表现 – LOWER LIKE vs iLIKE

以下两个查询组件的性能如何比较?

比较喜欢

... LOWER(description) LIKE '%abcde%' ...

我喜欢

... description iLIKE '%abcde%' ...
根据我的测试(每个查询中有10个),LOWER LIKE比iLIKE快17%.

说明

我创建了一行包含一些随机混合文本数据的行:

require 'securerandom'
inserts = []
1000000.times do |i|
        inserts << "(1, 'fake', '#{SecureRandom.urlsafe_base64(64)}')"
end
sql = "insert into books (user_id, title, description) values #{inserts.join(', ')}"
ActiveRecord::Base.connection.execute(sql)

验证行数:

my_test_db=# select count(id) from books ;
  count  
---------
 1000009

(是的,我有其他测试有九个额外的行 – 不是问题.)

示例查询和结果:

my_test_db=# SELECT "books".* FROM "books" WHERE "books"."published" = 'f'
my_test_db=# and (LOWER(description) LIKE '%abcde%') ;
   id    | user_id | title |                                      description                                       | published 
---------+---------+-------+----------------------------------------------------------------------------------------+------
 1232322 |       1 | fake  | 5WRGr7oCKABcdehqPKsUqV8ji61rsNGS1TX6pW5LJKrspOI_ttLNbaSyRz1BwTGQxp3OaxW7Xl6fzVpCu9y3fA | f
 1487103 |       1 | fake  | J6q0VkZ8-UlxIMZ_MFU_wsz_8MP3ZBQvkUo8-2INiDIp7yCZYoXqRyp1Lg7JyOwfsIVdpPIKNt1uLeaBCdelPQ | f
 1817819 |       1 | fake  | YubxlSkJOvmQo1hkk5pA1q2mMK6T7cOdcU3ADUKZO8s3otEAbCdEcmm72IOxiBdaXSrw20Nq2Lb383lq230wYg | f

LOWER LIKE的结果

my_test_db=# EXPLAIN ANALYZE SELECT "books".* FROM "books" WHERE "books"."published" = 'f' and (LOWER(description) LIKE '%abcde%') ;
                                                   QUERY PLAN                                                   
----------------------------------------------------------------------------------------------------------------
 Seq Scan on books  (cost=0.00..32420.14 rows=1600 width=117) (actual time=938.627..4114.038 rows=3 loops=1)
   Filter: ((NOT published) AND (lower(description) ~~ '%abcde%'::text))
   Rows Removed by Filter: 1000006
 Total runtime: 4114.098 ms

iLIKE的结果

my_test_db=# EXPLAIN ANALYZE SELECT "books".* FROM "books" WHERE "books"."published" = 'f' and (description iLIKE '%abcde%') ;
                                                   QUERY PLAN                                                   
----------------------------------------------------------------------------------------------------------------
 Seq Scan on books  (cost=0.00..29920.11 rows=100 width=117) (actual time=1147.612..4986.771 rows=3 loops=1)
   Filter: ((NOT published) AND (description ~~* '%abcde%'::text))
   Rows Removed by Filter: 1000006
 Total runtime: 4986.831 ms

数据库信息披露

Postgres版本:

my_test_db=# select version();
                                                                                 version
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 9.2.4 on x86_64-apple-darwin12.4.0, compiled by i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00), 64-bit

整理设置:

my_test_db=# select datcollate from pg_database where datname = 'my_test_db';
 datcollate  
-------------
 en_CA.UTF-8

表定义:

my_test_db=# \d books 
                                      Table "public.books"
   Column    |            Type             |                       Modifiers
-------------+-----------------------------+-------------------------------------------------------
 id          | integer                     | not null default nextval('books_id_seq'::regclass)
 user_id     | integer                     | not null
 title       | character varying(255)      | not null
 description | text                        | not null default ''::text
 published   | boolean                     | not null default false
Indexes:
    "books_pkey" PRIMARY KEY, btree (id)
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