如果根据EXPLAIN QUERY PLAN,为什么SQL查询比另一个慢？它应该相反？

Question

我有两张桌子

CREATE TABLE Categories (
   Category INTEGER,
   Id INTEGER,
   FOREIGN KEY (Category) REFERENCES CategoriesInfo(Category)
)
CREATE TABLE 'CategoriesInfo' (
   'Category' INTEGER PRIMARY KEY NOT NULL,
   'Name' TEXT
)

有索引

CREATE UNIQUE INDEX idxCategory ON Categories (Category, Id)

如果我跑

EXPLAIN QUERY PLAN
SELECT CategoriesInfo.Category, Name
FROM Categories, CategoriesInfo
Where Categories.Category=CategoriesInfo.Category AND Id=:id
ORDER BY Name

它说

Array
(
    [0] => Array
        (
            [selectid] => 0
            [order] => 0
            [from] => 1
            [detail] => SCAN TABLE CategoriesInfo (~1475 rows)
        )

    [1] => Array
        (
            [selectid] => 0
            [order] => 1
            [from] => 0
            [detail] => SEARCH TABLE Categories USING COVERING INDEX idxCategory (Category=? AND Id=?) (~1 rows)
        )

    [2] => Array
        (
            [selectid] => 0
            [order] => 0
            [from] => 0
            [detail] => USE TEMP B-TREE FOR ORDER BY
        )

)

但是如果我使用连接

EXPLAIN QUERY PLAN
SELECT CategoriesInfo.Category, CategoriesInfo.Name
FROM Categories
LEFT JOIN CategoriesInfo ON (Categories.Category=CategoriesInfo.Category)
WHERE Categories.Id=:id
ORDER BY Name

我明白了

Array
(
    [0] => Array
        (
            [selectid] => 0
            [order] => 0
            [from] => 0
            [detail] => SEARCH TABLE Categories USING AUTOMATIC COVERING INDEX (Id=?) (~6 rows)
        )

    [1] => Array
        (
            [selectid] => 0
            [order] => 1
            [from] => 1
            [detail] => SEARCH TABLE CategoriesInfo USING INTEGER PRIMARY KEY (rowid=?) (~1 rows)
        )

    [2] => Array
        (
            [selectid] => 0
            [order] => 0
            [from] => 0
            [detail] => USE TEMP B-TREE FOR ORDER BY
        )

)

然后,使用连接应该更快.但是当我用phpliteadmin运行两个代码时,

• 1号需要0秒
• 第二节需要0.3281秒

为什么？？

Answer 1

我会指出这两个查询不等同.

您的第一个查询有效地执行INNER JOIN,而您的第二个查询是LEFT JOIN.我打赌这是速度差异的原因.您的LEFT JOIN将要求所有来自Categories的记录都包含在输出中,这是您的第一个查询不需要的内容.

尝试将LEFT JOIN更改为INNER JOIN,然后比较速度.