Database Denormalisation

Add redundant data to the database to optimise queries and remove some joins.

For example, you have

• Posts table
• Comments table

For each post, you want to display all the list post number of associated comments. Doing this will need at least 2 query

1. One query to list the post
2. One query per post to count how many comments
3. ...

Which means several queries at once. Now what we can do is to add another number of comments field in the Posts table

Doing this you only need 1 query to list the post, and don't need to touch the Comments table as number of commenrts are already de-normalised to the Posts table

Now there are some costs

• Disk and memory as store the redundant data
• Each time add and remove a comment, have to
  • save/delete the comment
  • update the corresponding number in the Posts table

Note: this is good if your blog has a lot more people reading than writing comments (not so bad)