Reddit's database has only two tables
> Adding a column to 10 million rows takes locks and doesn’t work.It does not take locks, other than for very briefly.1. Make a new empty table that has the same structure as the table you wish to add a column to. Add your new column to the empty table.2. Put triggers on the old table that, whenever a row is added or updated, makes a copy of the row in the new table or updates the copy already there.3. Run a background process that goes through the old table doing dummy updates:    UPDATE table SET some_col = some_col WHERE ...

where the WHERE clause picks a small number of rows (e.g., just go through the primary key sequentially). Since you aren't actually modifying the table, all this does is trigger the trigger on the specified rows.4. When you've hit everything with a dummy update, rename the current table to a temp name, and rename the new table to the current table. This is the only step that needs a lock.There are tools for MySQL to automate much of this. There was a post either here or on Reddit a while back about this which linked to them. I'm sorry but I didn't save a link to it so you'll have to search if you want it.