question

从SQLite 网站，

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？

Answer 1

我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。

任何一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;