sed tricks

I helped a charity to rebuild a MySQL server and to restore a database with a lot of data of longblob type in the last two days. Fortunately there was a dump backup file for the database in question.

However, tables with longblob column(s) were not defined with “ROW_FORMAT=COMPRESSED”. I’d like to restore that database with row compression before inserting the data. Therefore I need to modify the dump sql file. The problem is that the file is 2.5 GB and the server only has 4 GB memory. So editing it is a challenge. Fortunately, Linux has sed to save the day. Don’t you love open source free software?

I am power Vi/Vim user, so I am familiar with sed and have used it in the past. But there are still a few things that I searched around for quick answers. So I’ll record noteworthy points here. I couldn’t remember how many times my own past blog entries helped me over the years. And I hope you’ll find this helpful too!

  • The -n switch is important. sed is a stream editor. In many cases you’d like to supress data streaming to stdout, and -n does that. This was especially important in my case, because a) the file is large, b) it contains blob that may or may not “fit to print”;
  • To see a particular line, say line a, use the p (print) command: sed -n 'ap' file
  • To see all lines between line a and b: sed -n 'a,bp' file
  • To see multiple, non-adjacent lines, say line a, e, g: sed -n 'ap;ep;gp' file
  • To edit big files, you’d like to make in-place changes. Therefore the -i switch. For example, to put in InnoDB row compression, this is the command I used: sed -i 's/CHARSET=utf8;/CHARSET=utf8 ROW_FORMAT=COMPRESSED;/' file
  • Similarly, to delete line a: sed -i 'ad' file You can also do range delete as well

By the way, when restore InnoDB database with a lot of blob data, it makes a lot of sense to enable the following settings in my.cnf, if they are not enabled already. It’ll make administration much easier down the road:
innodb_file_per_table
innodb_file_format = Barracuda

You may also need to tweak the max_allowed_packet and innodb_log_file_size parameters for successful restore.

Something else to pay attention to:
If you use:

mysql -uuser -p database < dump.sql to restore the database back, the program may report the wrong line where it had loading problems. In most cases, you need to search the surrounding lines to find where the problem is. Additionally, if you are in a hurry and want to load data in without troubleshooting loading issues, you can try adding -f switch to the command above so the restore ignores errors it encountered and jump to the next line.

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