To set up a hierarchical relationship in Firebase Firestore, you can use the reference data type. This creates a link between documents. Here's how you can do it:

• Step 1: Picture two collections: "Posts" and "Comments". Each post will have several comments.
• Step 2: When adding a new comment to a post, in the comments collection, create a new document. Include a "post" field, which is a reference to the specific post document. Use db.collection('Posts').doc('postId') to reference the post document.
• Step 3: This way, you create an interconnected relationship. Now, you can use this reference field to fetch data of the referenced post directly from a comment document.

Subcollections let us nest collections within documents, giving a natural hierarchical structure. Here's how to create a subcollection:

• Step 1: Using the same "Posts" and "Comments" example, imagine each post has multiple comments.
• Step 2: Instead of creating a separate "Comments" collection, add each comment as a document in a subcollection under each Post document. Use db.collection('Posts').doc('postId').collection('Comments').doc('commentId') to add documents to the Comments' subcollection.
• Step 3: This way, you can fetch comments directly from any specific post, providing a clear structure.

Sometimes, you need to access certain data frequently and quickly without going through its parent document. Denormalization, which involves duplicating data, can help with this.

• Step 1: Suppose you need to fetch comments' details often, without loading post information.
• Step 2: While adding a comment reference under a particular post, also add the comment as a document in a separate Comments' collection, including some basic post details like 'postId', 'postTitle', etc.
• Step 3: Even though this duplicates data, denormalization optimizes read performance, allowing you to access comments directly without fetching their parent post data.

Firestore transactions help maintain consistent data when managing hierarchical data.

• Step 1: Suppose when a comment is added to a post, you want to increase a 'commentCount' in the post document.
• Step 2: A transaction ensures that 'commentCount' is updated correctly even when multiple users are adding comments at the same time.
• Step 3: Start a transaction using db.runTransaction(). Then use 'transaction.get()' to fetch the post document. Increase the 'commentCount' by one and update the document using 'transaction.update()'.

Similarly, Batch writes ensure that multiple writing operations either all succeed or all fail, keeping data consistent. Use db.batch() to start a batch write.