why do we need 'select…for share' instead of a simple 'select'

(From chatgpt)
A simple SELECT query in PostgreSQL operates under the MVCC (Multi-Version Concurrency Control) model, which allows it to read data without locking the rows. This means it can see a snapshot of the data at the start of the transaction, regardless of whether other transactions are modifying that data at the same time. However, this also means that the data can change after you read it, and you wouldn't necessarily know about those changes unless you explicitly check again or use locking.

SELECT ... FOR SHARE introduces a row-level lock to prevent other transactions from making changes to the rows you are reading while you're still working with them. This is important in scenarios where you want consistency during a multi-step process that involves multiple queries. Let's dive deeper into why you'd want to use FOR SHARE over a simple SELECT.

Differences between a simple SELECT and SELECT ... FOR SHARE:

1. Simple SELECT:

   • Reads data without acquiring any locks that would block other transactions.
   • Uses MVCC to give you a consistent snapshot of the data as it was when the transaction started.
   • Other transactions can modify the rows you're reading (update, delete, etc.), and you won't be notified of those changes.
   • It works fine for purely read-only operations where you don't care if the data changes after your read.

2. SELECT ... FOR SHARE:

   • Acquires a shared lock on the rows you read.
   • Other transactions can still read these rows (with their own FOR SHARE), but they cannot modify or delete them until your transaction completes.
   • Prevents situations where another transaction modifies or deletes the rows you are reading before you've finished your own operation.
   • Useful when you need consistency across multiple steps in your transaction and want to ensure the rows you read remain unchanged while your transaction is ongoing.

Why not just use a simple SELECT?

A simple SELECT may not be sufficient in scenarios where the data you're reading must remain unchanged for the duration of your transaction or process. Without FOR SHARE, there’s no guarantee that another transaction won’t modify the data immediately after you read it, which could lead to:

1. Inconsistent Data: The data you just read might be modified by another transaction while you're still working, causing issues if your next operations depend on it being the same.

2. Lost Updates or Deletes: If you're planning to perform an action based on the data you just read, such as an update, without FOR SHARE, another transaction could modify or delete those rows before you execute your update, resulting in an error or unintended behavior.

Use Case Example:

Let’s say you have an e-commerce system where two transactions are happening:

• Transaction A wants to check the availability of a product and, if available, place an order.
• Transaction B wants to update the product's stock (e.g., after a restock or sale).

Without FOR SHARE, these transactions could interfere with each other. Here’s how things could go wrong with a simple SELECT:

Scenario without FOR SHARE:

1. Transaction A:

   BEGIN;
   SELECT stock FROM products WHERE product_id = 1;
   -- Stock is 5 units.
   -- Planning to place an order.
   

2. Transaction B (in parallel):

   BEGIN;
   UPDATE products SET stock = stock - 5 WHERE product_id = 1;
   -- Transaction B finishes and commits.
   COMMIT;
   

3. Transaction A continues:

   -- Transaction A continues based on the earlier read (which was 5 units), not knowing stock has changed.
   INSERT INTO orders (product_id, quantity) VALUES (1, 2);
   COMMIT;
   

   In this scenario, Transaction A was working with stale data. The stock was modified by Transaction B in the middle, but Transaction A wasn’t aware, resulting in potentially incorrect behavior.

Scenario with FOR SHARE:

If Transaction A used SELECT ... FOR SHARE:

1. Transaction A:

   BEGIN;
   SELECT stock FROM products WHERE product_id = 1 FOR SHARE;
   -- Stock is 5 units. Row is locked for modifications by other transactions.
   

2. Transaction B (in parallel) tries to modify the stock but is blocked:

   UPDATE products SET stock = stock - 5 WHERE product_id = 1;
   -- Blocked until Transaction A finishes.
   

3. Transaction A:

   -- Transaction A proceeds safely, knowing the data is still consistent.
   INSERT INTO orders (product_id, quantity) VALUES (1, 2);
   COMMIT;
   

In this case, Transaction B is blocked from modifying the stock until Transaction A completes, ensuring Transaction A is working with up-to-date and consistent data.

Conclusion:

• Use a simple SELECT when you just need to read data without caring if it changes afterward.
• Use SELECT ... FOR SHARE when you need to ensure that no other transaction modifies the data while your transaction is ongoing, but you're not making updates yourself. This is particularly important when your subsequent operations depend on the data staying consistent during a multi-step process.