海量数据,海明(simhash)距离高效检索(smlar) - 阿里云RDS PosgreSQL最佳实践
背景
http://www.cnblogs.com/lushilin/p/6549665.html
SimHash的应用
通过上面的步骤，我们可以利用SimHash算法为每一个网页生成一个向量指纹，那么问题来了，如何判断2篇文本的相似性？
这里面主要应用到是海明距离。
（1）什么是海明距离
两个码字的对应比特取值不同的比特数称为这两个码字的海明距离。在一个有效编码集中,任意两个码字的海明距离的最小值称为该编码集的海明距离。举例如下：10101和00110从第一位开始依次有第一位、第四、第五位不同，则海明距离为3。
（2）海明距离的几何意义
n位的码字可以用n维空间的超立方体的一个顶点来表示。两个码字之间的海明距离就是超立方体两个顶点之间的一条边，而且是这两个顶点之间的最短距离。
（3）海明距离的应用场景
用于编码的检错和纠错
经过SimHash算法提取来的指纹（Simhash对长文本500字+比较适用，短文本可能偏差较大，具体需要根据实际场景测试），最后使用海明距离，求相似，在google的论文给出的数据中，64位的签名，在海明距离为3的情况下，可认为两篇文档是相似的或者是重复的，当然这个值只是参考值，针对自己的应用可能又不同的测试取值
到这里相似度问题基本解决，但是按这个思路，在海量数据几百亿的数量下，效率问题还是没有解决的，因为数据是不断添加进来的，不可能每来一条数据，都要和全库的数据做一次比较，按照这种思路，处理速度会越来越慢，线性增长。
针对海量数据的去重效率，我们可以将64位指纹，切分为4份16位的数据块，根据抽屉原理在海明距离为3的情况，如果两个文档相似，那么它必有一个块的数据是相等的。
那么数据库是否支持这种高效率的检索呢？
反正PostgreSQL是支持的，通过黑科技smlar插件。
一、需求
快速从海量simhash中，搜索与某个simhash相似的simhash。
二、架构设计
在PostgreSQL中，从海量数据中，搜索海明距离小于N的数据，有多种设计手段。每种方法的能耗比都不一样，读者可以按需选择。
1 暴力计算
1、单机多核并行计算，暴力扫描。采用阿里云RDS PostgreSQL 10提供的多核并行能力，暴力扫描。
2、多机多核并行计算，暴力扫描。采用阿里云HybridDB for PostgreSQL提供的多级并行计算能力，暴力扫描。
3、利用GPU、FPGA加速暴力运算。
PostgreSQL提供了扩展接口，可以利用GPU，FPGA的能力对数据进行计算。
4、利用CPU向量计算指令，暴力计算。
PostgreSQL提供了扩展接口，可以利用CPU向量计算指令的能力加速计算。
2 索引
索引是高效的做法，例如PostgreSQL smlar插件，在阿里的导购平台就有使用，用于实时导购文的海量相似度查询。
如果要让smlar加速海明距离的搜索，需要采用更科学的方法，比如切片。
直接使用位置，会有问题，因为smlar的第一道工序是块级收敛，而海明码是bit64的编码，在一个数据块中，有若干条记录，任何位置都可能同时出现0和1，任何数据块都包含0和1，因此无法完成第一道过滤。
我们可以采用切片，减少这种可能性。例如每2个BIT一片，或者每4个BIT一片，或者更多。
通常海明距离大于3的，就没有什么相关性了。
三、DEMO与性能
1 暴力计算
1、全扫，并行扫描
创建测试表
create table hm (
id int, -- id
hmval bit(64) -- 海明HASH
);
写入1000万测试数据
postgres=# insert into hm select id, val::int8::bit(64) from (select id, sqrt(random())::numeric*9223372036854775807*2-9223372036854775807::numeric as val from generate_series(1,10000000) t(id)) t;
INSERT 0 10000000
postgres=# select * from hm limit 10;
id | hmval
----+------------------------------------------------------------------
1 | 0000101001110110110101010111101011100110101010000111100011110111
2 | 0110011100110101101000001010101111010001011101100111111011001110
3 | 1010110111001011011110110000111111101101101111010111111100101110
4 | 0110011110110000001011000010010000101011100101010100111000101001
5 | 0101110100101111010110010110000000101110000010001011010110110000
6 | 0011010000100000101011011100000101111110010110111101100001100001
7 | 1011110011101101101000011101011101010111011001011010110111101000
8 | 1110010011000101001101110010001111110100001101010101111101110010
9 | 0110111111110011101001001000101101011011111100010010111010001111
10 | 0011100011000010101011010001111000000110100011100100111011011001
(10 rows)
设置暴力并行度
postgres=# set force_parallel_mode = on;
postgres=# set min_parallel_table_scan_size = 0;
postgres=# set parallel_setup_cost = 0;
postgres=# set parallel_tuple_cost = 0;
postgres=# alter table hm set (parallel_workers = 128);
postgres=# set max_parallel_workers_per_gather = 64;
并行查询海明距离小于4的记录，耗时463毫秒。
postgres=# select * from hm where length(replace(bitxor(bit'0011110001011010110010001011010101001000111110000111110010010110', hmval)::text,'0','')) < 4;
id | hmval
----+------------------------------------------------------------------
16 | 0011110001011010110010001011010101001000111110000111110010010110
(1 row)
Time: 463.314 ms
非并行查询海明距离小于4的记录，耗时16秒。
postgres=# select * from hm where length(replace(bitxor(bit'0011110001011010110010001011010101001000111110000111110010010110', hmval)::text,'0','')) < 4;
id | hmval
----+------------------------------------------------------------------
16 | 0011110001011010110010001011010101001000111110000111110010010110
(1 row)
Time: 16791.215 ms (00:16.791)
求两个BIT的不同位数，还有更高效率的方法。理论上可以达到100毫秒以内。
https://www.postgresql.org/message-id/flat/ab1ea6540903121110l2a3021d4h6632b206e2419898%40mail.gmail.com#ab1ea6540903121110l2a3021d4h6632b206e2419898@mail.gmail.com
2 索引
阿里云RDS PostgreSQL提供了一个smlar插件，用于高效率的求数组的相似度。
我们需要将海明HASH，转换为数组，根据前面的设计，我们采用8个BIT一片的切法，支持索引查询海明距离为8以内的值。
切之前，验证一下切片后的过滤性：
postgres=# select relpages from pg_class where relname='hm';
relpages
----------
63695
(1 row)
1、单个片为1时，不用说，每个块都包含。
postgres=# select count(*) from (select substring(ctid::text,'(\d+),') from hm where substring(hmval,1,1)='0' group by 1)t;
count
-------
63695
(1 row)
2、单个片为8时，有接近一半的块包含。
postgres=# select count(*) from (select substring(ctid::text,'(\d+),') from hm where substring(hmval,1,8)='00000000' group by 1)t;
count
-------
29100
(1 row)
3、单个片为16时，只有100多个块包含了。
postgres=# select count(*) from (select substring(ctid::text,'(\d+),') from hm where substring(hmval,1,16)='0000000000000000' group by 1)t;
count
-------
160
(1 row)
8片切法的性能验证
创建插件
create extension smlar;
创建测试表
create table hm1 (id int, hmval bit(64), hmarr text[]);
生成1000万测试数据，生成测试数据时，按切分手段进行切分，记录为TEXT数组。
insert into hm1
select
id,
val::bit(64),
regexp_split_to_array('1_'||substring(val,1,8)||',2_'||substring(val,9,8)||',3_'||substring(val,17,8)||',4_'||substring(val,25,8)||',5_'||substring(val,33,8)||',6_'||substring(val,41,8)||',7_'||substring(val,49,8)||',8_'||substring(val,57,8), ',')
from
(select id, (sqrt(random())::numeric*9223372036854775807*2-9223372036854775807::numeric)::int8::bit(64)::text as val from generate_series(1,10000000) t(id)) t;
postgres=# select * from hm1 limit 10;
id | hmval | hmarr
----+------------------------------------------------------------------+-------------------------------------------------------------------------------------------
1 | 0000001110101101100110011000100111100100001100100101101010010011 | {1_00000011,2_10101101,3_10011001,4_10001001,5_11100100,6_00110010,7_01011010,8_10010011}
2 | 0001001000010101001100100010101010111001001000000110101101100100 | {1_00010010,2_00010101,3_00110010,4_00101010,5_10111001,6_00100000,7_01101011,8_01100100}
3 | 0011111111010100011001001010110110100010101110101001101111010000 | {1_00111111,2_11010100,3_01100100,4_10101101,5_10100010,6_10111010,7_10011011,8_11010000}
4 | 1100110010011001001110101110111111111111010000100000010011000010 | {1_11001100,2_10011001,3_00111010,4_11101111,5_11111111,6_01000010,7_00000100,8_11000010}
5 | 0011000011010001011111010101010111100110000110000011101100000101 | {1_00110000,2_11010001,3_01111101,4_01010101,5_11100110,6_00011000,7_00111011,8_00000101}
6 | 0111101101111110101000010110101101110011011110100100010111011001 | {1_01111011,2_01111110,3_10100001,4_01101011,5_01110011,6_01111010,7_01000101,8_11011001}
7 | 0010001011111111100010101011110001001101001011100100011000010000 | {1_00100010,2_11111111,3_10001010,4_10111100,5_01001101,6_00101110,7_01000110,8_00010000}
8 | 1110001111100011011110110111101111010101000111000100111111111101 | {1_11100011,2_11100011,3_01111011,4_01111011,5_11010101,6_00011100,7_01001111,8_11111101}
9 | 0111110010111000010111001000000101111000000110110110000011101110 | {1_01111100,2_10111000,3_01011100,4_10000001,5_01111000,6_00011011,7_01100000,8_11101110}
10 | 0111001101100010001101101111000000100100000000010001010011100101 | {1_01110011,2_01100010,3_00110110,4_11110000,5_00100100,6_00000001,7_00010100,8_11100101}
(10 rows)
创建smlar索引
postgres=# create index idx_hm1 on hm1 using gin(hmarr _text_sml_ops );
搜索海明距离小于等于1的VALUE。用到了smlar索引，耗时63毫秒。
postgres=# set smlar.type = overlap;
postgres=# set smlar.threshold = 7;
select
*,
smlar( hmarr, '{1_00000011,2_10101101,3_10011001,4_10001001,5_11100100,6_00110010,7_01011010,8_10010011}')
from
hm1
where
hmarr % '{1_00000011,2_10101101,3_10011001,4_10001001,5_11100100,6_00110010,7_01011010,8_10010011}'
and length(replace(bitxor(bit'0000001110101101100110011000100111100100001100100101101010010011', hmval)::text,'0','')) < 2
limit 100;
postgres=# explain (analyze,verbose,timing,costs,buffers) select
*,
smlar( hmarr, '{1_00000011,2_10101101,3_10011001,4_10001001,5_11100100,6_00110010,7_01011010,8_10010011}')
from
hm1
where
hmarr % '{1_00000011,2_10101101,3_10011001,4_10001001,5_11100100,6_00110010,7_01011010,8_10010011}'
and length(replace(bitxor(bit'0000001110101101100110011000100111100100001100100101101010010011', hmval)::text,'0','')) < 2
limit 100;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=117.83..420.48 rows=100 width=169) (actual time=62.928..62.929 rows=1 loops=1)
Output: id, hmval, hmarr, (smlar(hmarr, '{1_00000011,2_10101101,3_10011001,4_10001001,5_11100100,6_00110010,7_01011010,8_10010011}'::text[]))
Buffers: shared hit=166
-> Bitmap Heap Scan on public.hm1 (cost=117.83..10205.17 rows=3333 width=169) (actual time=62.927..62.927 rows=1 loops=1)
Output: id, hmval, hmarr, smlar(hmarr, '{1_00000011,2_10101101,3_10011001,4_10001001,5_11100100,6_00110010,7_01011010,8_10010011}'::text[])
Recheck Cond: (hm1.hmarr % '{1_00000011,2_10101101,3_10011001,4_10001001,5_11100100,6_00110010,7_01011010,8_10010011}'::text[])
Filter: (length(replace((bitxor(B'0000001110101101100110011000100111100100001100100101101010010011'::"bit", hm1.hmval))::text, '0'::text, ''::text)) < 2)
Heap Blocks: exact=1
Buffers: shared hit=166
-> Bitmap Index Scan on idx_hm1 (cost=0.00..117.00 rows=10000 width=0) (actual time=62.898..62.898 rows=1 loops=1)
Index Cond: (hm1.hmarr % '{1_00000011,2_10101101,3_10011001,4_10001001,5_11100100,6_00110010,7_01011010,8_10010011}'::text[])
Buffers: shared hit=165
Planning time: 0.147 ms
Execution time: 62.975 ms
(14 rows)
postgres=# select
*,
smlar( hmarr, '{1_00000011,2_10101101,3_10011001,4_10001001,5_11100100,6_00110010,7_01011010,8_10010011}')
from
hm1
where
hmarr % '{1_00000011,2_10101101,3_10011001,4_10001001,5_11100100,6_00110010,7_01011010,8_10010011}'
and length(replace(bitxor(bit'0000001110101101100110011000100111100100001100100101101010010011', hmval)::text,'0','')) < 2
limit 100;
id | hmval | hmarr | smlar
----+------------------------------------------------------------------+-------------------------------------------------------------------------------------------+-------
1 | 0000001110101101100110011000100111100100001100100101101010010011 | {1_00000011,2_10101101,3_10011001,4_10001001,5_11100100,6_00110010,7_01011010,8_10010011} | 8
(1 row)
Time: 61.227 ms
如果我们只需要查询4以内的海明距离，实际上可以使用16的分组，或者我们可以使用混合切法。
6片混合切法的性能验证
切法为8,16,8,8,16,8。支持海明距离6以内的查询。
create table hm2 (id int, hmval bit(64), hmarr text[]);
insert into hm2
select
id,
val::bit(64),
regexp_split_to_array('1_'||substring(val,1,8)||',2_'||substring(val,9,16)||',3_'||substring(val,25,8)||',4_'||substring(val,33,8)||',5_'||substring(val,41,16)||',6_'||substring(val,57,8), ',')
from
(select id, (sqrt(random())::numeric*9223372036854775807*2-9223372036854775807::numeric)::int8::bit(64)::text as val from generate_series(1,10000000) t(id)) t;
postgres=# select * from hm2 limit 10;
id | hmval | hmarr
----+------------------------------------------------------------------+-------------------------------------------------------------------------------------
1 | 1100111011000001100100100111111110100011100111111101101001101010 | {1_11001110,2_1100000110010010,3_01111111,4_10100011,5_1001111111011010,6_01101010}
2 | 0111111000101011000111010011011000000010010001111001000111011101 | {1_01111110,2_0010101100011101,3_00110110,4_00000010,5_0100011110010001,6_11011101}
3 | 0111111000101111000101011100100000001111011101101100110100000101 | {1_01111110,2_0010111100010101,3_11001000,4_00001111,5_0111011011001101,6_00000101}
4 | 0111010101010010100000110001100011110010111000001011000010010010 | {1_01110101,2_0101001010000011,3_00011000,4_11110010,5_1110000010110000,6_10010010}
5 | 1111101100110100101111000011001011111110111000100110101001100001 | {1_11111011,2_0011010010111100,3_00110010,4_11111110,5_1110001001101010,6_01100001}
6 | 0011110000100010101001000001100010000010111011100010011001000110 | {1_00111100,2_0010001010100100,3_00011000,4_10000010,5_1110111000100110,6_01000110}
7 | 0000111111001110100110011110000110001101110111111111111010111001 | {1_00001111,2_1100111010011001,3_11100001,4_10001101,5_1101111111111110,6_10111001}
8 | 0110100010010100111100110110000011101110101001001111010101011111 | {1_01101000,2_1001010011110011,3_01100000,4_11101110,5_1010010011110101,6_01011111}
9 | 0111001111001100101011001001100100000000111100000110110001000011 | {1_01110011,2_1100110010101100,3_10011001,4_00000000,5_1111000001101100,6_01000011}
10 | 1101111101011000111100101010101000100001101100101110100001111000 | {1_11011111,2_0101100011110010,3_10101010,4_00100001,5_1011001011101000,6_01111000}
(10 rows)
create index idx_hm2 on hm2 using gin(hmarr _text_sml_ops );
查询海明距离小于等于1的值，提高到2毫秒了。
postgres=# set smlar.type = overlap;
postgres=# set smlar.threshold = 5;
postgres=# select
*,
smlar( hmarr, '{1_11001110,2_1100000110010010,3_01111111,4_10100011,5_1001111111011010,6_01101010}')
from
hm2
where
hmarr % '{1_11001110,2_1100000110010010,3_01111111,4_10100011,5_1001111111011010,6_01101010}'
and length(replace(bitxor(bit'1100111011000001100100100111111110100011100111111101101001101010', hmval)::text,'0','')) < 2
limit 100;
id | hmval | hmarr | smlar
----+------------------------------------------------------------------+-------------------------------------------------------------------------------------+-------
1 | 1100111011000001100100100111111110100011100111111101101001101010 | {1_11001110,2_1100000110010010,3_01111111,4_10100011,5_1001111111011010,6_01101010} | 6
(1 row)
Time: 1.954 ms
postgres=# explain (analyze,verbose,timing,costs,buffers) select
*,
smlar( hmarr, '{1_11001110,2_1100000110010010,3_01111111,4_10100011,5_1001111111011010,6_01101010}')
from
hm2
where
hmarr % '{1_11001110,2_1100000110010010,3_01111111,4_10100011,5_1001111111011010,6_01101010}'
and length(replace(bitxor(bit'1100111011000001100100100111111110100011100111111101101001101010', hmval)::text,'0','')) < 2
limit 100;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=103.83..406.06 rows=100 width=153) (actual time=2.414..2.416 rows=1 loops=1)
Output: id, hmval, hmarr, (smlar(hmarr, '{1_11001110,2_1100000110010010,3_01111111,4_10100011,5_1001111111011010,6_01101010}'::text[]))
Buffers: shared hit=102
-> Bitmap Heap Scan on public.hm2 (cost=103.83..10177.17 rows=3333 width=153) (actual time=2.414..2.415 rows=1 loops=1)
Output: id, hmval, hmarr, smlar(hmarr, '{1_11001110,2_1100000110010010,3_01111111,4_10100011,5_1001111111011010,6_01101010}'::text[])
Recheck Cond: (hm2.hmarr % '{1_11001110,2_1100000110010010,3_01111111,4_10100011,5_1001111111011010,6_01101010}'::text[])
Filter: (length(replace((bitxor(B'1100111011000001100100100111111110100011100111111101101001101010'::"bit", hm2.hmval))::text, '0'::text, ''::text)) < 2)
Heap Blocks: exact=1
Buffers: shared hit=102
-> Bitmap Index Scan on idx_hm2 (cost=0.00..103.00 rows=10000 width=0) (actual time=2.374..2.374 rows=1 loops=1)
Index Cond: (hm2.hmarr % '{1_11001110,2_1100000110010010,3_01111111,4_10100011,5_1001111111011010,6_01101010}'::text[])
Buffers: shared hit=101
Planning time: 0.149 ms
Execution time: 2.463 ms
(14 rows)
4片切法的性能验证
create table hm3 (id int, hmval bit(64), hmarr text[]);
insert into hm3
select
id,
val::bit(64),
regexp_split_to_array('1_'||substring(val,1,16)||',2_'||substring(val,17,16)||',3_'||substring(val,33,16)||',4_'||substring(val,41,16), ',')
from
(select id, (sqrt(random())::numeric*9223372036854775807*2-9223372036854775807::numeric)::int8::bit(64)::text as val from generate_series(1,10000000) t(id)) t;
postgres=# select * from hm3 limit 10;
id | hmval | hmarr
----+------------------------------------------------------------------+-------------------------------------------------------------------------------
1 | 0101011111111010000001001011101101100011111101111101101100000011 | {1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011}
2 | 1101011000010000000000000000111011011111011101110100000010101011 | {1_1101011000010000,2_0000000000001110,3_1101111101110111,4_0111011101000000}
3 | 0101000010110110110010001010100010101001001010111111011000110011 | {1_0101000010110110,2_1100100010101000,3_1010100100101011,4_0010101111110110}
4 | 0111000111100011111000100111000011101111110000011110101101000100 | {1_0111000111100011,2_1110001001110000,3_1110111111000001,4_1100000111101011}
5 | 0010111010101011111010011110110010011110111111110011101110010011 | {1_0010111010101011,2_1110100111101100,3_1001111011111111,4_1111111100111011}
6 | 0110111110011100100110010111010000000011100011000011110001010110 | {1_0110111110011100,2_1001100101110100,3_0000001110001100,4_1000110000111100}
7 | 0100110100111001110011011110100111101110101001000101010110110110 | {1_0100110100111001,2_1100110111101001,3_1110111010100100,4_1010010001010101}
8 | 0110010111001100111000011011011100001100111111101111011010100010 | {1_0110010111001100,2_1110000110110111,3_0000110011111110,4_1111111011110110}
9 | 0110111010110000001010101111000101110000010011100011100101000100 | {1_0110111010110000,2_0010101011110001,3_0111000001001110,4_0100111000111001}
10 | 0101101000000110100101100011111111000101110001010011100110101011 | {1_0101101000000110,2_1001011000111111,3_1100010111000101,4_1100010100111001}
(10 rows)
create index idx_hm3 on hm3 using gin(hmarr _text_sml_ops );
查询海明距离小于等于1的值，提高到0.2毫秒了。
postgres=# set smlar.type = overlap;
postgres=# set smlar.threshold = 3;
postgres=# select
*,
smlar( hmarr, '{1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011}')
from
hm3
where
hmarr % '{1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011}'
and length(replace(bitxor(bit'0101011111111010000001001011101101100011111101111101101100000011', hmval)::text,'0','')) < 2
limit 100;
id | hmval | hmarr | smlar
----+------------------------------------------------------------------+-------------------------------------------------------------------------------+-------
1 | 0101011111111010000001001011101101100011111101111101101100000011 | {1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011} | 4
(1 row)
postgres=# explain (analyze,verbose,timing,costs,buffers) select
*,
smlar( hmarr, '{1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011}')
from
hm3
where
hmarr % '{1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011}'
and length(replace(bitxor(bit'0101011111111010000001001011101101100011111101111101101100000011', hmval)::text,'0','')) < 2
limit 100;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=99.83..401.19 rows=100 width=134) (actual time=0.169..0.170 rows=1 loops=1)
Output: id, hmval, hmarr, (smlar(hmarr, '{1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011}'::text[]))
Buffers: shared hit=14
-> Bitmap Heap Scan on public.hm3 (cost=99.83..10144.17 rows=3333 width=134) (actual time=0.168..0.169 rows=1 loops=1)
Output: id, hmval, hmarr, smlar(hmarr, '{1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011}'::text[])
Recheck Cond: (hm3.hmarr % '{1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011}'::text[])
Filter: (length(replace((bitxor(B'0101011111111010000001001011101101100011111101111101101100000011'::"bit", hm3.hmval))::text, '0'::text, ''::text)) < 2)
Heap Blocks: exact=1
Buffers: shared hit=14
-> Bitmap Index Scan on idx_hm3 (cost=0.00..99.00 rows=10000 width=0) (actual time=0.145..0.145 rows=1 loops=1)
Index Cond: (hm3.hmarr % '{1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011}'::text[])
Buffers: shared hit=13
Planning time: 0.101 ms
Execution time: 0.200 ms
(14 rows)
查询海明距离小于等于4的，依旧在毫秒返回。
postgres=# set smlar.type = overlap;
postgres=# set smlar.threshold = 0;
postgres=# select
*,
smlar( hmarr, '{1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011}')
from
hm3
where
hmarr % '{1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011}'
and length(replace(bitxor(bit'0101011111111010000001001011101101100011111101111101101100000011', hmval)::text,'0','')) < 5
limit 100;
id | hmval | hmarr | smlar
----+------------------------------------------------------------------+-------------------------------------------------------------------------------+-------
1 | 0101011111111010000001001011101101100011111101111101101100000011 | {1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011} | 4
(1 row)
Time: 6.983 ms
不使用索引，23秒。
postgres=# select * from hm3 where length(replace(bitxor(bit'0101011111111010000001001011101101100011111101111101101100000011', hmval)::text,'0','')) < 5;
id | hmval | hmarr
----+------------------------------------------------------------------+-------------------------------------------------------------------------------
1 | 0101011111111010000001001011101101100011111101111101101100000011 | {1_0101011111111010,2_0000010010111011,3_0110001111110111,4_1111011111011011}
(1 row)
Time: 22954.686 ms
相比没有索引的情况，性能从23秒提升到了0.2毫秒。提升了11.48万倍。
自动切分
有人会说，怎么自动生成simhash切分后的数组呢？
不用担心，PostgreSQL提供了强大的UDF功能，可以建立UDF和TRIGGER，在写入数据时，自动生成切分后的数组。
例子
create table hm4 (id int, hmval bit(64), hmarr text[]);
create or replace function sp(val bit(64)) returns text[] as $$
select regexp_split_to_array('1_'||substring(val::text,1,10)||',2_'||substring(val::text,11,10)||',3_'||substring(val::text,21,10)||',4_'||substring(val::text,31,10)||',5_'||substring(val::text,41,10)||',6_'||substring(val::text,51,14), ',') ;
$$ language sql strict;
postgres=# select sp(123::bit(64));
sp
-------------------------------------------------------------------------------------
{1_0000000000,2_0000000000,3_0000000000,4_0000000000,5_0000000000,6_00000001111011}
(1 row)
-- 写入1亿记录
insert into hm4
select
id,
val::bit(64),
sp(val::bit(64))
from
(select id, (sqrt(random())::numeric*9223372036854775807*2-9223372036854775807::numeric)::int8::bit(64)::text as val from generate_series(1,100000000) t(id)) t;
-- 索引
create index idx_hm4 on hm4 using gin(hmarr _text_sml_ops );
-- 查询海明距离小于等于1的记录，性能杠杠的
postgres=# set smlar.type = overlap;
postgres=# set smlar.threshold = 5;
select
*,
smlar( hmarr, sp(123::bit(64))) -- 查询与123::bit(64)海明距离小于2的记录
from
hm3
where
hmarr % sp(123::bit(64)) -- 查询与123::bit(64)海明距离小于2的记录
and length(replace(bitxor(123::bit(64), hmval)::text,'0','')) < 2 -- 查询与123::bit(64)海明距离小于2的记录
limit 100;
postgres=# explain (analyze,verbose,timing,costs,buffers) select
*,
smlar( hmarr, sp(123::bit(64))) -- 查询与123::bit(64)海明距离小于2的记录
from
hm3
where
hmarr % sp(123::bit(64)) -- 查询与123::bit(64)海明距离小于2的记录
and length(replace(bitxor(123::bit(64), hmval)::text,'0','')) < 2 -- 查询与123::bit(64)海明距离小于2的记录
limit 100;
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=109.83..411.19 rows=100 width=134) (actual time=0.078..0.078 rows=0 loops=1)
Output: id, hmval, hmarr, (smlar(hmarr, '{1_0000000000,2_0000000000,3_0000000000,4_0000000000,5_0000000000,6_00000001111011}'::text[]))
Buffers: shared hit=19
-> Bitmap Heap Scan on public.hm3 (cost=109.83..10154.17 rows=3333 width=134) (actual time=0.076..0.076 rows=0 loops=1)
Output: id, hmval, hmarr, smlar(hmarr, '{1_0000000000,2_0000000000,3_0000000000,4_0000000000,5_0000000000,6_00000001111011}'::text[])
Recheck Cond: (hm3.hmarr % '{1_0000000000,2_0000000000,3_0000000000,4_0000000000,5_0000000000,6_00000001111011}'::text[])
Filter: (length(replace((bitxor(B'0000000000000000000000000000000000000000000000000000000001111011'::bit(64), hm3.hmval))::text, '0'::text, ''::text)) < 2)
Buffers: shared hit=19
-> Bitmap Index Scan on idx_hm3 (cost=0.00..109.00 rows=10000 width=0) (actual time=0.074..0.074 rows=0 loops=1)
Index Cond: (hm3.hmarr % '{1_0000000000,2_0000000000,3_0000000000,4_0000000000,5_0000000000,6_00000001111011}'::text[])
Buffers: shared hit=19
Planning time: 0.592 ms
Execution time: 0.117 ms
(13 rows)
创建触发器，写入simhash时，自动写入切分数组
create or replace function tg() returns trigger as $$
declare
begin
NEW.hmarr := sp(NEW.hmval);
return NEW;
end;
$$ language plpgsql strict;
postgres=# create trigger tg before insert or update on hm4 for each row execute procedure tg();
CREATE TRIGGER
-- 效果很赞
postgres=# truncate hm4;
TRUNCATE TABLE
postgres=# insert into hm4 values (1,1::bit(64));
INSERT 0 1
postgres=# select * from hm4;
id | hmval | hmarr
----+------------------------------------------------------------------+-------------------------------------------------------------------------------------
1 | 0000000000000000000000000000000000000000000000000000000000000001 | {1_0000000000,2_0000000000,3_0000000000,4_0000000000,5_0000000000,6_00000000000001}
(1 row)
postgres=# update hm4 set hmval=123456::bit(64);
UPDATE 1
postgres=# select * from hm4;
id | hmval | hmarr
----+------------------------------------------------------------------+-------------------------------------------------------------------------------------
1 | 0000000000000000000000000000000000000000000000011110001001000000 | {1_0000000000,2_0000000000,3_0000000000,4_0000000000,5_0000000111,6_10001001000000}
(1 row)
爽就点个赞吧。
GIN索引本例相似算法介绍
1、cosine
相似值算法
case ST_COSINE:
{
int cnt;
double power;
power = ((double)(sa->nelems)) * ((double)(sb->nelems));
cnt = numOfIntersect(sa, sb);
PG_RETURN_FLOAT4( ((double)cnt) / sqrt( power ) );
}
GIN一重过滤算法
case ST_COSINE:
{
double power;
power = sqrt( ((double)(sa->nelems)) * ((double)(cnt)) );
if ( ((double)cnt) / power >= GetSmlarLimit() )
res = true;
}
2、overlap
相似值算法
case ST_OVERLAP:
{
float4 res = (float4)numOfIntersect(sa, sb);
PG_RETURN_FLOAT4(res);
}
GIN一重过滤算法
case ST_OVERLAP:
if (cnt >= GetSmlarLimit())
res = true;
3、tfidf
目前仅支持词频tf=idf的gin索引检索(否则不支持gin索引)，IDF写在表里。(也就是说索引 支持词的权重IDF，但是不支持词频TF。算法本身是同时支持TF和IDF的)
如果要让GIN支持TF，需要改一下smlar代码。(不过目前GiST索引已支持词频TF了。所以你可以选择GiST索引)
《PostgreSQL结合余弦、线性相关算法 在文本、图片、数组相似 等领域的应用 - 2 smlar插件详解》
可用于短文本的相似度判断。
相似值算法
考虑了IDF的权重(一张IDF TABLE来记录权重)。相比cosine算法，更适合短文本相似判断。 但是请注意不考虑TF。
例如一个短文的关键词向量为 (我1, 爱1, 中国5),这里的tf 115全部不计。
static double
TFIDFSml(SimpleArray *a, SimpleArray *b)
{
int cmp;
Datum *aptr = a->elems,
*bptr = b->elems;
ProcTypeInfo info = a->info;
double res = 0.0;
double suma = 0.0, sumb = 0.0;
Assert( a->info->typid == b->info->typid );
Assert( a->df );
Assert( b->df );
getFmgrInfoCmp(info);
while( aptr - a->elems < a->nelems && bptr - b->elems < b->nelems )
{
cmp = cmpArrayElem(aptr, bptr, info);
if ( cmp < 0 )
{
suma += a->df[ aptr - a->elems ] * a->df[ aptr - a->elems ];
aptr++;
}
else if ( cmp > 0 )
{
sumb += b->df[ bptr - b->elems ] * b->df[ bptr - b->elems ];
bptr++;
}
else
{
res += a->df[ aptr - a->elems ] * b->df[ bptr - b->elems ];
suma += a->df[ aptr - a->elems ] * a->df[ aptr - a->elems ];
sumb += b->df[ bptr - b->elems ] * b->df[ bptr - b->elems ];
aptr++;
bptr++;
}
}
/*
* Compute last elements
*/
while( aptr - a->elems < a->nelems )
{
suma += a->df[ aptr - a->elems ] * a->df[ aptr - a->elems ];
aptr++;
}
while( bptr - b->elems < b->nelems )
{
sumb += b->df[ bptr - b->elems ] * b->df[ bptr - b->elems ];
bptr++;
}
if ( suma > 0.0 && sumb > 0.0 )
res = res / sqrt( suma * sumb );
else
res = 0.0;
return res;
}
GIN一重过滤算法
case ST_TFIDF:
{
double weight = 0.0, /* exact weight of union */
saSum = 0.0, /* exact length of query */
siSum = 0.0; /* lower limit of length of indexed value */
if ( getTFMethod() != TF_CONST )
elog(ERROR,"GIN supports only smlar.tf_method = \"const\"" );
Assert(sa->df);
for(i=0; i<sa->nelems; i++)
{
/*
* With smlar.tf_method = "const" sa->df[i] is
* equal to its idf, so lookup of StatElem is not needed
*/
if ( check[i] )
{
weight += sa->df[i] * sa->df[i];
siSum += sa->df[i] * sa->df[i];
}
saSum += sa->df[i] * sa->df[i];
}
if ( saSum > 0.0 && siSum > 0.0 && weight / sqrt(saSum * siSum ) > GetSmlarLimit() )
res = true;
}
根据TFIDF查找相似的，可以参考
《PostgreSQL结合余弦、线性相关算法 在文本、图片、数组相似 等领域的应用 - 2 smlar插件详解》
四、技术点
1、阿里云RDS PostgreSQL smlar插件，使用GIN索引，块级收敛，二重过滤。0.2毫秒的响应速度，1000万数据中，检索海明距离2以内的记录。
2、阿里云RDS PostgreSQL 10，使用多核并行，暴力扫描，1000万数据，检索海明距离为N以内的数据，约450毫秒。
3、阿里云HybridDB for PostgreSQL，使用多机并行，横向扩展计算能力，也可以做到暴力扫描。根据实际的节点数计算查询效率。
五、云端产品
六、类似场景、案例
《电商内容去重\内容筛选应用(实时识别转载\盗图\侵权?) - 文本、图片集、商品集、数组相似判定的优化和索引技术》
《基于 阿里云 RDS PostgreSQL 打造实时用户画像推荐系统》
七、小结
采用阿里云RDS PostgreSQL的SMLAR插件，对千万量级的simhash数据检索相似文本，（更多数据量的测试后续提供，响应速度应该在毫秒级），相比没有索引的情况，性能从23秒提升到了0.2毫秒。提升了11.48万倍。
开不开心，意不意外。
八、参考
《从难缠的模糊查询聊开 - PostgreSQL独门绝招之一 GIN , GiST , SP-GiST , RUM 索引原理与技术背景》
《PostgreSQL结合余弦、线性相关算法 在文本、图片、数组相似 等领域的应用 - 3 rum, smlar应用场景分析》
《PostgreSQL结合余弦、线性相关算法 在文本、图片、数组相似 等领域的应用 - 2 smlar插件详解》
《17种文本相似算法与GIN索引 - pg_similarity》
《电商内容去重\内容筛选应用(实时识别转载\盗图\侵权?) - 文本、图片集、商品集、数组相似判定的优化和索引技术》
《从相似度算法谈起 - Effective similarity search in PostgreSQL》
《PostgreSQL (varbit, roaring bitmap) VS pilosa(bitmap库)》
《阿里云RDS for PostgreSQL varbitx插件与实时画像应用场景介绍》
《基于 阿里云 RDS PostgreSQL 打造实时用户画像推荐系统》
《块级(ctid)扫描在IoT(物联网)极限写和消费读并存场景的应用》
Fast Search in Hamming Space with Multi-index Hashing