renaming column in ClickHouse

If your CH server supports ALTER TABLE UPDATE(if no, sorry but it's time to migrate to new version)  here is trick:

!Before doing it please test on your test data!

create new column

ALTER TABLE my_super_table
    ADD COLUMN
    new_name UInt32 DEFAULT CAST(0, 'UInt32');

later set value

ALTER TABLE my_super_table
    UPDATE new_name = old_name WHERE 1;

Not sure but I think it's better to wait until you merges are finished

drop column

ALTER TABLE my_super_table
    DROP COLUMN old_name;



That's it!

Percona Live 2018

This year I visited US Percona Live 2018. A lot of amazing topics, from my perspective the best one was from ClickHouse Team: "ClickHouse Deep Dive".

Also good ones:

• "Designing and launching the next-generation database system @ Slack: from whiteboard to production"
• "The Evolution of Backup/Restore System in Facebook" 

Comparing mydumper vs pure mysqldump

Today I tested mydumper.

It showed very cool results, see below.

Data size + index size is 9054MB. Here is timing of dumping/restoring the same database using mydumper vs mysqldump.

     |  mysqldump/restore dump |mydumper/loader
dump   | real    1m36.642s     |real    0m49.342s
restore| real    32m32.732s     |real    20m32.215s

Also during compilation I faced with issue:

mydumper.c:2814: error: ‘MYSQL_TYPE_JSON’ undeclared (first use in this function)
mydumper.c:2814: error: (Each undeclared identifier is reported only once
mydumper.c:2814: error: for each function it appears in.)
make[2]: *** [CMakeFiles/mydumper/mydumper.c.o] Error 1
make[1]: *** [CMakeFiles/mydumper/all] Error 2
make: *** [all] Error 2

Probably because I am using Percoan 5.6 dev package and JSON data type was introduced only in 5.7. I have commented 2 lines with the 

./mydumper.c: /* if (fields[i].type == MYSQL_TYPE_JSON) g_string_append(statement_row, "CONVERT("); */

./mydumper.c: /* if (fields[i].type == MYSQL_TYPE_JSON) g_string_append(statement_row, " USING UTF8MB4)"); */

I hope it will not affect me unless I am using 5.7+.

Доходчиво про индексы MySQL

Ссылка для прочтения.