Question

有人可以告诉我为什么'CASE WHEN'会让它变得如此缓慢以及如何优化/修复它，好吗？

需要将固定的项目放在结果中并按顺序排在第一位。

我可能会在sql查询之后执行此操作，但我认为如果在sql查询中完成此排序，它会更快，如果完成的话。

慢查询〜490ms

SELECT 
    places.id AS place_id,
    url,
    title,
    thumbnails.score AS score,
    thumbnails.clipping AS clipping,
    thumbnails.lastModified AS lastModified,
    EXISTS (SELECT 1 FROM pinned pi WHERE pi.place_id = places.id) AS pinned
FROM places
    LEFT JOIN thumbnails ON (thumbnails.place_id = places.id)
    LEFT JOIN pinned j ON (j.place_id = places.id) WHERE (hidden == 0)
ORDER BY case when j.id is null then 1 else 0 end,
    j.id,
    frecency DESC LIMIT 24

删除'CASE WHEN'部分：查询〜6ms

SELECT
    places.id AS place_id,
    url,
    title,
    thumbnails.score AS score,
    thumbnails.clipping AS clipping,
    thumbnails.lastModified AS lastModified,
    EXISTS (SELECT 1 FROM pinned pi WHERE pi.place_id = places.id) AS pinned
FROM places
    LEFT JOIN thumbnails ON (thumbnails.place_id = places.id) WHERE (hidden == 0)
ORDER BY frecency DESC LIMIT 24

表格信息：

var Create_Table_Places =
    'CREATE TABLE places (' +
        'id INTEGER PRIMARY KEY,' +
        'url LONGVARCHAR,' +
        'title LONGVARCHAR,' +
        'visit_count INTEGER DEFAULT 0,' +
        'hidden INTEGER DEFAULT 0 NOT NULL,' +
        'typed INTEGER DEFAULT 0 NOT NULL,' +
        'frecency INTEGER DEFAULT -1 NOT NULL,' +
        'last_visit_date INTEGER,' +
        'dateAdded INTEGER,' +
        'lastModified INTEGER' +
    ')';

var Create_Table_Thumbnails =
    'CREATE TABLE thumbnails (' +
        'id INTEGER PRIMARY KEY,' +
        'place_id INTEGER UNIQUE,' +
        'data LONGVARCHAR,' +
        'score REAL,' +
        'clipping INTEGER,' +
        'dateAdded INTEGER,' +
        'lastModified INTEGER' +
    ')';

var Create_Table_Pinned =
    'CREATE TABLE pinned (' +
        'id INTEGER PRIMARY KEY,' +
        'place_id INTEGER UNIQUE,' +
        'position INTEGER,' +
        'dateAdded INTEGER,' +
        'lastModified INTEGER' +
    ')';

Answer 1

要确定查询执行是否存在根本差异，请使用EXPLAIN QUERY PLAN。在SQLite 3.7.almost15中，您的查询具有以下计划：

selectid order from detail
-------- ----- ---- ------
0        0     0    SCAN TABLE places (~100000 rows)
0        1     1    SEARCH TABLE thumbnails USING INDEX sqlite_autoindex_thumbnails_1 (place_id=?) (~1 rows)
0        2     2    SEARCH TABLE pinned AS j USING COVERING INDEX sqlite_autoindex_pinned_1 (place_id=?) (~1 rows)
0        0     0    EXECUTE CORRELATED SCALAR SUBQUERY 1
1        0     0    SEARCH TABLE pinned AS pi USING COVERING INDEX sqlite_autoindex_pinned_1 (place_id=?) (~1 rows)
0        0     0    USE TEMP B-TREE FOR ORDER BY

selectid order from detail
-------- ----- ---- ------
0        0     0    SCAN TABLE places (~100000 rows)
0        1     1    SEARCH TABLE thumbnails USING INDEX sqlite_autoindex_thumbnails_1 (place_id=?) (~1 rows)
0        0     0    EXECUTE CORRELATED SCALAR SUBQUERY 1
1        0     0    SEARCH TABLE pinned AS pi USING COVERING INDEX sqlite_autoindex_pinned_1 (place_id=?) (~1 rows)
0        0     0    USE TEMP B-TREE FOR ORDER BY

这两个计划几乎相同，但重复的pinned查找除外。如果您的SQLite不以这种方式执行查询，请更新它。

在第一个查询中，您可以删除pinned字段的子查询，因为您已经加入pinned表，并且您正在执行与为连接执行的完全相同的查找;请改用j.id IS NOT NULL。

您的CASE WHEN的目的是在其他值之后对NULL进行排序。您可以通过converting all NULLs获得与sorted after numbers的某个值相同的效果，例如字符串：

... ORDER BY IFNULL(j.id, ''), frecency DESC

但是，从理论上讲，这不应该与CASE WHEN有很大的运行时差异。

WebSQL查询优化帮助 - CASE WHEN使查询速度减慢400毫秒

1 个答案: