Performance of query from this schema Essentially all comments are stored in this schema (dbml)

table comments {
  commentID BINARY(16) [not null] //UUID
  post_date timestamp
  postID BINARY(16) [not null] //UUID
  has_reply bool [default: false]
  // replyTo either points to commentID, or null
  // Essentially won't be loaded as a 'root' comment
  reply_to varchar [default: null]
  comment_content varchar [not null]
  // dislikes will decrease, likes increase
  likes int [default: 0]
  visible bool [default: true]
}


As the user goes through the posts, essentially the database would be queried as following
SELECT * FROM foo.comments WHERE postID = "ID of the post the USER is currently on" LIMIT 0, 20

Because of this setup, I realized this table would grow quite large housing all the comments.
After a while (say ambitiously) 1 million comments are created.  How would mariaDB handle this? Would it be dreadfully slow, if so how should it be modeled? 

Will the WHERE clause save me from performance issues?
What is the PRIMARY KEY of the table?  Probably commentID is unique to each row??  If so, add PRIMARY KEY(commentID).

For WHERE postID = "...", you should add INDEX(postID).  That will make that query fast even for a billion rows.

Read about "indexes" (aka, "keys").