Search is not working with lowercase LIKE for russian characters - postgresql

Question

What is the reason for this query give no results:
SELECT name FROM users WHERE LOWER(name) LIKE LOWER('%после%');
When this works fine:
SELECT name FROM users WHERE LOWER(name) LIKE LOWER('%После%');
Name is 'Последователь'. If i use name like 'Post', then search works fine.
Version: PostgreSQL 11.2 (Ubuntu 11.2-100) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0, 64-bit
Server and db encoding is UTF8. Client encoding is UNICODE.

Answer 1

The lower function works according to the database collation, so the result will vary depending on how your database is defined.
It will work well in this case:
test=# CREATE DATABASE rus TEMPLATE template0
ENCODING UTF8 LC_COLLATE "ru_RU.utf8" LC_CTYPE "ru_RU.utf8";
CREATE DATABASE
test=# \c rus
You are now connected to database "rus" as user "postgres".
rus=# SELECT LOWER('%после%') = LOWER('%После%');
?column?
----------
t
(1 row)
But it won't work with the C collation, because that does not know how to properly lower case cyrillic characters:
rus=# \c test
You are now connected to database "test" as user "postgres".
test=# DROP DATABASE rus;
DROP DATABASE
test=# CREATE DATABASE rus TEMPLATE template0
ENCODING UTF8 LC_COLLATE "C" LC_CTYPE "C";
CREATE DATABASE
test=# \c rus
You are now connected to database "rus" as user "postgres".
rus=# SELECT LOWER('%после%') = LOWER('%После%');
?column?
----------
f
(1 row)
So if you want your query to work well, make sure that you are using a collation that knows how to convert the characters to upper and lower case.

Answer 2

Functions LOWER and UPPER do not work correct if database has wrong locale.
Step-1. View locale of databases in Postgres:
\l
Step-2. View available locales in terminal:
locale -a
or by your language, for example ru:
locale -a | grep ru
Step-3. Use locale name from Step-2(for example ru_RU.utf8) in updating database YOUR_DATABASE_NAME in Postgres:
update pg_database set datcollate='ru_RU.utf8', datctype='ru_RU.utf8' where datname='YOUR_DATABASE_NAME';
or you can create new database with locale:
CREATE DATABASE NEW_DATABASE_NAME TEMPLATE template0 ENCODING UTF8 LC_COLLATE "ru_RU.UTF-8" LC_CTYPE "ru_RU.UTF-8";
PS My macOS has name ru_RU.UTF-8, Ubuntu has name ru_RU.utf8

Answer 3

Look Closer. LIKE statement is sometimes case sensitive. Depens on the program you use
The first statement is :
SELECT name FROM users WHERE LOWER(name) LIKE LOWER('%после%');
while the second is :
SELECT name FROM users WHERE LOWER(name) LIKE LOWER('%После%');
What you seek is
П оследователь
note that in the first statement you wrote with (п) while the second one is (П)