digoal
非常欢迎数据库用户提出场景, 在此issue回复即可, 一起来建设沉浸式数据库学习教学素材库, 帮助开发者用好数据库, 提升开发者职业竞争力, 同时为企业降本提效.
本文的实验可以使用永久免费的阿里云云起实验室来完成.
如果你本地有docker环境也可以把镜像拉到本地来做实验:
x86_64机器使用以下docker image:
ARM机器使用以下docker image:
很多业务场景中需要判断商标侵权, 避免纠纷. 例如
而且商标侵权通常还有相似的情况, 避免不法分子蹭大品牌的流量, 导致大品牌名誉受损.
例如postgresql是个商标, 如果你使用posthellogresql、postgresqlabc也可能算侵权.
以跨境电商为力, 为了避免侵权, 在发布内容时需要商品描述中出现的品牌名、产品名等是否与已有的商标库有相似.
对于跨境电商场景, 由于店铺和用户众多, 商品的修改、发布是比较高频的操作, 所以需要实现高性能的字符串相似匹配功能.
创建一张品牌表, 用于存储收集好的注册商标(通常最终转换为文字).
create unlogged table tbl_ip ( -- 测试使用unlogged table, 加速数据生成
id serial primary key, -- 每一条品牌信息的唯一ID
n text -- 品牌名
);
使用随机字符模拟生成1000万条品牌名.
insert into tbl_ip (n) select md5(random()::text) from generate_series(1,10000000);
再放入几条比较容易识别的:
insert into tbl_ip(n) values ('polardb'),('polardbpg'),('polardbx'),('alibaba'),('postgresql'),('mysql'),('aliyun'),('apsaradb'),('apple'),('microsoft');
postgres=# select * from tbl_ip limit 10;
id | n
----+----------------------------------
1 | f4cd4669d249c1747c1d31b0b492d84e
2 | 2e29f32460485698088f4ab0632d86b7
3 | a8460622db4a3dc4ab70a8443a2c2a1a
4 | c4554856e259d3dfcccfb3c9872ab1d0
5 | b3a6041c5838d70d95a1316eea45bea3
6 | fc2d701eca05c74905fd1a604f072006
7 | f3dc443060e33bb672dc6a3b79bc1acd
8 | 1305b6092f9e798453e9f60840b8db2a
9 | 9b07cad251661627e15f239e5b122eaf
10 | 8b5d2a468435febe417b17d0d0442b86
(10 rows)
postgres=# select count(*) from tbl_ip;
count
----------
10000010
(1 row)
传统方法只能使用like全模糊查询, 但是局部侵权的可能性非常多, 使用模糊查询需要很多很多组合, 性能会非常差.
例如postgresql是个商标, 如果用户使用了一个字符串为以下组合, 都可能算侵权:
写成SQL应该是这样的
select * from tbl_ip where
n like '%post%' or
n like '%postgres%' or
n like '%sql%' or
n like '%gresql%' or
n like '%postgresql%' or
n like '%postgre%';
结果
id | n
----------+------------
10000005 | postgresql
10000006 | mysql
(2 rows)
耗时如下
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Seq Scan on tbl_ip (cost=0.00..333336.00 rows=5999 width=37) (actual time=2622.461..2622.463 rows=2 loops=1)
Filter: ((n ~~ '%post%'::text) OR (n ~~ '%postgres%'::text) OR (n ~~ '%sql%'::text) OR (n ~~ '%gresql%'::text) OR (n ~~ '%postgresql%'::text) OR (n ~~ '%postgre%'::text))
Rows Removed by Filter: 10000008
Planning Time: 1.381 ms
JIT:
Functions: 2
Options: Inlining false, Optimization false, Expressions true, Deforming true
Timing: Generation 1.442 ms, Inlining 0.000 ms, Optimization 1.561 ms, Emission 6.486 ms, Total 9.489 ms
Execution Time: 2624.001 ms
(9 rows)
使用pg_trgm插件, gin索引, 以及它的字符串相似查询功能,
创建插件
postgres=# create extension if not exists pg_trgm;
NOTICE: extension "pg_trgm" already exists, skipping
CREATE EXTENSION
创建索引
postgres=# create index on tbl_ip using gin (n gin_trgm_ops);
设置相似度阈值, 仅返回相似度大于0.9的记录
postgres=# set pg_trgm.similarity_threshold=0.9;
SET
使用相似度查询
select *,
similarity(n, 'post'),
similarity(n, 'postgres'),
similarity(n, 'sql'),
similarity(n, 'gresql'),
similarity(n, 'postgresql'),
similarity(n, 'postgre')
from tbl_ip
where
n % 'post' or
n % 'postgres' or
n % 'sql' or
n % 'gresql' or
n % 'postgresql' or
n % 'postgre';
结果
id | n | similarity | similarity | similarity | similarity | similarity | similarity
----------+------------+------------+------------+------------+------------+------------+------------
10000005 | postgresql | 0.33333334 | 0.6666667 | 0.15384616 | 0.3846154 | 1 | 0.5833333
(1 row)
耗时如下
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on tbl_ip (cost=996.70..7365.20 rows=5999 width=37) (actual time=0.180..0.183 rows=1 loops=1)
Recheck Cond: ((n % 'post'::text) OR (n % 'postgres'::text) OR (n % 'sql'::text) OR (n % 'gresql'::text) OR (n % 'postgresql'::text) OR (n % 'postgre'::text))
Heap Blocks: exact=1
-> BitmapOr (cost=996.70..996.70 rows=6000 width=0) (actual time=0.140..0.141 rows=0 loops=1)
-> Bitmap Index Scan on tbl_ip_n_idx (cost=0.00..115.30 rows=1000 width=0) (actual time=0.053..0.053 rows=0 loops=1)
Index Cond: (n % 'post'::text)
-> Bitmap Index Scan on tbl_ip_n_idx (cost=0.00..200.00 rows=1000 width=0) (actual time=0.019..0.019 rows=0 loops=1)
Index Cond: (n % 'postgres'::text)
-> Bitmap Index Scan on tbl_ip_n_idx (cost=0.00..93.30 rows=1000 width=0) (actual time=0.007..0.007 rows=0 loops=1)
Index Cond: (n % 'sql'::text)
-> Bitmap Index Scan on tbl_ip_n_idx (cost=0.00..157.10 rows=1000 width=0) (actual time=0.011..0.011 rows=0 loops=1)
Index Cond: (n % 'gresql'::text)
-> Bitmap Index Scan on tbl_ip_n_idx (cost=0.00..242.90 rows=1000 width=0) (actual time=0.035..0.035 rows=1 loops=1)
Index Cond: (n % 'postgresql'::text)
-> Bitmap Index Scan on tbl_ip_n_idx (cost=0.00..179.10 rows=1000 width=0) (actual time=0.013..0.013 rows=0 loops=1)
Index Cond: (n % 'postgre'::text)
Planning Time: 4.682 ms
Execution Time: 0.272 ms
(18 rows)
使用了pg_trgm后, 即使是like查询响应速度也飞快:
postgres=# explain analyze select * from tbl_ip where
n like '%post%' or
n like '%postgres%' or
n like '%sql%' or
n like '%gresql%' or
n like '%postgresql%' or
n like '%postgre%';
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on tbl_ip (cost=612.80..6981.30 rows=5999 width=37) (actual time=0.122..0.126 rows=2 loops=1)
Recheck Cond: ((n ~~ '%post%'::text) OR (n ~~ '%postgres%'::text) OR (n ~~ '%sql%'::text) OR (n ~~ '%gresql%'::text) OR (n ~~ '%postgresql%'::text) OR (n ~~ '%postgre%'::text))
Heap Blocks: exact=1
-> BitmapOr (cost=612.80..612.80 rows=6000 width=0) (actual time=0.099..0.101 rows=0 loops=1)
-> Bitmap Index Scan on tbl_ip_n_idx (cost=0.00..50.40 rows=1000 width=0) (actual time=0.047..0.048 rows=1 loops=1)
Index Cond: (n ~~ '%post%'::text)
-> Bitmap Index Scan on tbl_ip_n_idx (cost=0.00..136.20 rows=1000 width=0) (actual time=0.011..0.011 rows=1 loops=1)
Index Cond: (n ~~ '%postgres%'::text)
-> Bitmap Index Scan on tbl_ip_n_idx (cost=0.00..29.50 rows=1000 width=0) (actual time=0.003..0.003 rows=2 loops=1)
Index Cond: (n ~~ '%sql%'::text)
-> Bitmap Index Scan on tbl_ip_n_idx (cost=0.00..93.30 rows=1000 width=0) (actual time=0.014..0.014 rows=1 loops=1)
Index Cond: (n ~~ '%gresql%'::text)
-> Bitmap Index Scan on tbl_ip_n_idx (cost=0.00..179.10 rows=1000 width=0) (actual time=0.014..0.014 rows=1 loops=1)
Index Cond: (n ~~ '%postgresql%'::text)
-> Bitmap Index Scan on tbl_ip_n_idx (cost=0.00..115.30 rows=1000 width=0) (actual time=0.008..0.008 rows=1 loops=1)
Index Cond: (n ~~ '%postgre%'::text)
Planning Time: 0.571 ms
Execution Time: 0.207 ms
(18 rows)
品牌数 | 传统like查询耗时 ms | pg_trgm近似查询耗时 ms | pg_trgm like查询耗时 ms |
---|---|---|---|
1000万条 | 2624.001 | 0.272 | 0.207 |
1、pg_trgm
https://www.postgresql.org/docs/16/pgtrgm.html
如何计算两个字符串的相似度:
将字符串转换生成token的例子:
-- 第一步得到two和words, 然后得到" two "和" words ", 然后得到以下.
postgres=# select show_trgm('two ,words');
show_trgm
-------------------------------------------------------
{" t"," w"," tw"," wo","ds ",ord,rds,two,"wo ",wor}
(1 row)
postgres=# select show_trgm('two , words');
show_trgm
-------------------------------------------------------
{" t"," w"," tw"," wo","ds ",ord,rds,two,"wo ",wor}
(1 row)
postgres=# select show_trgm(' two , words ');
show_trgm
-------------------------------------------------------
{" t"," w"," tw"," wo","ds ",ord,rds,two,"wo ",wor}
(1 row)
-- 结果token会去重
postgres=# select show_trgm('two two1');
show_trgm
-----------------------------------
{" t"," tw","o1 ",two,"wo ",wo1}
(1 row)
postgres=# select show_trgm('two');
show_trgm
-------------------------
{" t"," tw",two,"wo "}
(1 row)
postgres=# select show_trgm('words');
show_trgm
---------------------------------
{" w"," wo","ds ",ord,rds,wor}
(1 row)
postgres=# select show_trgm('abc');
show_trgm
-------------------------
{" a"," ab",abc,"bc "}
(1 row)
postgres=# select show_trgm('abc hello');
show_trgm
-------------------------------------------------------
{" a"," h"," ab"," he",abc,"bc ",ell,hel,llo,"lo "}
(1 row)
比较两个字符串相似性的算法: 详见 contrib/pg_trgm/trgm_op.c
1: similarity (%
) (t % 'word' ==> 计算相似性对应 similarity(t, 'word')
)
相似性 = 两个字符串的token交集去重后的个数 / 两个字符串的token并集去重后的个数
大致可以表达 两个字符串的整体相似性.
阈值参数: pg_trgm.similarity_threshold (real)
2: word_similarity (<% and %>
) ('word' <% t ==> 计算相似性对应 word_similarity('word', t)
)
word_similarity(string1, string2)
== count.匹配string1 token的(token(substring(string2中的任意连续的word组))) / count(token(string1))
大致可以表达 字符串2的若干连续字符与字符串1的相似度.
阈值参数: pg_trgm.word_similarity_threshold (real)
3: strict_word_similarity (<<% and %>>
) ('word' <<% t ==> 计算相似性对应 strict_word_similarity('word', t)
)
strict_word_similarity(string1, string2)
== max( similarity(string1, string2中的任意连续的word组) )
大致可以表达 字符串2的若干连续单词与字符串1的相似度.
相似度阈值参数, 相似度大于阈值时, 对应的相似操作符返回true的结果.
阈值参数: pg_trgm.strict_word_similarity_threshold (real)
计算两个字符串相似度的例子:
postgres=# select similarity('abc','abc hello');
similarity
------------
0.4
(1 row)
postgres=# select similarity('abc hello','abc');
similarity
------------
0.4
(1 row)
word_similarity
postgres=# select word_similarity('abc','abc hello');
word_similarity
-----------------
1
(1 row)
postgres=# select word_similarity('abc hello','abc');
word_similarity
-----------------
0.4
(1 row)
strict_word_similarity
postgres=# select strict_word_similarity('abc','abc hello');
strict_word_similarity
------------------------
1
(1 row)
postgres=# select strict_word_similarity('abc hello','abc');
strict_word_similarity
------------------------
0.4
(1 row)
postgres=# select similarity('word', 'wor ord');
similarity
------------
0.625
(1 row)
postgres=# select similarity('word', 'ord wor');
similarity
------------
0.625
(1 row)
postgres=# select word_similarity('word', 'ord wor');
word_similarity
-----------------
1
(1 row)
postgres=# select word_similarity('word', 'wor ord');
word_similarity
-----------------
0.625
(1 row)
postgres=# select strict_word_similarity('word', 'wor ord');
strict_word_similarity
------------------------
0.625
(1 row)
postgres=# select strict_word_similarity('word', 'ord wor');
strict_word_similarity
------------------------
0.625
(1 row)
为什么传统方法与pg_trgm相比性能相差这么大?
字符串近似查询还可以应用于哪些场景?
如果将相似度调低, 性能还能这么好吗?
如果想返回最相似的一条, 怎么优化查询效果最佳?
和smlar相比, 搜索算法是否有相似之处?
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