SQLite 慢选查询

Question

我正在运行以下选择查询：

SELECT "entry"."id" AS "entry_id",
"entry"."input" AS "entry_input",
"entry"."output" AS "entry_output",
"entry"."numOfWords" AS "entry_numOfWords",
"entry"."times_seen" AS "entry_times_seen",
"word_class"."value" AS "word_class_value",
"dominant_noun"."noun" AS "dominant_noun_noun",
"dominant_noun"."article" AS "dominant_noun_article",
"dominant_noun"."isPluaral" AS "dominant_noun_isPluaral",
"subject"."subjectIndex" AS "subject_subjectIndex",
"last_time_visited"."value" AS "last_time_visited_value"
FROM "entry" "entry"
LEFT JOIN "word_class" "word_class" ON "word_class"."entryId"="entry"."id"
LEFT JOIN "dominant_noun" "dominant_noun" ON "dominant_noun"."entryId"="entry"."id"
LEFT JOIN "subject_entries_entry" "subject_entry" ON "subject_entry"."entryId"="entry"."id"
LEFT JOIN "subject" "subject" ON "subject"."id"="subject_entry"."subjectId"
LEFT JOIN "last_time_visited" "last_time_visited" ON "last_time_visited"."entryId"="entry"."id"
WHERE "entry"."inputLang" = 31
AND ("entry"."input" like '% hilfe %' OR "entry"."input" like 'hilfe %' OR "entry"."input" like '% hilfe')
ORDER BY "word_class"."value" DESC, "entry"."numOfWords" ASC;

时间结果：

real    0m15.100s
user    0m14.072s
sys     0m1.024s

针对此数据库架构：

CREATE TABLE sqlite_sequence(name,seq);
CREATE TABLE IF NOT EXISTS "subject" ("id" integer PRIMARY KEY AUTOINCREMENT NOT NULL, "subjectIndex" tinyint NOT NULL);
CREATE TABLE IF NOT EXISTS "entry" ("id" integer PRIMARY KEY AUTOINCREMENT NOT NULL, "inputLang" tinyint NOT NULL, "outputLang" tinyint NOT NULL, "input"
varchar NOT NULL, "output" varchar NOT NULL, "numOfWords" tinyint NOT NULL, "times_seen" integer NOT NULL DEFAULT (0));
CREATE TABLE IF NOT EXISTS "abbr" ("id" integer PRIMARY KEY AUTOINCREMENT NOT NULL, "value" varchar NOT NULL, "entryId" integer, CONSTRAINT "REL_ca935aaf7
66cba1e7bfbe90275" UNIQUE ("entryId"), CONSTRAINT "FK_ca935aaf766cba1e7bfbe902757" FOREIGN KEY ("entryId") REFERENCES "entry" ("id"));
CREATE TABLE IF NOT EXISTS "word_class" ("id" integer PRIMARY KEY AUTOINCREMENT NOT NULL, "value" integer NOT NULL, "entryId" integer, CONSTRAINT "REL_941
45442deb2b2209bd943a787" UNIQUE ("entryId"), CONSTRAINT "FK_94145442deb2b2209bd943a7874" FOREIGN KEY ("entryId") REFERENCES "entry" ("id"));
CREATE TABLE IF NOT EXISTS "dominant_noun" ("id" integer PRIMARY KEY AUTOINCREMENT NOT NULL, "noun" varchar NOT NULL, "article" tinyint NOT NULL, "isPluar
al" boolean NOT NULL, "entryId" integer, CONSTRAINT "REL_f493eeedea653d8a89f595c82c" UNIQUE ("entryId"), CONSTRAINT "FK_f493eeedea653d8a89f595c82c4" FOREI
GN KEY ("entryId") REFERENCES "entry" ("id"));
CREATE TABLE IF NOT EXISTS "last_time_visited" ("id" integer PRIMARY KEY AUTOINCREMENT NOT NULL, "value" datetime NOT NULL DEFAULT (CURRENT_TIMESTAMP), "e
ntryId" integer, CONSTRAINT "REL_e631a6f55d59214f8e6aaa6447" UNIQUE ("entryId"), CONSTRAINT "FK_e631a6f55d59214f8e6aaa64478" FOREIGN KEY ("entryId") REFER
ENCES "entry" ("id"));
CREATE TABLE IF NOT EXISTS "subject_entries_entry" ("subjectId" integer NOT NULL, "entryId" integer NOT NULL, CONSTRAINT "FK_d2eaa7a84a7963ed94e472cef0b"FOREIGN KEY ("subjectId") REFERENCES "subject" ("id") ON DELETE CASCADE, CONSTRAINT "FK_5f940450dd4c681a9fecf0b14b2" FOREIGN KEY ("entryId") REFERENCES "entry" ("id") ON DELETE CASCADE, PRIMARY KEY ("subjectId", "entryId"));
CREATE INDEX "IDX_3091789786b922bee00bbb44b1" ON "entry" ("inputLang") ;
CREATE INDEX "IDX_36ab3550b9e3ef647d1230affc" ON "entry" ("outputLang") ;
CREATE INDEX "IDX_1b0f6266dffb9a7e6343e7faa4" ON "entry" ("input") ;
CREATE INDEX "IDX_a77c7936ea412ec1958007154a" ON "entry" ("numOfWords") ;
CREATE INDEX "IDX_b32699a03d36223ff9bad94ea6" ON "entry" ("times_seen") ;

解释结果：

addr  opcode         p1    p2    p3    p4             p5  comment
----  -------------  ----  ----  ----  -------------  --  -------------
0     Init           0     109   0                    00  Start at 109
1     SorterOpen     6     14    0     k(2,-B,B)      00
2     OpenRead       0     12    0     7              00  root=12 iDb=0; entry
3     OpenRead       1     2     0     3              00  root=2 iDb=0; word_class
4     OpenRead       7     3     0     k(2,,)         02  root=3 iDb=0; sqlite_autoindex_word_class_1
5     OpenRead       2     5     0     5              00  root=5 iDb=0; dominant_noun
6     OpenRead       8     6     0     k(2,,)         02  root=6 iDb=0; sqlite_autoindex_dominant_noun_1
7     OpenRead       3     10    0     2              00  root=10 iDb=0; subject_entries_entry
8     OpenRead       4     9     0     2              00  root=9 iDb=0; subject
9     OpenRead       5     7     0     3              00  root=7 iDb=0; last_time_visited
10    OpenRead       9     8     0     k(2,,)         02  root=8 iDb=0; sqlite_autoindex_last_time_visited_1
11    Rewind         0     92    0                    00
12      Column         0     1     1                    00  r[1]=entry.inputLang
13      Ne             2     91    1     (BINARY)       54  if r[1]!=r[2] goto 91
14      Column         0     3     4                    00  r[4]=entry.input
15      Function0      1     3     1     like(2)        02  r[1]=func(r[3..4])
16      If             1     23    0                    00
17      Column         0     3     6                    00  r[6]=entry.input
18      Function0      1     5     1     like(2)        02  r[1]=func(r[5..6])
19      If             1     23    0                    00
20      Column         0     3     8                    00  r[8]=entry.input
21      Function0      1     7     1     like(2)        02  r[1]=func(r[7..8])
22      IfNot          1     91    1                    00
23      Integer        0     9     0                    00  r[9]=0; init LEFT JOIN no-match flag
24      Rowid          0     10    0                    00  r[10]=rowid
25      SeekGE         7     87    10    1              00  key=r[10]
26      IdxGT          7     87    10    1              00  key=r[10]
27      DeferredSeek   7     0     1                    00  Move 1 to 7.rowid if needed
28      Integer        1     9     0                    00  r[9]=1; record LEFT JOIN hit
29      Integer        0     11    0                    00  r[11]=0; init LEFT JOIN no-match flag
30      Rowid          0     12    0                    00  r[12]=rowid
31      SeekGE         8     83    12    1              00  key=r[12]
32      IdxGT          8     83    12    1              00  key=r[12]
33      DeferredSeek   8     0     2                    00  Move 2 to 8.rowid if needed
34      Integer        1     11    0                    00  r[11]=1; record LEFT JOIN hit
35      Once           0     44    0                    00
36      OpenAutoindex  10    3     0     k(3,B,,)       00  nColumn=3; for subject_entries_entry
37      Rewind         3     44    0                    00
38        Column         3     1     13                   00  r[13]=subject_entries_entry.entryId
39        Column         3     0     14                   00  r[14]=subject_entries_entry.subjectId
40        Rowid          3     15    0                    00  r[15]=rowid
41        MakeRecord     13    3     1                    00  r[1]=mkrec(r[13..15])
42        IdxInsert      10    1     0                    10  key=r[1]
43      Next           3     38    0                    03
44      Integer        0     16    0                    00  r[16]=0; init LEFT JOIN no-match flag
45      Rowid          0     17    0                    00  r[17]=rowid
46      SeekGE         10    80    17    1              00  key=r[17]
47        IdxGT          10    80    17    1              00  key=r[17]
48        Integer        1     16    0                    00  r[16]=1; record LEFT JOIN hit
49        Integer        0     18    0                    00  r[18]=0; init LEFT JOIN no-match flag
50        Column         10    1     19                   00  r[19]=subject_entries_entry.subjectId
51        SeekRowid      4     76    19                   00  intkey=r[19]
52        Integer        1     18    0                    00  r[18]=1; record LEFT JOIN hit
53        Integer        0     20    0                    00  r[20]=0; init LEFT JOIN no-match flag
54        Rowid          0     21    0                    00  r[21]=rowid
55        SeekGE         9     72    21    1              00  key=r[21]
56        IdxGT          9     72    21    1              00  key=r[21]
57        DeferredSeek   9     0     5                    00  Move 5 to 9.rowid if needed
58        Integer        1     20    0                    00  r[20]=1; record LEFT JOIN hit
59        Rowid          0     24    0                    00  r[24]=rowid
60        Column         0     3     25                   00  r[25]=entry.input
61        Column         0     4     26                   00  r[26]=entry.output
62        Column         0     6     27    0              00  r[27]=entry.times_seen
63        Column         2     1     28                   00  r[28]=dominant_noun.noun
64        Column         2     2     29                   00  r[29]=dominant_noun.article
65        Column         2     3     30                   00  r[30]=dominant_noun.isPluaral
66        Column         4     1     31                   00  r[31]=subject.subjectIndex
67        Column         5     1     32                   00  r[32]=last_time_visited.value
68        Column         1     1     22                   00  r[22]=word_class.value
69        Column         0     5     23                   00  r[23]=entry.numOfWords
70        MakeRecord     22    11    35                   00  r[35]=mkrec(r[22..32])
71        SorterInsert   6     35    22    11             00  key=r[35]
72        IfPos          20    76    0                    00  if r[20]>0 then r[20]-=0, goto 76
73        NullRow        5     0     0                    00
74        NullRow        9     0     0                    00
75        Goto           0     58    0                    00
76        IfPos          18    79    0                    00  if r[18]>0 then r[18]-=0, goto 79
77        NullRow        4     0     0                    00
78        Goto           0     52    0                    00
79      Next           10    47    0                    00
80      IfPos          16    83    0                    00  if r[16]>0 then r[16]-=0, goto 83
81      NullRow        10    0     0                    00
82      Goto           0     48    0                    00
83      IfPos          11    87    0                    00  if r[11]>0 then r[11]-=0, goto 87
84      NullRow        2     0     0                    00
85      NullRow        8     0     0                    00
86      Goto           0     34    0                    00
87      IfPos          9     91    0                    00  if r[9]>0 then r[9]-=0, goto 91
88      NullRow        1     0     0                    00
89      NullRow        7     0     0                    00
90      Goto           0     28    0                    00
91    Next           0     12    0                    01
92    OpenPseudo     11    36    14                   00  14 columns in r[36]
93    SorterSort     6     108   0                    00
94      SorterData     6     36    11                   00  r[36]=data
95      Column         11    10    34                   00  r[34]=last_time_visited_value
96      Column         11    9     33                   00  r[33]=subject_subjectIndex
97      Column         11    8     32                   00  r[32]=dominant_noun_isPluaral
98      Column         11    7     31                   00  r[31]=dominant_noun_article
99      Column         11    6     30                   00  r[30]=dominant_noun_noun
100     Column         11    0     29                   00  r[29]=word_class_value
101     Column         11    5     28                   00  r[28]=entry_times_seen
102     Column         11    1     27                   00  r[27]=entry_numOfWords
103     Column         11    4     26                   00  r[26]=entry_output
104     Column         11    3     25                   00  r[25]=entry_input
105     Column         11    2     24                   00  r[24]=entry_id
106     ResultRow      24    11    0                    00  output=r[24..34]
107   SorterNext     6     94    0                    00
108   Halt           0     0     0                    00
109   Transaction    0     0     348   0              01  usesStmtJournal=0
110   Integer        31    2     0                    00  r[2]=31
111   String8        0     3     0     % hilfe %      00  r[3]='% hilfe %'
112   String8        0     5     0     hilfe %        00  r[5]='hilfe %'
113   String8        0     7     0     % hilfe        00  r[7]='% hilfe'
114   Goto           0     1     0                    00

解释查询计划输出：

QUERY PLAN
|--SCAN TABLE entry AS entry
|--SEARCH TABLE word_class AS word_class USING INDEX sqlite_autoindex_word_class_1 (entryId=?)
|--SEARCH TABLE dominant_noun AS dominant_noun USING INDEX sqlite_autoindex_dominant_noun_1 (entryId=?)
|--SEARCH TABLE subject_entries_entry AS subject_entry USING AUTOMATIC COVERING INDEX (entryId=?)
|--SEARCH TABLE subject AS subject USING INTEGER PRIMARY KEY (rowid=?)
|--SEARCH TABLE last_time_visited AS last_time_visited USING INDEX sqlite_autoindex_last_time_visited_1 (entryId=?)
`--USE TEMP B-TREE FOR ORDER BY

分析输出：

subject||1437631
entry|IDX_b32699a03d36223ff9bad94ea6|2348382 2348382
entry|IDX_a77c7936ea412ec1958007154a|2348382 67097
entry|IDX_1b0f6266dffb9a7e6343e7faa4|2348382 2
entry|IDX_36ab3550b9e3ef647d1230affc|2348382 1174191
entry|IDX_3091789786b922bee00bbb44b1|2348382 1174191
abbr|sqlite_autoindex_abbr_1|42575 1
dominant_noun|sqlite_autoindex_dominant_noun_1|823071 1
word_class|sqlite_autoindex_word_class_1|2005516 1
subject_entries_entry|sqlite_autoindex_subject_entries_entry_1|1437631 1 1

通常需要 10 多秒才能获得结果。虽然这是我第一次使用 SQLite，但 20 秒的回复时间似乎很奇怪。如果我应该提供额外信息以解决问题，请添加评论？

Answer 1

您看到的长查询时间的因素是subject_entries_entry 表。这是一个标准联结表，用于将entry 表中的行与subject 表中的行相关联。表定义使用主键，将主题 ID 放在首位，然后是条目 ID (PRIMARY KEY ("subjectId", "entryId"))。

另一方面，您的查询首先连接表中的条目 ID，然后连接主题 ID - 与键中的顺序相反。 Sqlite 可以并且确实对连接中的表进行重新排序以尽可能提高效率，但在这种情况下它没有这样做。转到EXPLAIN QUERY PLAN 输出：

SEARCH TABLE subject_entries_entry AS subject_entry USING AUTOMATIC COVERING INDEX (entryId=?)

SEARCH 表示它正在查找索引中的特定行，而不是查看每一行 (SCAN)，这是您想要的，但 AUTOMATIC COVERING INDEX 部分不好。 AUTOMATIC 表示查询规划器没有找到它可以使用的现有索引，但认为使用索引比必须扫描表更好 - 因此它构建了一个仅为该查询而存在的临时索引。看起来subject_entries_entry 表有很多行，所以这可能需要一段时间。

以与它们在连接中使用的相同顺序重新创建具有主键列的表可以大大缩短时间（与翻转列的单独索引一样，以使用更多磁盘空间为代价）。

我对这张桌子的另一个建议是把它变成WITHOUT ROWID 一张。普通 sqlite 表使用 64 位整数主键（称为 rowid），而不管表定义使用什么；非INTEGER PRIMARY KEY 只是此类表中的普通UNIQUE 索引。使用WITHOUT ROWID，主键是表的实际主键，这样在实际rowid 没有实际用途的情况下可以节省空间。它没有一个表和一个复制每一行内容的索引，它只有一个表。不过，这种优化不会影响查询速度，因为它使用的 covering 索引已经在索引中包含所有需要的信息；实际的表甚至没有像现在这样在查询中查看。

我不确定是否会进一步加速 - 查看查询计划，它对其余表使用预先存在的索引，并且连接子句都很简单。我有点惊讶它没有使用entry(inputLang) 上的索引来进行搜索而不是对该表进行扫描。也许如果您可以在打开SQLITE_ENABLE_STAT4 的情况下重建sqlite 库，然后使用PRAGMA optimize 来重建统计表，但这取决于您使用的语言（在C 或C++ 中很容易，在其他）。

编辑：

其他一些值得探索的事情：

• 使用 PRAGMA threads=N 启用多线程排序，其中 N 是您正在使用的内核数。
• 使用PRAGMA mmap_size=X 启用内存映射数据库。