Many Small Queries Are Efficient In SQLite

1.执行摘要

每个网页的SQL语句对于MySQL，PostgreSQL或SQL Server等客户端/服务器数据库引擎来说过多。

但对于SQLite，每个网页200或更多的SQL语句不成问题。

SQLite也可以有效地处理大型和复杂的查询，就像客户端/服务器数据库一样。但SQLite也可以高效地完成许多更小的查询。应用程序开发人员可以使用最适合当前任务的技术。

2.感知问题

适用于SQLite页面的说法是，SQLite网站上的动态页面通常每个都会执行大约200条SQL语句。这引起了读者的批评。例子：

“200个SQL语句对单个页面而言是一个可笑的数字”

“对于大多数网站来说，200个查询的方式太多了。”

“这是糟糕的设计”

这种批评对于传统的客户端/服务器数据库引擎（如MySQL，PostgreSQL或SQL Server）来说是有根据的。在客户端/服务器数据库中，每条SQL语句都需要从应用程序到数据库服务器并返回到应用程序的消息。按顺序执行超过200次往返消息可能会造成严重的性能下降。这有时被称为“N + 1查询问题”或“N + 1选择问题”，它是一种反模式。

3. N + 1查询不是SQLite的问题

SQLite不是客户端/服务器，但是。SQLite数据库与应用程序在相同的进程地址空间中运行。查询不涉及消息往返，只涉及函数调用。SQLite中单个SQL查询的延迟要少得多。因此，使用SQLite进行大量查询不是问题。

4.每个网页需要超过200条SQL语句

SQLite网站上的动态网页大多由Fossil版本控制系统生成。一个典型的动态页面将是一个时间轴，例如https://www.sqlite.org/src/timeline。时间线使用的所有SQL的日志如下所示。

日志中的第一组查询是从Fossil数据库的“config”和“global_config”表中提取显示选项。然后是一个复杂的查询，提取要在时间轴上显示的所有元素的列表。这个“时间轴”查询表明，SQLite可以轻松处理涉及多个表，子查询和复杂WHERE子句约束的复杂关系数据库查询，并且它可以有效利用索引以最少的磁盘I / O解决查询。

在单个大时间轴查询之后，每个时间轴元素都有其他查询。Fossil正在使用“N + 1查询”模式，而不是试图尽可能少地查询所有信息。但这没关系，因为没有不必要的IPC开销。在每个时间线页面的底部，Fossil显示了生成页面需要多长时间。对于50条时间线，延迟通常小于25毫秒。分析表明，在数据库引擎内花费的毫秒数很少。

在Fossil中使用N + 1查询模式不会损害应用程序。但是N + 1查询模式确实有好处。首先，创建时间线查询的代码部分可以与准备显示每个时间线条目的部分完全分开。这提供了责任分离，有助于保持代码简单并易于维护。其次，显示所需的信息以及提取该信息所需的查询根据要显示的对象类型而变化。签入需要一组查询。门票需要另一组查询。Wiki页面需要不同的查询。等等。通过按需执行这些查询以及处理各实体的代码部分，进一步分离责任并简化总体代码库。

因此，SQLite能够执行一两个大而复杂的查询，或者它可以执行许多更小更简单的查询。两者都是有效的。应用程序可以使用其中一种或两种技术，具体取决于最适合当前情况的应用程序。

以下是用于生成一个特定时间线的所有SQL日志（2016-09-16捕获）：

-- sqlite3_open: /home/drh/sqlite/sqlite/.fslckout
PRAGMA foreign_keys=OFF;
SELECT sql FROM localdb.sqlite_master WHERE name=='vfile';
-- sqlite3_open: /home/drh/.fossil
PRAGMA foreign_keys=OFF;
SELECT value FROM vvar WHERE name='repository';
ATTACH DATABASE '/home/drh/www/repos/sqlite.fossil' AS 'repository' KEY '';
SELECT value FROM config WHERE name='allow-symlinks';
SELECT value FROM global_config WHERE name='allow-symlinks';
SELECT value FROM config WHERE name='aux-schema';
SELECT 1 FROM config WHERE name='baseurl:http://';
SELECT value FROM config WHERE name='ip-prefix-terms';
SELECT value FROM global_config WHERE name='ip-prefix-terms';
SELECT value FROM config WHERE name='localauth';
SELECT value FROM vvar WHERE name='default-user';
SELECT uid FROM user WHERE cap LIKE '%s%';
SELECT login FROM user WHERE uid=1;
SELECT cap FROM user WHERE login = 'nobody';
SELECT cap FROM user WHERE login = 'anonymous';
SELECT value FROM config WHERE name='public-pages';
SELECT value FROM global_config WHERE name='public-pages';
SELECT value FROM config WHERE name='header';
SELECT value FROM config WHERE name='project-name';
SELECT value FROM config WHERE name='th1-setup';
SELECT value FROM global_config WHERE name='th1-setup';
SELECT value FROM config WHERE name='redirect-to-https';
SELECT value FROM global_config WHERE name='redirect-to-https';
SELECT value FROM config WHERE name='index-page';
SELECT mtime FROM config WHERE name='css';
SELECT mtime FROM config WHERE name='logo-image';
SELECT mtime FROM config WHERE name='background-image';
CREATE TEMP TABLE IF NOT EXISTS timeline(
  rid INTEGER PRIMARY KEY,
  uuid TEXT,
  timestamp TEXT,
  comment TEXT,
  user TEXT,
  isleaf BOOLEAN,
  bgcolor TEXT,
  etype TEXT,
  taglist TEXT,
  tagid INTEGER,
  short TEXT,
  sortby REAL
)
;
INSERT OR IGNORE INTO timeline SELECT
  blob.rid AS blobRid,
  uuid AS uuid,
  datetime(event.mtime,toLocal()) AS timestamp,
  coalesce(ecomment, comment) AS comment,
  coalesce(euser, user) AS user,
  blob.rid IN leaf AS leaf,
  bgcolor AS bgColor,
  event.type AS eventType,
  (SELECT group_concat(substr(tagname,5), ', ') FROM tag, tagxref
    WHERE tagname GLOB 'sym-*' AND tag.tagid=tagxref.tagid
      AND tagxref.rid=blob.rid AND tagxref.tagtype>0) AS tags,
  tagid AS tagid,
  brief AS brief,
  event.mtime AS mtime
 FROM event CROSS JOIN blob
WHERE blob.rid=event.objid
 AND NOT EXISTS(SELECT 1 FROM tagxref WHERE tagid=5 AND tagtype>0 AND rid=blob.rid)
 ORDER BY event.mtime DESC LIMIT 50;
-- SELECT value FROM config WHERE name='timeline-utc';
SELECT count(*) FROM timeline WHERE etype!='div';
SELECT min(timestamp) FROM timeline;
SELECT julianday('2016-09-15 14:54:51',fromLocal()
SELECT EXISTS (SELECT 1 FROM event CROSS JOIN blob WHERE blob.rid=event.objid AND mtime<=2457647.121412037
SELECT max(timestamp) FROM timeline;
SELECT julianday('2016-09-24 17:42:43',fromLocal()
SELECT EXISTS (SELECT 1 FROM event CROSS JOIN blob WHERE blob.rid=event.objid AND mtime>=2457656.238009259
SELECT value FROM config WHERE name='search-ci';
SELECT value FROM vvar WHERE name='checkout';
SELECT value FROM config WHERE name='timeline-max-comment';
SELECT value FROM global_config WHERE name='timeline-max-comment';
SELECT value FROM config WHERE name='timeline-date-format';
SELECT value FROM config WHERE name='timeline-truncate-at-blank';
SELECT value FROM global_config WHERE name='timeline-truncate-at-blank';
SELECT * FROM timeline ORDER BY sortby DESC;
SELECT value FROM config WHERE name='hash-digits';
SELECT value FROM global_config WHERE name='hash-digits';
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=68028;
SELECT pid FROM plink WHERE cid=68028 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM tagxref WHERE rid=68028 AND tagid=9 AND tagtype>0;
SELECT value FROM config WHERE name='timeline-block-markup';
SELECT value FROM config WHERE name='timeline-plaintext';
SELECT value FROM config WHERE name='wiki-use-html';
SELECT value FROM global_config WHERE name='wiki-use-html';
SELECT 1 FROM private WHERE rid=68028;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=68026;
SELECT pid FROM plink WHERE cid=68026 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=68026;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=68024;
SELECT pid FROM plink WHERE cid=68024 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=68024;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=68018;
SELECT pid FROM plink WHERE cid=68018 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=68018;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=68012;
SELECT pid FROM plink WHERE cid=68012 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=68012;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=68011;
SELECT value FROM config WHERE name='details';
SELECT pid FROM plink WHERE cid=68011 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM tagxref WHERE rid=68011 AND tagid=9 AND tagtype>0;
SELECT 1 FROM private WHERE rid=68011;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=68008;
SELECT pid FROM plink WHERE cid=68008 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=68008;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=68006;
SELECT pid FROM plink WHERE cid=68006 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=68006;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=68000;
SELECT pid FROM plink WHERE cid=68000 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=68000;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67997;
SELECT pid FROM plink WHERE cid=67997 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67997;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67992;
SELECT pid FROM plink WHERE cid=67992 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67992;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67990;
SELECT pid FROM plink WHERE cid=67990 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67990;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67989;
SELECT pid FROM plink WHERE cid=67989 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67989;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67984;
SELECT pid FROM plink WHERE cid=67984 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67984;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67983;
SELECT pid FROM plink WHERE cid=67983 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67983;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67979;
SELECT pid FROM plink WHERE cid=67979 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67979;
SELECT value FROM config WHERE name='ticket-closed-expr';
SELECT status='Closed' OR status='Fixed' FROM ticket  WHERE tkt_uuid>='1ec41379c9c1e400' AND tkt_uuid<'1ec41379c9c1e401';
SELECT 1 FROM private WHERE rid=67980;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67977;
SELECT pid FROM plink WHERE cid=67977 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT status='Closed' OR status='Fixed' FROM ticket  WHERE tkt_uuid>='1ec41379c9c1e400' AND tkt_uuid<'1ec41379c9c1e401';
SELECT 1 FROM private WHERE rid=67977;
SELECT status='Closed' OR status='Fixed' FROM ticket  WHERE tkt_uuid>='1ec41379c9c1e400' AND tkt_uuid<'1ec41379c9c1e401';
SELECT 1 FROM private WHERE rid=67974;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67971;
SELECT pid FROM plink WHERE cid=67971 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67971;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67972;
SELECT pid FROM plink WHERE cid=67972 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67972;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67969;
SELECT pid FROM plink WHERE cid=67969 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67969;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67966;
SELECT pid FROM plink WHERE cid=67966 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67966;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67962;
SELECT pid FROM plink WHERE cid=67962 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67962;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67960;
SELECT pid FROM plink WHERE cid=67960 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67960;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67957;
SELECT pid FROM plink WHERE cid=67957 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67957;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67955;
SELECT pid FROM plink WHERE cid=67955 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67955;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67953;
SELECT pid FROM plink WHERE cid=67953 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT status='Closed' OR status='Fixed' FROM ticket  WHERE tkt_uuid>='5990a1bdb4a073' AND tkt_uuid<'5990a1bdb4a074';
SELECT 1 FROM blob WHERE uuid>='5990a1bdb4a073' AND uuid<'5990a1bdb4a074';
SELECT 1 FROM private WHERE rid=67953;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67941;
SELECT pid FROM plink WHERE cid=67941 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67941;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67940;
SELECT pid FROM plink WHERE cid=67940 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67940;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67938;
SELECT pid FROM plink WHERE cid=67938 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67938;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67935;
SELECT pid FROM plink WHERE cid=67935 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67935;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67934;
SELECT pid FROM plink WHERE cid=67934 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67934;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67932;
SELECT pid FROM plink WHERE cid=67932 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67932;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67930;
SELECT pid FROM plink WHERE cid=67930 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67930;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67928;
SELECT pid FROM plink WHERE cid=67928 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM tagxref WHERE rid=67928 AND tagid=9 AND tagtype>0;
SELECT 1 FROM private WHERE rid=67928;
SELECT status='Closed' OR status='Fixed' FROM ticket  WHERE tkt_uuid>='0eab1ac7591f511d' AND tkt_uuid<'0eab1ac7591f511e';
SELECT 1 FROM private WHERE rid=67919;
SELECT status='Closed' OR status='Fixed' FROM ticket  WHERE tkt_uuid>='01874d252ac44861' AND tkt_uuid<'01874d252ac44862';
SELECT 1 FROM blob WHERE uuid>='01874d252ac44861' AND uuid<'01874d252ac44862';
SELECT 1 FROM private WHERE rid=67918;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67916;
SELECT pid FROM plink WHERE cid=67916 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT status='Closed' OR status='Fixed' FROM ticket  WHERE tkt_uuid>='0eab1ac759' AND tkt_uuid<'0eab1ac75:';
SELECT 1 FROM private WHERE rid=67916;
SELECT status='Closed' OR status='Fixed' FROM ticket  WHERE tkt_uuid>='a49bc0a8244feb08' AND tkt_uuid<'a49bc0a8244feb09';
SELECT 1 FROM blob WHERE uuid>='a49bc0a8244feb08' AND uuid<'a49bc0a8244feb09';
SELECT 1 FROM private WHERE rid=67914;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67913;
SELECT pid FROM plink WHERE cid=67913 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT status='Closed' OR status='Fixed' FROM ticket  WHERE tkt_uuid>='0eab1ac7591f' AND tkt_uuid<'0eab1ac7591g';
SELECT 1 FROM private WHERE rid=67913;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67911;
SELECT pid FROM plink WHERE cid=67911 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67911;
SELECT status='Closed' OR status='Fixed' FROM ticket  WHERE tkt_uuid>='0eab1ac7591f511d' AND tkt_uuid<'0eab1ac7591f511e';
SELECT 1 FROM private WHERE rid=67909;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67907;
SELECT pid FROM plink WHERE cid=67907 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67907;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67899;
SELECT pid FROM plink WHERE cid=67899 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67899;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67897;
SELECT pid FROM plink WHERE cid=67897 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67897;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67895;
SELECT pid FROM plink WHERE cid=67895 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67895;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67893;
SELECT pid FROM plink WHERE cid=67893 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67893;
SELECT value FROM tagxref WHERE tagid=8 AND tagtype>0 AND rid=67891;
SELECT pid FROM plink WHERE cid=67891 AND pid NOT IN phantom ORDER BY isprim DESC;
SELECT 1 FROM private WHERE rid=67891;
SELECT count(*) FROM plink
 WHERE pid=67928 AND isprim
   AND coalesce((SELECT value FROM tagxref
                  WHERE tagid=8 AND rid=plink.pid), 'trunk')
      =coalesce((SELECT value FROM tagxref
                  WHERE tagid=8 AND rid=plink.cid), 'trunk')
;
SELECT count(*) FROM plink
 WHERE pid=68011 AND isprim
   AND coalesce((SELECT value FROM tagxref
                  WHERE tagid=8 AND rid=plink.pid), 'trunk')
      =coalesce((SELECT value FROM tagxref
                  WHERE tagid=8 AND rid=plink.cid), 'trunk')
;
SELECT count(*) FROM plink
 WHERE pid=68028 AND isprim
   AND coalesce((SELECT value FROM tagxref
                  WHERE tagid=8 AND rid=plink.pid), 'trunk')
      =coalesce((SELECT value FROM tagxref
                  WHERE tagid=8 AND rid=plink.cid), 'trunk')
;
SELECT value FROM config WHERE name='show-version-diffs';
SELECT value FROM config WHERE name='adunit-omit-if-admin';
SELECT value FROM global_config WHERE name='adunit-omit-if-admin';
SELECT value FROM config WHERE name='adunit-omit-if-user';
SELECT value FROM global_config WHERE name='adunit-omit-if-user';
SELECT value FROM config WHERE name='adunit';
SELECT value FROM global_config WHERE name='adunit';
SELECT value FROM config WHERE name='auto-hyperlink-delay';
SELECT value FROM global_config WHERE name='auto-hyperlink-delay';
SELECT value FROM config WHERE name='footer';
PRAGMA database_list;
PRAGMA database_list;
PRAGMA localdb.freelist_count;
PRAGMA localdb.page_count;

 SQLite在公共领域。